Plant Life

Acid Precipitation

2011-12-27T02:29:00.000-08:00

Acid precipitation is rain, snow, or mist which has a pH lower than unpolluted precipitation. Increased levels of acid precipitation have significant effects on food chains and ecosystems.

Precipitation—rain, snow, hail, sleet, or mist—is naturally acidified by carbonic acid (H₂CO₃). Carbon dioxide (CO₂) in the atmosphere reacts with water molecules, lowering the pH of precipitation to 5.6. A pH scale is used to measure a solution’s acidity or alkalinity; pH is defined as the negative logarithm of the concentration of hydrogen ions, H⁺ . A solution with a pH of 7.0 is neutral. A pH lower than 7 is acidic, and a pH greater than 7 is alkaline.

Other acidic substances are also present in the atmosphere, causing "unpolluted" precipitation to have a pH approaching 5.0. Solutions with a pH of 5.0 or less have concentrations of hydroxyl ion, or OH^– , and carbonate ion, or CO3^– , approaching zero.

Acid precipitation is the name given to rain or snow contaminated with oxides of sulfur (SO_x) and oxides of nitrogen (NO_x). These chemicals combine with water droplets to form sulfuric acid and nitric acid. SO_x is formed by combustion of materials containing sulfur, and NO_x is formed by oxidation of molecular nitrogen in the atmosphere during combustion. SO_x sometimes arises fromnatural sources such as volcanoes and geyser fields, and NO_x is formed by lightning.

Downwind of smelting facilities, hydrochloric acid (HCl) and hydrofluoric acid (HF)may also contribute to acid precipitation. Acid precipitation may detrimentally change soil chemistry, either by stripping nutrients, especially magnesium and calcium, or mobilizing phytotoxic trace elements (elements toxic to plants).

Geographic Extent of Damage

Acid precipitation is a regional problem. SO_x and NO_x can travel many thousands of kilometers in the atmosphere after being emitted by large, stationary sources, especially those that have very high smoke stacks.

These pollutants are slowly transformed into sulfuric and nitric acid aerosols and are incorporated into precipitation, which eventually makes contact with the earth’s surface. Acid precipitation in the eastern United States contains more SO_x than precipitation in the western United States, which contains more NO_x.

In North America, acid precipitation and dry deposition (of acid aerosol particles) are major environmental problems in New England and New York State and in Ontario and Quebec. These regions attribute much of their acid precipitation to emissions from large coal-burning plants in the American Ohio Valley.

Scandinavian activists blame coal-burning power plants and factory emissions in the British Isles for that region’s acid rain problems. Central Europe—including Poland, the Czech Republic, Slovakia, and eastern Germany—has many power plants and factories that burn high-sulfur coal. Acid-laden pollution plumes stretch thousands of kilometers downwind from smokestacks in that region.

Controlled Studies

Controlled experiments on individual plant species have revealed short-term damage to a limited number of those species. Experiments using simulated acid rain (SAR) are difficult to extrapolate to field conditions, where the specific pollutants and pH levels vary widely over time.

In controlled conditions, studies showed no link between SAR and yield in Amsoy soybeans. However, field studies demonstrated that acid deposition does decrease yield in Amsoy soybeans.

Acid precipitation influences plant diseases by acting on both pathogens and host organisms. Seedlings of Pinus rigida, Pinus echinata, Pinus taeda, and Pinus strobus exposed to SAR of pH 3.0 had a 100 percent mortality rate because of fungal damping-off, a diseased condition of seedlings marked by wilting or rotting.

Red spruce seedlings subjected to dilute sulfuric acid mist developed brown lesions on their needles, followed by needle drop. Studies showed a reduction in the growth of sugar maple seedlings following exposure to low pH moisture, and that seedling survival decreased with increasing acidity.

Crop and Forest Decline

In field experiments, soybeans have shown reduced yields with decreasing pH (increasing acidity) of moisture applied. Yields of seed and seed protein are both reduced in soybeans exposed to high acidity. A lower number of seed pods were found in plants exposed to high acidity, compared to control plants.

Acid precipitation causes detrimental long-term effects in most ecosystems, especially forests. Root systems under acidic stress show great variability in tolerance and injury. Acidic stress on roots decreases root growth, measured by a reduction in root length, and severely damaged trees have more fine roots with opaque tip zones than do slightly damaged trees. Some scientists have suggested that the radical growth rate in yellow pines in the southeastern United States may be reduced by acid precipitation.

Since the 1960’s Central European soils have been progressively acidified, altering soil buffering capacities. Acid rain containing nitrates (which are not immobilized in soil) played an important role in this soil acidification.

Acidification has reduced the magnesium, calcium, and potassium available for nutrient uptake by plants and has affected root growth. One-quarter of European forests are moderately or severely damaged by acid precipitation, with dry deposition believed (by scientists and politicians) to be largely responsible.

This pattern of damage, first detected in the 1980’s, has been called neuartige Waldschäden (literally, "new-type forest decline"). It has been detected throughout Central Europe at all elevations and on all soil types. Waldschäden is most pronounced downwind of major air pollution sources.

Abnormally high numbers of red spruce have died in the high-elevation northern Appalachian Mountains since the 1960’s. This die-off has been attributed to high rates of acid deposition (up to 4 kilo equivalents of hydronium ions per hectare per year) and exposure to acid fog droplets for up to two thousand hours per year. Very high levels of trace metals (known to be phytotoxic) have accumulated in the region.

Aquatic Ecosystems

Most freshwater ecosystems range from pH 6.0 to pH 8.0. In limestone terrain, acid precipitation is neutralized by dissolution of calcium carbonate. As a freshwater environment becomes acidified, the number of species it supports declines. When conditions are more acidic than pH5.5, dissolved inorganic carbon exists only as dissolved carbon dioxide. Planktonic algae, which can use low levels of dissolved inorganic carbon, are favored in these environments.

Acid environments greatly reduce the numbers of herbivores that graze on aquatic plants; this is thought to explain why filamentous green algae are found in most acidified lakes. Scientists point out that it is difficult to separate the effects that acidification alone produces in an aquatic ecosystem.

Adaptations

2011-12-27T00:48:00.000-08:00

The results of natural selection in which succeeding generations of organisms become better able to live in their environments are called adaptations. Many of the features that are most interesting and beautiful in biology are adaptations. Specialized structures, physiological processes, and behaviors are all adaptations when they allow organisms to cope successfully with the special features of their environments.

Adaptations ensure that individuals in populations will reproduce and leave well-adapted offspring, thus ensuring the survival of the species. Adaptations arise through mutations—inheritable changes in an organism’s genetic material.

These rare events are usually harmful, but occasionally they give specific survival advantages to the mutated organism and its offspring. When certain individuals in a population possess advantageous mutations, they are better able to cope with their specific environmental conditions and, as a result, will contribute more offspring to future generations than those individuals that lack the mutation.

Over time, the number of individuals that have the advantageous mutation will increase in the population at the expense of those that do not have it. Individuals with an advantageous mutation are said to have a higher fitness than those without it, because they tend to have comparatively higher survival and reproductive rates. This is natural selection.

Natural Selection

Over very long periods of time, evolution by natural selection results in increasingly better adaptations to environmental circumstances. Natural selection is the primary mechanism of evolutionary change, and it is the force that either favors or selects against mutations.

Although natural selection acts on individuals, a population gradually changes as those with adaptations become better represented in the total population. Most flowering plants, for example, are unable to grow in soil containing high concentrations of certain elements (for example, heavy metals) commonly found in mine tailings.

Therefore, an adaptation that conferred resistance to these elements would open up a whole new habitat where competition with other plants would be minimal. Natural selection would favor the mutations, which confer specific survival advantages to those that carry them and impose limitations on individuals lacking these advantages.

Thus, plants with special adaptations for resistance to the poisonous effects of heavy metals would have a competitive advantage over those that find heavy metals toxic. These attributes would be passed to their more numerous offspring and, in evolutionary time, resistance to heavy metals would increase in the population.

Types of Adaptations

Although natural selection serves as the instrument of change in shaping organisms to very specific environmental features, highly specific adaptations may ultimately be a disadvantage. Adaptations that are specialized may not allow sufficient flexibility (generalization) for survival in changing environmental conditions.

The degree of adaptative specialization is ultimately controlled by the nature of the environment. Environments, such as the tropics, that have predictable, uniform climates and have had long, uninterrupted periods of climatic stability are biologically complex and have high species diversity.

Scientists generally believe that this diversity results, in part, from complex competition for resources and fromintense predator-prey relationships. Because of these factors, many narrowly specialized adaptations have evolved when environmental stability and predictability prevail.

By contrast, harsh physical environments with unpredictable or erratic climates seem to favor organisms with general adaptations, or adaptations that allow flexibility. Regardless of the environment type, organisms with both general and specific adaptations exist because both types of adaptation enhance survival under different environmental circumstances.

Metabolism is the sum of all chemical reactions taking place in an organism, whereas physiology consists of the processes involved in an organism carrying out its function. Physiological adaptations are changes in the metabolism or physiology of organisms, giving them specific advantages for a given set of environmental circumstances.

Because organisms must cope with the rigors of their physical environments, physiological adaptations for temperature regulation, water conservation, varying metabolic rate, and dormancy allow organisms to adjust to the physical environment or respond to changing environmental conditions.

Adaptations and Environment

Desert environments, for example, pose a special set of problems for organisms. Hot, dry environments require physiological mechanisms that enable organisms to conserve water and resist prolonged periods of high temperature.

Evolution has favored a specialized form of photosynthesis in cacti and other succulents inhabiting arid regions. Crassulacean acid metabolism (CAM) photosynthesis allows plants with this physiological adaptation to absorb carbon dioxide at night, when relative humidity is comparatively high and air temperatures relatively low.

Taking in carbon dioxide during the day would dehydrate plants, because opening the pores through which gas exchange takes place allows water to escape from the plant. CAM photosynthesis, therefore, allows these plants to exchange the atmospheric gases essential for their metabolism at night, when the danger of dehydration is minimized.

Because organisms must also respond and adapt to an environment filled with other organisms— including potential predators and competitors— adaptations that minimize the negative effects of biological interactions are favored by natural selection. Often the interaction among species is so close that each species strongly influences the others and serves as the selective force causing change.

Under these circumstances, species evolve together in a process called coevolution. The adaptations resulting from coevolution have a common survival value to all the species involved in the interaction. The coevolution of flowers and their pollinators is a classic example of these tight associations and their resulting adaptations.

Speciation

Adaptations can be general or highly specific. General adaptations define broad groups of organisms whose lifestyles are similar. At the species level, however, adaptations are more specific and give narrow definition to those organisms that are more closely related to one another.

Slight variations in a single characteristic, such as bill size in the seed-eating Galapágos finches, are adaptive in that they enhance the survival of several closely related species. An understanding of how adaptations function to make species distinct also furthers the knowledge of how species are related to one another.

Why so many species exist is one of the most intriguing questions of biology. The study of adaptations offers biologists an explanation. Because there are many ways to cope with the environment, and because natural selection has guided the course of evolutionary change for billions of years, the vast variety of species existing on the earth today is simply an extremely complicated variation on the theme of survival.

Active Transport

2011-12-27T00:17:00.000-08:00

Active transport is the process by which cells expend energy to move atoms or molecules across membranes, requiring the presence of a protein carrier, which is activated by ATP. Cotransport is active transport that uses a carrier that must simultaneously transport two substances in the same direction. Countertransport is active transport that employs a carrier that must transport two substances in opposite directions at the same time.

Biologists in nearly every field of study have discovered that one of the major methods by which organisms regulate their metabolisms is by controlling the movement of molecules into cells or into organelles such as the nucleus.

This regulation is possible because of the semipermeable nature of cellular membranes. The membranes of all living cells are fluid mosaic structures composed primarily of lipids and proteins. The lipid molecules are aliphatic, which means that their molecular structure exhibits both a hydrophilic (water-attracted) and a hydrophobic (water-repelling) portion.

These aliphatic molecules form a double layer: The hydrophilic heads are arranged opposite one another on the inner and outer surfaces, and the hydrophobic tails are aligned across from one another within the interior, sandwiched between the hydrophilic heads. The protein in the membrane is interspersed periodically throughout the lipid bilayer.

Some of the protein, referred to as peripheral protein, penetrates only one of the lipid layers. The integral protein, as the remaining protein is called, extends through both layers of lipid to interface with the environment on both the internal and external surfaces of the membrane. These integral proteins can serve as transport channels and carriers.

Cellular Energy

Transport across the membrane is accomplished by three different mechanisms: simple diffusion, facilitated diffusion, and active transport. The first two mechanisms are referred to as passive processes because they do not require the direct input of cellular energy, and they involve transport down a concentration gradient, that is, from the side with a higher concentration to the side with a lower concentration of the substance being transported.

In many instances, however, substances are transported across a membrane from the side with a low concentration to the side containing a greater concentration. This "uphill" movement across membranes is called active transport, and it requires the expenditure of cellular energy.

Cellular energy, produced by the biological oxidation of fuels such as carbohydrates, is stored as adenosine triphosphate (ATP). When this high-energy phosphate is hydrolyzed, the stored energy is released to drive cellular reactions such as active transport. The ATPase protein located in membranes belongs to one of the groups of enzymes which hydrolyze ATP.

The mechanism has not been completely deciphered, but it appears as though a protein carrier molecule binds with the substance to be transported at the surface on one side of the membrane. This binding occurs at a specific activated region on the carrier protein called the active site. After combining with the carrier, the substance is moved across the membrane and released at the surface on the other side of the membrane.

ATP is then hydrolyzed by an ATPase, and the energy released in this reaction prepares the protein carrier for attachment to another molecule to be transported by reactivating the active site. There is some question as to whether ATPase is a component of the carrier molecule or functions separately from it. Regardless of the spatial arrangement, the two molecules are intimately related in the active transport process.

Cotransport and Countertransport

There are two important modifications of the active transport process: cotransport and countertransport. Cotransport, or symport, involves a specialized protein molecule referred to as a symport carrier. Asymport carrier has two attachment sites. One site is for the attachment of the molecule to be transported, and the other is for the attachment of a second molecule, which can be referred to as the synergist.

Both the molecule to be transported and the synergist must be bound to the symport carrier before transport across the membrane can take place. The synergist is moved down a concentration gradient, and this downhill flow of the synergist drives the carrier to transport both molecules.

In order to keep the synergist moving down a concentration gradient when attached to the symport carrier, the synergist must be pumped back across the membrane. This movement of the synergist in the opposite direction is mediated by a protein carrier activated by the energy released from the hydrolysis of ATP by an ATPase.

Countertransport, or antiport, also utilizes a specialized carrier with two attachment sites. This antiport carrier binds with the molecule to be transported at one of the attachment sites, and a second molecule, which can be called the antagonist, binds at the other.

The carrier moves the molecule to be transported across the membrane while simultaneously moving the antagonist in the opposite direction. Again, both molecules must be attached to the antiport carrier before either can be transported, and the flow of the antagonist down a concentration gradient drives the transport by the carrier in both directions.

The antagonist is pumped back across the membrane by a protein carrier activated by the energy released from the hydrolytic action of an ATPase on ATP. This action maintains a concentration gradient favorable for transport when the antagonist is attached to the antiport carrier.

Transport in Action

The presence of these three active transport mechanisms has been well documented. Calcium, for example, has been shown to be pumped from the cell by a carrier protein activated by the hydrolysis of ATP. Sugars for energy and carbohydrate structure must be cotransported into the cell by a symport carrier that utilizes the sodium ion as a synergist. At least two countertransport ion pumps have been identified.

One pumps the potassium ion into the cell at the same time that it pumps the hydrogen ion out. The second pumps the potassium ion into the cell while the antagonist, sodium, is moved in the opposite direction. It is likely that numerous other active transport systems exist that have not yet been positively identified.

A protein carrier is one of the basic components of any active transport mechanism. Although no specific carrier molecule has yet been positively identified, there is ample indirect evidence to support the presence of such a protein. Much of this evidence comes from studies showing that active transport exhibits saturation kinetics.

This means that the transport of a specific ion will increase as the concentration increases, up to a certain point. At this point, further increases in concentration will have no effect on transport. These results strongly suggest that the ion is binding with another molecule in the membrane, such as a carrier protein, which is limited in concentration and becomes saturated.

Studies have also shown the transport of some substances to be competitively inhibited by the presence of another, very similar, substance. This indicates that both substances are competing for the same site on a membrane molecule, such as a protein carrier.

Role of Active Transport

The ability to accumulate substances against a concentration gradient is necessary for the normal function and survival of cells. There are numerous examples, however, of active transport being intimately involved in the regulation of some important biological processes. In the plant kingdom, sugar is produced by photosynthesis in the green leaves.

This sugar must be transported out of the leaves and into nonphotosynthetic tissues, such as roots or fruit, through specialized transport cells in the phloem. The loading of sugars into the phloem is dependent on an active cotransport mechanism.

Almost every field of life science is concerned with gene regulation. Genes are continually being induced (activated) or repressed (deactivated) as organisms develop and change from the time of their conception until their death.

Repression is usually caused by the presence of a protein molecule in the cell nucleus, but induction may very often be the result of metabolites being actively transported into the cell or nucleus. Hence, the active transport mechanisms may be a very important component of gene regulation.

Adaptive Radiation

2011-12-26T23:07:00.000-08:00

In adaptive radiation, numerous species evolve from a common ancestor introduced into an environment with diverse ecological niches. The progeny evolve genetically into customized variations of themselves, each adapting to survive in a particular niche.

In 1898 Henry F. Osborn identified and developed the evolutionary phenomenon known as adaptive radiation, whereby different forms of a species evolve, quickly in evolutionary terms, from a common ancestor.

According to the principles of natural selection, organisms that are the best adapted (most fit) to compete will live to reproduce and pass their successful traits on to their offspring. The process of adaptive radiation illustrates one way in which natural selection can operate when members of one population of a species are cut off or migrate to a different environment that is isolated from the first.

Such isolation can occur from one patch of plantings to another, from one mountain top or hillside to another, from pond to pond, or from island to island. Faced with different environments, the group will diverge from the original population and in time become different enough to form a new species.

Genetic Changes

In a divergent population, the relative numbers of one form of allele (characteristic) decrease, while the relative numbers of a different allele increase. New environmental pressures will select for favorable alleles that may not have been favored in the old environment.

Over successive generations, therefore, a new gene created by random mutation (change) may replace the original form of the gene if, for example, the trait encoded by that gene allows the divergent group to cope better with environmental factors, such as food sources, predators, or temperature.

The result in the long term is that deoxyribonucleic acid (DNA) changes sufficiently through the growth of divergent populations to allow new generations to become significantly different from the original population. In time, they are unable to reproduce with members of the original species and become a new species.

Galápagos Islands Case Study

Adaptive radiation occurs dramatically when a species migrates from one landmass to another. This may occur between islands or between continents and islands. A classic example of adaptive radiation is the evolution of finches noted by Charles Darwin during his trips to the Galápagos Islands off the west coast of South America.

Several species of plants and animals had migrated to these islands from the South American mainland by means of flight, wind, ocean debris, or other means of transport. Finches from the mainland—perhaps aided by winds—settled on fifteen of the islands in the Galápagos group and began to adapt to the various unoccupied ecological niches on those islands, which differed.

Over several generations, natural selection favored a variety of finch species with beaks adapted for the different types of foods available on the different islands. As a result, several species of different finches evolved, roughly simultaneously, on these islands.

Hawaiian Silversword Alliance

Although plants seem unable to "migrate" as birds and other animals do, adaptive radiation occurs in the plant world as well. In the Hawaiian Islands, for example, twenty-eight species of the Asteraceae family are known together as the Hawaiian silversword alliance. The entire group appears to be traceable to one ancestor, thought to have arrived on the island of Kauai from western North America.

The silverswords—which compose three genera, Argyroxiphium, Dubautia, and Wilkesia— have since evolved into twenty-eight species, and this speciation came about due to major ecological shifts. These plants are therefore prime examples of adaptive radiation.

Within the silversword alliance, different species have adapted to widely varying ecosystems found throughout the islands. Argyroxiphium sandwicense, for example, is endemic to the island of Maui and grows at high elevations from 6,890 to 9,843 feet (2,100 - 3,000 meters) on the dry, alpine slopes of the volcano Haleakala.

This species has succulent leaves covered with silver hairs. It is thought that the hairs lessen the pace of evaporative moisture loss and protect the leaves from the sun. In contrast, species of the genus Dubautia that grow in wet, shady forests have large leaves that lack hairs.

Despite their "customized" physiologies, the silverswords that have evolved in Hawaii are all closely related to one another, so much so that any two can hybridize. Studies of the silverswords have provided what geneticist Michael Purugganan called a "genetic snapshot of plant evolution". Adaptive radiation is one window into how new plant structures arise.

African Agriculture

2011-12-26T22:48:00.000-08:00

Soil and climatic conditions throughout Africa determine not only agricultural practices, such as which crops can be grown, but also whether plant life is capable of sustaining livestock on the land and enabling fishing of the oceans.

Rainfall—the dominant influence on agricultural output—varies greatly among Africa’s fifty-six countries. Without irrigation, agriculture requires a reliable annual rainfall of more than 30 inches (75 centimeters). Portions of Africa have serious problems from lack of rainfall, such as increasing desertification and periods of drought.

Food output has declined, with per capita food production 10 percent less in the 1990’s than it was in the 1980’s. In most African countries, however, more than 50 percent, and often 80 percent, of the population works in agriculture, mostly subsistence agriculture. Large portions of the continent, such as Mali and the Sudan, have the potential of becoming granaries to much of the continent and producing considerable food exports.

Traditional African Agriculture

Traditionally, agriculture in Africa has been subsistence farming in small plots. It has been labor-intensive, relying upon family members. New land for farming was obtained by the slash-and-burn method (shifting cultivation). The trees in a forested area would be cut down and burned where they fell.

The ashes from the burned trees fertilized the soil. Bothmen andwomenworked at such farming. Slash-and-burn agriculture is common not only in Africa but also in tropical areas around the world. In areas of heavy rainfall, the rainswash out the nutrients from soil and burned trees in a period of two to three years.

The crops grown depend upon the region. In the very dry, yet habitable, parts of Africa—such as the Sudano-Sahelian region that stretches from Senegal and Mali in the west of Africa to the Sudan in the east—a key subsistence crop is green millet, a grain. Ground into a type of flour, it can be made into a bread-like substance.

In moister areas, traditional crops are root and tuber crops, such as yams and cassava. Cassava has an outer surface or skin that is poisonous, but it can be treated to remove the poison. The tuber then can be ground and used tomake a bread-like substance.Other important traditional crops are rice and corn, which were introduced by Europeans when they came to Africa.

Animal husbandry, or seminomadic herding, is another form of traditional agriculture. Problems that have arisen with this type of agriculture are the availability of water and grass or hay for cattle. Regions that are very moist, such as the Gulf of Guinea, which has rain forest, are not good for cattle because of the tsetse fly, which carries diseases such as sleeping sickness.

Crops

The most widely grown crop is rice, which is grown onmore than one-third of the irrigated crop area in Africa. Cultivated mostly in wetlands and valley bottoms, rice is the most common crop in the humid areas of the Gulf of Guinea and Eastern Africa. It is also grown on the plateaus of Madagascar.

In the northern and southern regions, rice represents only a small portion of the total crops under water management. Wheat and corn are cultivated and irrigated, mostly in Egypt, Morocco, South Africa, Sudan, and Somalia.

Vegetables, including root and tuber crops, are present in all regions and almost every country. Vegetables are grown on about 8 percent of the cultivated areas under water management. In Algeria, Mauritania, Kenya, Burundi, and Rwanda, they are the most widespread crops under water management.

Arboriculture (growing of fruit trees), which represents 5 percent of the total irrigated crops, is concentrated in the northern region and consists mostly of citrus fruits. Commercial crops (for cash and export) are grown mostly in the Sudan and in the countries of the southern region and consist mostly of cotton and oilseeds.

Other commercial crops in Africa are sugarcane, coffee, cocoa, oil and date palm, bananas, tobacco, and cut flowers. Sugarcane is grown in all countries except in the northern region. The other commercial crops are concentrated in a few countries.

North Africa

In Morocco, Algeria, Tunisia, Libya, and Egypt, the region’s agricultural resources are limited by its dry climate. Its products are those typical of the Mediterranean, steppe, and desert regions: wheat, barley, olives, grapes, citrus fruits, some vegetables, dates, sheep, and goats.

Agriculture employs less than 20 percent of the working population in Libya and as much as 55 percent in Egypt. From about the middle of the twentieth century, North Africa’s production failed to keep pace with its population growth and remained susceptible to large annual fluctuations.

Cropland occupies about 33 percent of Tunisia but less than 3 percent of Algeria, Egypt, and Libya. Some export crops, such as citrus fruits, tobacco, and cotton, have suffered from strong international competition.

The northern region is not a major contributor to the continent’s fish catch. Morocco, however, with its cool, plankton-rich Atlantic waters and access to the Mediterranean Sea, is one of the world’s largest fish producers.

Sudano-Sahelian Region

This region comprises Mauritania, the western Sahara, Senegal, Gambia, Mali, Burkina-Faso, Niger, Chad, and the Sudan. Because of the region’s extreme dryness, mostly subsistence farming and seminomadic herding are practiced. Millet is the primary crop.

In the late twentieth century, this region was devastated by long droughts that caused famine and starvation. Mali and the Sudan have the Niger and Nile Rivers flowing through them. These great rivers provide plenty of water for irrigation of fields.

During the rainy season in Mali—typically June through September—the Niger River widens into a great, extensive floodplain. This area is good for the growing of rice. Similarly, in the Sudan the Blue and White Niles meet at Khartoum to form the Nile River.

Gulf of Guinea

This region comprises Guinea-Bissau, Cape Verde, Guinea, Liberia, Sierra Leone, Côte d’Ivoire, Togo, Ghana, Benin, and Nigeria. With the exception of Nigeria, agriculture there is dominated by rice cultivation. The percentage of total land area that is under cultivation ranges from 60 percent in Liberia to just 9 percent in Sierra Leone.

The total cultivable area of Ghana is 39,000 square miles (100,000 square kilometers), or 42 percent of its total land area. Only 4.8 percent of the total land area was under cultivation at the end of the twentieth century. Much of the cultivation is subsistence farming of yams and other crops.

Ghana’s efforts in agriculture have been hampered by droughts. Additional problems are that organic matter has been leached out of the soils by heavy rainfall and that increasing deforestation has led to additional erosion. This is the situation in much of the Gulf of Guinea and the central regions.

About half of Nigeria’s available land is under cultivation. Increasing rainfall from the semiarid north to the tropically forested south allows for great crop diversity. Principal food crops are corn, millet, yams, sorghum, cassava, rice, potatoes, and vegetables.

Nigeria was the world’s fourth-largest exporter of cocoa beans in 1990-1991, accounting for about 7.1 percent of world trade in this commodity. However, Nigeria’s share of the world cocoa market has been substantially reduced because of aging trees, low prices, black pod disease, smuggling, and labor shortages.

Central Region

This region comprises the Central African Republic,Cameroon,Congo-Brazzaville,Congo-Kinshasa, Gabon, Equatorial Guinea, Burundi, Rwanda, and São Tomé and Príncipe. Cameroon has 14.7 million acres of arable land.

In 1997, 55,000 tons of rice were produced, but the country imported 124,000 tons in 1995. In the central region, the percentage of arable land ranges from 0.4 percent for the Congo-Brazzaville to 47 percent for Rwanda.

Cassava is harvested in Congo-Brazzaville, Congo-Kinshasa, Equatorial Guinea, and Gabon. Corn is harvested in Congo-Brazzaville, Congo-Kinshasa, and Burundi. In Rwanda, 17 percent of the harvested land is used to grow sweet potatoes. Agriculture is not important in the economy of São Tomé and Príncipe.

Eastern Region

This region comprises Eritrea, Djibouti, Ethiopia, Somalia, Kenya, Uganda, and Tanzania. Agriculture employs about 80 percent of the labor force in Uganda and Ethiopia. Approximately 2.5 million small farms dominate agriculture in both countries.

About 84 percent of Uganda’s land is suitable for agriculture—a high percentage compared to the majority of African countries, such as Ethiopia with only 12 percent. Food crops account for about 74 percent of agricultural production.

Only one-third is marketed; the rest is for home consumption. In four years out of five, the minimum needed rainfall may be expected in 78 percent of Uganda but in only 15 percent of Kenya. Somalia and Ethiopia receive almost none of the needed minimum.

Tanzania has almost four million farms. Traditional export crops include coffee, cotton, cashew nuts, tobacco, and tea. Major staple foods (corn, rice, and wheat) are exported in times of surplus.

Tanzania’s climatic growing conditions are favorable for the production of a wide range of fruits, vegetables, and flowers. Drought-resistant crops (sorghum, millet, and cassava) and other substaples such as onions, Irish potatoes, sweet potatoes, bananas, and plantains are also produced.

Areas that have 20-30 inches (50-75 centimeters) of rainfall per year rely on a mixture of agriculture and livestock herding. Regions with a smaller annual rainfall or a long dry season can support only drought-resistant crops such as sorghum, millet, and cassava. Over large areas of eastern Africa, rainfall is inadequate for crop cultivation.

The whole of Somalia and 70 percent of Kenya receive less than 20 inches (50 centimeters) of rain four years out of five. In these areas, the only feasible use of land is for raising livestock. Agriculture is not an important factor in the economies of Eritrea and Djibouti.

Southern Region

This region comprises Angola, Namibia, Zambia, Zimbabwe, Malawi, Mozambique, Botswana, Lesotho, Swaziland, and South Africa. The arable percentage of the total land area ranges from 14 percent in Malawi to just 1 percent in Namibia. With the exception of Mozambique, where cassava predominates, corn is the major crop in the countries in this region.

About 13 percent of South Africa’s land area can be used for crop production. Rainfall varies across the country, and varied climatic zones and terrains enable the production of almost any kind of crop.

The largest area of farmland is planted with corn, followed by wheat, then oats, sugarcane, and sunflowers. The nation is well known for the high quality of its fruits, such as apples and citrus.

Agriculture is the predominant economic activity in Zimbabwe, accounting for 40 percent of total export earnings—about 22 percent of the total economy—and employing more than 60 percent of the country’s labor force.

The main export crops are tobacco, cotton, and oilseeds. Zimbabwe is usually self-sufficient in food production. Its main food crops are corn, soybeans, oilseeds, fruits and vegetables, and sugar.

Mozambique’s agriculture has been badly hindered by civil war. However, the country has considerable potential for irrigation due to the Zambezi and Limpopo Rivers. The irrigation potential is estimated to be 7.5 million acres. In the 1990’s, only 110,000 acres were irrigated, growing rice, sugarcane, corn, and citrus.

Agriculture and livestock production employ about 62 percent of Botswana’s labor force. Most of the country has semidesert conditions with erratic rainfall and poor soil conditions, making it more suitable to grazing than to crop production. The principal food crops are sorghum and corn.

Namibia’s cultivated area is only 506,000 acres—only 0.8 percent of the cultivable area. Agriculture makes up approximately 10 percent of the economy but employs more than 80 percent of the population. The major irrigated crops are corn, wheat, and cotton.

Indian Ocean Islands

This region comprises Madagascar, Mauritius, the Comoros, and the Seychelles. During the 1990’s an estimated 8.7 million people lived in the rural areas, 65 percent of whom lived at the subsistence level.

Only 5.2 percent of Madagascar’s total land area (7.4 million acres) was under cultivation. Of the total land area, 50.7 percent supported livestock production, while 16 percent (1.2 million acres) of the land under cultivation was irrigated.

Cassava, planted almost everywhere on the island, is grown as well as corn and sweet potatoes, with smaller quantities of cotton, bananas, and cloves. The fisheries sector, especially the export of shrimp, has been the most rapidly growing area of the agricultural economy in the Indian Ocean Islands region.

Mauritius has 30,000 acres of sugarcane plantations that have had one of the highest sugarcane and sugar yields in the world. The Seychelles have a total land area of only 72 squaremiles (187 square kilometers), of which only 3,000 acres are cultivated.

This 3 percent of the land area accounts for only 4 percent of the island nation’s economy. The Comoros’ agriculture is heavily weighted toward rice, the staple food of the populace.

African Flora

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With few exceptions, Africa’s flora (vegetation) is tropical or subtropical. This is primarily because none of the African continent extends far from the equator, and there are only a few high-elevation regions that support more temperate plants.

Listed in order of decreasing land area, the three main biomes of Africa are subtropical desert, tropical savanna, and tropical forest. The flora in southern Africa has been most studied. The flora of central and northern Africa is less known.

The subtropical desert biome is the driest of the biomes in Africa and includes some of the driest locations on earth. The largest desert region is the Sahara in northern Africa. It extends from near the west coast of Africa to the Arabian Peninsula and is part of the largest desert system in the world, which extends into south central Asia.

A smaller desert region in southern Africa includes the Namib Desert, located along the western half of southern Africa, especially near the coast, and the Kalihari Desert, which is primarily inland and east of the Namib Desert.

Where more moisture is available, grasslands predominate, and as rainfall increases, grasslands gradually become tropical savanna. The difference between a grassland and a savanna is subjective but is in part determined by tree growth, with more trees characterizing a savanna. The grassland/ tropical savanna biome forms a broad swath across much of central Africa and dominates much of eastern and southern Africa.

Tropical forests make up a much smaller area of Africa than the other two biomes. They are most abundant in the portions of central Africa not dominated by the grassland/tropical savanna biome and are not far from the coast of central West Africa. Scattered tropical forest regions also occur along major river systems of West Africa, from the equator almost to southern Africa.

Subtropical Desert

The subtropical deserts of Africa seem, at first, to be nearly devoid of plants. While this is true for some parts of the Sahara and Namib Deserts that are dominated by sand dunes or bare, rocky outcrops, much of the desert has a noticeable amount of plant cover.

The Sahara is characterized by widely distributed species of plants that are found in similar habitats. The deserts of southern Africa have more distinctive flora, with many species endemic to specific local areas.

Succulents of the Subtropical Desert

To survive the harsh desert climate, plants use several adaptations. Mesembryanthemum, whose species include ice plant and sea figs, is a wide-spread genus, with species occurring in all of Africa’s deserts. It typically has thick, succulent leaves.

Such succulents store water in their leaves or stems, which they retain by using a specialized type of photosynthesis. Most plants open their stomata (small openings in the leaves) during the day to get carbon dioxide from the surrounding air.

This would lead to high amounts of water loss in a desert environment, so succulents open their stomata at night. Through a biochemical process, they store carbon dioxide until the next day, when it is released inside the plant so photosynthesis can occur without opening the stomata.

To prevent water loss, many succulents have no leaves at all. Anabasis articulata, found in the Sahara desert, is a leafless succulent with jointed stems. Cacti are found only in North and South America, but a visitor to the Sahara would probably be fooled by certain species in the spurge family that resemble cacti.

For example, Euphorbia echinus, another Saharan plant, has succulent, ridged stems with spines. The most extreme adaptation in succulents is found in the living stones of southern Africa. Their plant body is reduced to two plump, rounded leaves that are very succulent.

They hug the ground, sometimes partially buried, and have camouflaged coloration so that they blend in with the surrounding rocks and sand, thus avoiding being eaten by grazing animals. Other succulents, such as the quiver tree, attain the size and appearance of trees.

Water-Dependent Plants of the Subtropical Desert

Water-dependent plants are confined to areas near a permanent water source, such as a spring. The most familiar of these plants is the date palm, which is a common sight at desert oases.

Tamarind and acacia are also common where water is available. A variety of different sedges and rushes occur wherever there is abundant permanent freshwater, the most famous of these being the papyrus, or bulrush.

Ephemerals of the Subtropical Desert

Annuals whose seeds germinate when moisture becomes available and quickly mature, set seed, and die, are called ephemerals. These plants account for a significant portion of the African desert flora.

A majority of the ephemerals are grasses. Ephemerals are entirely dependent on seasonal or sporadic rains. A few days after a significant rain the desert turns bright green, and after several more days flowers, often in profusion, appear.

Some ephemerals germinate with amazing speed, such as the pillow cushion plant, which germinates and produces actively photosynthesizing seed leaves only ten hours after being wetted. Reproductive rates for ephemerals, and even for perennial plants, are rapid. Species of morning glory can complete an entire life cycle in three to six weeks.

Tropical Savanna

Tropical savanna ranges from savanna grassland, which is dominated by tall grasses lacking trees or shrubs, to thicket and scrub communities, which are composed primarily of trees and shrubs of a fairly uniform size.

The most common type of savanna in Africa is the savanna woodland, which is composed of tall, moisture-loving grasses and tall, deciduous or semi deciduous trees that are unevenly distributed and generally well spaced. The type of savanna familiar to viewers of African wildlife documentaries is the savanna parkland, which is primarily tall grass with widely spaced trees.

Savanna Grasses and Herbs

Grasses represent the majority of plant cover beneath and between the trees. In some types of savanna, the grass can be more than 6 feet (1.8 meters) high. Although much debated, two factors seem to perpetuate the dominance of grasses: seasonal moisture with long intervening dry spells and periodic fires.

Given excess moisture and lack of fire, savannas seem inevitably to become forests. Activities by humans, such as grazing cattle or cutting trees, also perpetuate, or possibly promote, grass dominance.

A variety of herbs exist in the savanna, but they are easily overlooked, except during flowering periods. Many of them also do best just after a fire, when they are better exposed to the sun and to potential pollinators.

Plants such as hibiscus and coleus are familiar garden and house plants popular the world over. Vines related to the sweet potato are also common. Many species from the legume or pea and sunflower families are present. Wild ginger often displays its showy blossoms after a fire.

Savanna Trees and Shrubs

Trees of the African savanna often have relatively wide-spreading branches that all terminate at about the same height, giving the trees a flattopped appearance. Many are from the legume family, most notably species of Acacia, Brachystegia, Julbernardia, and Isoberlinia. With the exception of acacias, these are not well known outside Africa.

There is an especially large number of Acacia species ranging from shrubs to trees, many with spines. A few also have a symbiotic relationship with ants that protect them from herbivores. The hashab tree, a type of acacia that grows in more arid regions, is the source of gum arabic.

Although not as prominent, the baobab tree is renowned for its large size and odd appearance and occurs in many savanna regions. It has an extremely thick trunk with smooth, gray bark and can live for up to two thousand years. Many savanna trees also have showy flowers, such as the flame tree and the African tulip tree.

Tropical Forest

The primary characteristics of African tropical forests are their extremely lush growth, high species diversity, and complex structure. The diversity is often so great that a single tree species cannot be identified as dominant in an area.

Relatively large trees, such as ironwood, iroko, and sapele, predominate. Forest trees grow so close together that their crowns overlap, forming a canopy that limits the amount of light that falls beneath them. A few larger trees, called emergent trees, break out above the thick canopy.

A layer of smaller trees live beneath the main canopy. A few smaller shrubs and herbs grow near the ground level, but the majority of the herbs and other perennials are epiphytes, that is, plants that grow on other plants.

On almost every available space on the trunks and branches of the canopy trees there are epiphytes that support an entire, unique community. All this dense plant growth is supported by a monsoon climate in which 60 inches (150 centimeters) or more of rain often falls annually, most of it in the summer.

Lianas and Epiphytes

Lianas are large, woody vines that cling to trees, many of them hanging down near to the ground. They were made famous by Tarzan movies. Many lianas belong to families with well-known temperate vine species, such as the grape family, morning glory family, and cucumber family. Other, related plants remain intimately connected to the trunks of trees. One of these, the strangler fig, is a strong climber that begins life in the canopy.

The fruits are eaten by birds ormonkeys, and the seeds are deposited in their feces on branches high in the canopy. The seeds germinate and send a stem downward to the ground. Once the stem reaches the ground, it roots; additional stems then develop and growupward along the trunk of the tree.

After many years, a strangler fig can so thoroughly surround a tree that it prevents water and nutrients from flowing up the trunk. Eventually, the host tree dies and rots away, leaving a hollow tube of mostly strangler fig. Other climbers include members of the Araceae family, the most familiar being the ornamental philodendron.

Themost common epiphytes are bryophytes, lower plants related to mosses, and lichens, a symbiotic combination of algae (or cyanobacteria) and fungus. The most abundant higher plants are ferns and orchids. As these plants colonize the branches of trees, they gradually trap dust and decaying materials, eventually leading to a thin soil layer that other plants can use.

Accumulations of epiphytes can be so great in some cases that tree branches break from their weight. Epiphytes are not parasites (although there are some parasitic plants that grow on tree branches); they simply use the host tree for support.

Tropical Forest Floor Plants

Grasses are almost entirely absent from the forest floor; those that grow there have much broader leaves than usual. Some forest-floor herbs are able to grow in the deep shade beneath the canopy, occasionally being so highly adapted to the low light that they can be damaged if exposed to full sunlight.

Some popular house plants have come from among these plants, because they do not need direct sunlight to survive. Still, the greatest numbers of plants occur beneath breaks in the canopy, where more light is available.

Experimental Crops

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Experimental crops are foodstuffs with the potential to be grown in a sustainable manner, produce large yields, and reduce people’s reliance on the traditional crops wheat, rice, and corn.

Shifting from a hunter-gatherer society to an agrarian society led to increasingly larger-scale agricultural production that involved selecting local crops for domestication. In recent history there has been a reduction in the number of agricultural crops grown for human consumption.

There are estimated to be at least 20,000 species of edible plants on earth, out of more than 350,000 known species of higher plants. However, only a handful of crops feed most of the world’s people.

These include wheat, rice, corn, potatoes, sugar beets, sugarcane, cassava, barley, soybeans, tomatoes, and sorghum. Rice, wheat, and corn together account for a majority of calories consumed. In the effort to develop experimental crops, agricultural goals include expanding the diversity of plant food in the human diet.

Recent Successes

Soybeans (Glycine max) are a relatively newcrop that gained worldwide acceptance and widespread cultivation in the second half of the twentieth century. Originally cultivated in China, soybeans gradually spread throughout Asia and became a staple food there.

High in protein, soybeans were first grown in the Western world as animal feed. Concerted breeding efforts have resulted in many locally adapted varieties. Today, soybeans as both meal and oil are common place. Worldwide soybean production is now the greatest of any legume.

Triticale (x Triticosecale) is a hybrid created to combine the ruggedness and high protein content of rye (Secale cereale) with the high yield of wheat (Triticumaestivum). Triticale has not replaced wheat or rye in bread-making due to its rather low gluten content but is used to supplement bread flours. Triticale is also adaptable to marginal agricultural soils.

Kiwifruit (Actinidia deliciosa) is another recent success story. Apreviously little-known fruit originally called Chinese gooseberry, it was introduced to New Zealand at the turn of the twentieth century and renamed kiwifruit. The name change was a marketing strategy that led to worldwide popularity.

Today kiwifruit cultivation and consumption are increasing worldwide. Kiwifruit grows on a de- ciduous vine, much like grapes. It can be harvested and then stored for several months without loss of quality.

Grains and Cereals

Quinoa (Chenopodium quinoa) is a grain native to the Andes Mountains of South America. It has been a staple in the diets of people living in that region for centuries. Although the leaves are edible, it is principally the tiny seed which is consumed.

The seeds contain high amounts of protein, calcium, phosphorus, and the essential amino acid lysine, which is typically lacking in other cereals such as wheat, rye, and barley.

Quinoa seeds must be washed or otherwise processed to remove the bitter saponins contained in the pericarp and can then be cooked and eaten much like rice. Quinoa can also be ground into flour as a supplement for bread making. Cultivation and use of quinoa have increased steadily since the 1980’s.

Grain amaranths (Amaranth) are being rediscovered and developed as a potential new source of grain. Amaranth was a staple crop for centuries in Mexico, Central America, and South America. Amaranth is grown as an annual and yields thick, heavy seed heads containing numerous tiny seeds.

The hard seed coat is removed by heating or boiling and can be prepared much like corn. Amaranth is comparable to other grains in protein, contains high amounts of lysine, and can be consumed by those allergic to typical grains.

Breeding efforts over the last few decades involving A. hypochondriacus, A. cruentus, and A. hybridus have greatly increased seed yield as well as desirable plant growth habits. Another important characteristic is amaranth’s drought resistance.

Legumes

Members of the Leguminosae family are particularly valuable as food sources because they contain high levels of protein. This is in part due to their ability to fix atmospheric nitrogen in root nodules that contain nitrogen-fixing bacteria. This symbiotic relationshipwith the bacteriameans relatively little nitrogenous fertilizer is required for agricultural production of legumes.

Tarwi (Lupinus mutabilis) is a legume native to the South American Andes that has a high protein and oil content, similar to the soybean. Tarwi is also high in the essential amino acid lysine. It grows well in poor soils and is drought-resistant. Current breeding efforts focus on reducing the bitter alkaloids, which can be removed by rinsing in water.

The winged bean (Psophocarpus tetragonolobus), native of tropical Asia, is entirely edible—leaves, flowers, seeds, pods, and tuberous roots. Like most legumes, the winged bean has a high protein content. This species could have tremendous potential in many tropical regions of the world, rivaling the success of the soybean.

A native of North America, the groundnut (Apios americana)was a major food source of many American Indian tribes. It is purported to have been offered to the Pilgrims to avert starvation. The numerous underground tubers can be prepared (cooking is necessary) like potatoes yet have a much higher protein content.

Several other legumes whose use and acceptance are likely to increase include the tepary bean (Phaseolus acutiflius), the pigeon pea (Cajanus cajan), and the bambara groundnut (Voandzeia subterranea).

Other Crops

There are many other potential food crops. Most have been cultivated on a small scale for years and are being rediscovered and researched for commercial production. Some of these include potato-like oca tubers (Oxalis tuberosa), fruits such as cherimoya (Annona cherimola), pepino (Solanum muricatum), and feijo (Acca sellowiana), and nuts such as egg nut (Couepia longipendula).

Agricultural Revolution

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The agricultural revolution marked the transition by humans from hunting and gathering all their food to domesticating plants for food.

People first obtained their food by scavenging kills made by other animals, by hunting animals, and by gathering wild food plants. Between ten thousand and twelve thousand years ago, people began to use plants in new ways. Some scientists and historians call this period of time the "agricultural revolution".

Agricultural Beginnings

Before the 1960’s,many scientists and historians believed that hunter-gatherers abruptly switched from foraging to farming. Those who thought that agriculture arose quickly coined the term "agricultural revolution".

They suggested that this revolution spread rapidly because it was a tremendous improvement over the old foraging lifestyle, with the availability of cultivated food sources far more dependable than those of wild sources.

Since the 1960’s, scientists and historians have challenged this view of agricultural beginnings. Later studies have shown that modern hunting-gathering societies have a remarkably sophisticated knowledge about native plants and plants’ life cycles.

Gatherers use a large number of plant species for food. Hunting and gathering cultures today do not have to plant seeds intentionally to keep from starving and most likely did not have to do so in the past.

Domesticated and Wild Plants

Domesticated plants are genetically distinct from their wild ancestors. Domestication involves processes by which a wild plant adapts to the needs of the farmer. The traits that make a plant desirable as a human food plant may not be ones that confer survival value on plants in their natural habitats and may actually be detrimental to the plant’s survival in the wild.

People who gathered wild plants looked for traits that made gathering easier and more profitable. They would have gathered grasses, for example, that had bigger seeds or plants, had more seeds or fruits or edible parts, or had seed heads that did not shatter easily. If seeds from such plants are the ones that were planted, accidentally or on purpose, their useful traits would be reinforced in successive generations.

The appearance of a domesticated plant in the archaeological record is the end result of generations of cumulative genetic transformations that might have taken hundreds or even thousands of years. Therefore, it becomes difficult to pick a single point in the past for any continent or geographic region that signals the beginning of an agricultural economy.

Geography of Agricultural Origins

The area of the Middle East called the Fertile Crescent (between the Tigris and Euphrates Rivers in what is now Iraq) seems to be the first area where formal agriculture began. The native grasses were highly productive.

Wild wheat and barley grew in dense stands and were valuable food sources before cultivation began. It was this use of gathered wild grasses for food that probably led to their early domestication.

Along with the grasses, complementary sources of protein were adopted, namely leguminous crops such as pea and lentil, and animals were domesticated. The plants that were domesticated in the Middle East include einkorn wheat, emmer wheat, bread wheat, barley, lentil, pea, vetch, the fava (or broad) bean, chickpea, lupine, and flax.

Agriculture originated in northeast China with the Yang Shao culture around six thousand years ago and spread quickly into Korea and Japan. Some of the plants brought under cultivation include barley, barnyard millet, common or broomcorn millet, foxtail millet (or foxtail grass), soybean, adzuki and mung beans, hemp, buckwheat, bottle gourd, Chinese cabbage, great burdock, lacquer tree, paper mulberry, and a number of fruit trees, including apricot, pear, peach, and plum.

In Southeast Asia, as well as the Pacific Islands and India, cultivated plants included sesame, the pigeon pea, eggplant, rice, sugarcane, bananas, plantains, coconuts, oranges, mango, Asian yam, betel nut, pepper, taro, bitter gourd, winter melon, snake gourd, luffa, mangosteen, durian, rambutan, breadfruit, and bamboo.

In Africa, plant domestication took place south of the Sahara Desert and north of the equator. Many crops were grown, including various kinds of millet, sorghum, okra, coffee, watermelon, several species of yam, African rice, cowpea, African oil palm, and cola nut. In Ethiopia, Ethiopian oats, coffee, enset, tef, noog, and chat were cultivated.

In Central America, archaeological evidence suggests that squash and pumpkins may have been cultivated before corn, especially in the Oaxaca region. InOaxaca and Tamaulipas, along with squash and pumpkins, people were cultivating beans and bottle gourds, followed later by corn.

In the Tehuacán Valley of Central Mexico, corn, chile peppers, avocado, beans, amaranth, and foxtail grass were among the very earliest cultivated plants.

Cultivated plants either originating or cultivated early in South America include quinoa, white potato, peanut, cacao, jicama, lima bean, common bean, pineapple, chile pepper, papaya, sweet potato, yucca, and avocado. Tomatoes were cultivated in both Central and South America.

In North America, prior to the diffusion of the corn-squash-beans complex fromthe southwest after 1000 c.e., Indians of Eastern North America were cultivating a number of plants, including bottle gourd, erect knotweed, sumpweed, goosefoot, maygrass, little barley, and sunflower.

Marine Agriculture

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Marine agriculture uses techniques of artificial cultivation, such as growing, managing, and harvesting, and applies them to marine plants and animals. The products are then used for human consumption.

Marine agriculture is also known as mariculture or aquaculture, although aquaculture is a more general term referring to both freshwater and marine farming of organisms. The world’s oceans cover approximately three-fourths of the globe, including vast regions of unexplored life and landforms.

The potential for exploiting the oceans agriculturally is great but currently meets significant obstacles. Because of the expense of equipment and personnel involved, most marine species are not cultivated.

Coastal pollution, habitat destruction, competition for land use, and economics all limit mariculture programs. Nevertheless, mariculture does offer several food, medical, and other products that are currently being marketed.

Food

Seaweeds are edible, especially the red and brown algae. The three most common types of seaweeds are known by their Japanese names: nori (Porphyra), a red seaweed high in vitamin C and digestible protein; kombu (Laminaria); and wakame (Undaria), high in calcium.

They are eaten raw, cooked, or dried and have several vitamins and minerals as well as protein. Seaweeds are low in fats, and 35 to 50 percent of the dry weight of red seaweeds is protein.

Seaweeds can be used to add taste and variety to foods. They are used as a hot vegetable, boiled and formed into cakes and fried, in salads, and in preparing desserts, breads, soups, casseroles, sandwiches, teas, and candy.

The world’s yearly harvest of seaweeds is approximately 8.4 million tons of green seaweed, 2.8 million tons of brown seaweed, and 1.2 million tons of red seaweed. The total seaweed market in 1998 was worth more than $5 billion, with $600 million deriving from food additives alone.

China is the leading harvester and the world’s biggest seaweed consumer. Japan is the leading seaweed importer and, at the end of the twentieth century, employed more than thirty-five thousand people in the industry. Harvesting and marketing edible seaweed is a growing business in the United States, especially on the West Coast.

Seaweeds produce several types of phycocolloids, starchlike chemicals used in food processing and manufacturing. An important type called algin, which makes up alginic acid and alginates, is used inmanufacturing dairy products such as ice cream, cheese, and toppings aswell as to prevent frostings and pies from desiccation.

Another extract is agar, used to form jellies and protect fish and meats during canning. Agar is also used in low-calorie foods and as a thickener. Red algae is a source of the agglutinant carrageenan, which is used in many food products as an emulsifier to give body to dairy products and other processed foods, including instant puddings. Additionally, seaweed-based food additives are common in prepared and fast foods, including hamburgers and yogurt.

Kelp farming is a major livelihood in the eastern Pacific, with approximately 140,000 tons harvested each year for the extraction of alginates used in food and food additives. Kelp is a good source of calcium, potassium, iron, iodine, bromine, and zinc. It is also low-fat, has some protein, and is a natural tenderizer. Kelp flakes are used as a low-sodium salt substitute.

Medicine

The use ofmarine plants inmedicine is still in the early stages of exploration and faces many challenges, including identification of useful chemicals and the cultivation of significant quantities.

Dinoflagellates and other microalgae are being investigated for compounds that might fight cancerous tumors. Diluted algae toxins from red tides can be used to inhibit the growth of most bacteria. Green algae has halosphaerin, a strong antibiotic.

Seaweed is used in wound dressings in hospitals and as a source of iodine, A, B, D, and E vitamins, calcium, magnesium, potassium, sodium, sulfur, and trace antioxidants such as selenium and zinc. The seaweed extract agar is used in laxatives and as a medium to grow bacteria and molds.

Kelp is rich in chlorophyll, which can help detoxify the body, fight inflammations, and increase the formation of oxygen-carrying red blood cells. Chlorophyll is also used to fight bad breath and as an ingredient in deodorants.

Kelp is used to reduce cholesterol, treat gastrointestinal, respiratory, and genitourinary disorders, and lower blood pressure. The alginic acid produced by kelp can rid the body of radioactive strontium, the most dangerous to humans of all components in the fallout from atomic explosions.

Other Uses

Marine plants are used for a variety of other purposes. Seaweed is used as a component of many fertilizers, as a food additive in animal feed, and to reduce soil acidity.

Research on cattle and swine has revealed that the addition of seaweed to animal feed can enhance the immune system and makes the meat a more desirable color. It can also save cattle from the effects of fungus-infected grass.

Seaweed is used as an ingredient in cosmetics as well as to nourish, revitalize, condition, and improve the skin, hair, and body. It is used in cleansers, toners, moisturizers, scrubs, body lotions, and hair and bath products.

The giant kelp (Macrocystis) is a major source of algin for commercial uses, as is the brown algae Laminaria, which is harvested in the north Atlantic. Algin is used in shampoos, shaving cream, plastics, pesticides, rubber products, paper, paints, and cosmetics.

Additionally, kelp is used in emulsifiers for toothpastes and printing inks. Kelp has even been used to make fishing lines. Some research has been done on using kelp as a fuel to produce a clean-burning methane gas. Kelp can be used to ferment human waste and garbage, which can then be sold as fertilizers.

Modern Agriculture Problems

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Many current problems in agriculture are not new. Erosion and pollution, for example, have been around as long as agriculture. However, agriculture has changed drastically within its ten-thousand-year history, especially since the dawn of the Industrial Revolution in the seventeenth century.

Erosion and pollution are now bigger problems than before and have been joined by a host of other issues that are equally critical—not all related to physical deterioration.

Monoculture

Modern agriculture emphasizes crop specialization, also known as monoculture. Farmers, especially in industrialized regions, often grow a single crop on much of their land. Problems associated with this practice are exacerbated when a single variety or cultivar of a species is grown. Such a strategy allows the farmer to reduce costs, but it also makes the crop, and thus the farmand community, susceptible to widespread crop failure.

The corn blight of 1970 devastated more than 15 percent of the North American corn crop. The cornwas particularly susceptible to the harmful organisms because 70 percent of the crop being grown was of the same high-yield variety. Chemical antidotes can fight pests, but they increase pollution.

Maintaining species diversity or varietal diversity—growing several different crops instead of one or two—allows for crop failures without jeopardizing the entire economy of a farm or region that specializes in a particular monoculture, such as tobacco, coffee, or bananas.

Genetic Engineering

Growing genetically modified (GM) crops is one attempt to replace post-infestation chemical treatments. Recombinant technologies used to splice genes into varieties of rice or potatoes from other organisms are becoming increasingly common. The benefits of such GM crops include more pest-resistant plants and higher crop yields.

However, environmentalists fear new genes could trigger unknown side effects with more serious, long-term environmental and economic consequences than the problems they were used to solve. GM plants designed to resist herbicide applications could potentially pass the resistant gene to closely related wild weed species that would then become "super weeds".

Also, pests, just as they can develop resistance to pesticides, may also become resistant to defenses engineered into GM plants. The high cost of recombinant technologies calls into question the feasibility of continuing development of GM plants.

Erosion

An age-old problem, soil loss from erosion occurs all over the world.As soil becomes unproductive or erodes away, more land is plowed. The newly plowed lands usually are considered marginal, meaning they are too steep, nonporous or too sandy, or deficient in some other way.

When natural vegetative cover blankets these soils, it protects them from erosive agents: water, wind, ice, or gravity. Plant cover "catches" rainwater that seeps downward into the soil rather than running off into rivers. As marginal land is plowed or cleared to grow crops, erosion increases.

Expansion of land under cultivation is not the only factor contributing to erosion. Fragile grasslands in dry areas also are being used more intensively. Grazing more livestock than these pastures can handle decreases the amount of grass in the pasture and exposes more of the soil to wind, the primary erosive agent in dry regions.

Overgrazing can affect pastureland in tropical regions too. Thousands of acres of tropical forest have been cleared to establish cattle-grazing ranges in Latin America. Tropical soils, although thick, are not very fertile. After one or two growing seasons, crops grown in these soils will yield substantially less than before.

Tropical fields require fallow periods of about ten years to restore the soil after it is depleted. That is why tropical farmers using slash-and-burn agriculture move to new fields every few years in a cycle that returns them to the same place years later, after their particular lands have regenerated.

Where there is heavy forest cover, soils are protected from exposure to the massive amounts of rainfall. Organic material for crops is present as long as the forest remains in place.

When the forest is cleared, however, the resulting grassland cannot provide the adequate protection, and erosion accelerates. Lands that are heavily grazed provide even less protection from heavy rains, and erosion accelerates even more.

The use of machines also promotes erosion, and modern agriculture relies on machinery such as tractors, harvesters, trucks, balers, and ditchers. In industrialized nations, machinery is used intensely. Machinery use is on the rise in developing countries such as India, China, Mexico, and Indonesia, where traditional, nonmechanized farming methods are the norm.

Farming machines, in gaining traction, loosen topsoil and inhibit vegetative cover growth, especially when farm implements designed to rid the soil of weeds are attached. The soil is then more prone to erode.

Eco-fallow farming has become more popular in the United States and Europe as a way to reduce erosion. This method of agriculture, which leaves the crop residue in place over the fallow (non-growing) season, does not root the soil in place as well as living plants do.

As a result, some erosion continues. Additionally, eco-fallow methods require heavy use of chemicals, such as herbicides, to "burn down" weed growth at the start of the growing season. This contributes to increased erosion and pollution.

Pollution and Silt

Besides causing resistance among harmful bacteria, insects, and weeds, pesticides inevitably wash into, and contaminate, surface and groundwater supplies. Chemicals, although problematic, are not as difficult to contend with as the increasingly heavy silt load choking the life out of streams and rivers.

Accelerated erosion from water runoff carries silt particles into streams, where they remain suspended and inhibit the growth of many forms of plant and animal life.

The silt load inAmerican streams has become so heavy that the Mississippi River Delta is growing faster than it once did. Heavy silt loads, combined with chemical residues, are creating an expanded dead zone. By taxing the capabilities of ecosystems around the Delta, sediments are filtered out slowly, plant absorption of nutrients is decreased, and salinity levels for aquatic life cannot be stabilized.

Most of the world’s population lives in coastal zones, and 80 percent of the world’s fish catch comes from coastal waters over continental shelves that are most susceptible to this form of pollution.

Pesticide Resistance

With the onset of the Green Revolution, the use of herbicides, insecticides, and other pesticides increased dramatically all over the world. An increasing awareness of problems caused by overuse of pesticides extends even to household antibacterial cleaning agents and other products. Mutations among the genes of bacteria and plants have allowed these organisms to resist the effects of chemicals that were toxic to their ancestors.

Use of pesticides leads to a cycle wherein more, or different combinations of, chemicals are used, and more pests develop resistance to these toxins. Additionally, the development of herbicide-resistant crop plants enables greater use of herbicides to kill undesirable weeds on croplands.

Increasing interest in biopesticides may slow the cycle of pesticide resistance. Types of biopesticides include beneficial microbes, fungi, and insects such as ladybugs that can be released in infested areas to prey upon specific pests. Biopesticides used today include naturally occurring and genetically modified organisms. Their use also avoids excessive reliance on chemical pesticides.

Fertilizers and Eutrophication

Increased use of fertilizers was another result of the Green Revolution. Particulate amounts of most fertilizers enter the hydrologic cycle through run-off. As a result, bodies of water become enriched in dissolved nutrients, such as nitrates and phosphates.

The growth of aquatic plants in rivers and lakes is overstimulated, and this results in the depletion of dissolved oxygen. This process of eutrophication can harm all aquatic life in these ecosystems.

Water Depletion

With an increasing reliance on irrigation, groundwater resources are mismanaged and overtapped. The rate of groundwater recharge is slow, usually between 0.1 and 0.3 percent per year. When the amount of water pumped out of the ground exceeds the recharge rate, it is referred to as aquifer overdraft. An aquifer is a water-bearing stratum of permeable rock, sand, or gravel.

In Tamil Nadu, India, groundwater levels dropped 25 to 30 meters during the 1970’s due to excessive pumping for irrigation. In Tianjin, China, the groundwater level declines 4.4 meters per year. In the United States, aquifer overdraft averages 25 percent over the replacement rate.

The Ogallala aquifer under Kansas, Nebraska, and Texas represents an extreme example of overdraft: Depletion is 130 to 160 percent above the replacement rate annually. At this rate, this aquifer, which supplies water to countless communities and farms, has been projected to become nonproductive by 2030.

Soil Salinization

In addition, continued irrigation of arid regions can lead to soil problems. Soil salinization is widespread in the small-grained soils of these regions, which have a high water absorption capacity and a low infiltration rate.

Some irrigation practices add large amounts of salts into the soil, increasing its natural rate of salinization. This can also occur at the base of a hill slope. Soil salinization has been recognized as a major process of land degradation.

Although surface and groundwater resources cannot be enriched by technology, conservation and improved environmental management can make the use of precious freshwater more efficient. In agriculture, for example, drip irrigation can reduce water use by nearly 50 percent. In developing countries, though, equipment and installation costs often limit the availability of these more efficient technologies.

Urban Sprawl

As more farms become mechanized, the need for farmers and farm workers is being drastically reduced. From a peak in 1935 of about 6.8 million farmers farming 1.1 billion acres, the United States at the end of the twentieth century counted fewer than 2 million farmers farming 950 million acres.

Urban sprawl converts a tremendous amount of cropland into parking lots, malls, industrial parks, and suburban neighborhoods. If cities were located in marginal areas, then concern about the loss of farmland to commercial development would be nominal.

However, the cities attracting the greatest numbers of people have too often replaced the best cropland. Taking the best cropland out of primary production imposes a severe economic penalty.

Traditional Agriculture

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Two agricultural practices that are widespread among the world’s traditional cultures, slash-and-burn agriculture and nomadism, share several features. Both are ancient forms of agriculture, both involve farmers not remaining in a fixed location, and both can pose serious environmental threats if practiced in a nonsustainable fashion.

The most significant difference between the two is that slash-and-burn is associated with raising field crops, while nomadism usually involves herding livestock.

Slash-and-Burn Agriculture

Farmers have practiced slash-and-burn agriculture, which is also referred to as shifting cultivation or swidden agriculture, in almost every region of the world where farming is possible.

Although at the end of the twentieth century slash-and-burn agriculture was most commonly found in tropical areas such as the Amazon River basin in South America, swidden agriculture once dominated agriculture in more temperate regions, such as northern Europe. Swidden agriculture was, in fact, common in Finland and northern Russia well into the early decades of the twentieth century.

Slash-and-burn acquired its name from the practice of farmers who cleared land for planting crops by cutting down the trees or brush on the land and then burning the fallen timber on the site. The farmers literally slash and burn.

The ashes of the burnt wood add minerals to the soil, which temporarily improves its fertility.Crops the first year following clearing and burning are generally the best crops the site will provide. Each year after that, the yield diminishes slightly as the fertility of the soil is depleted.

Farmers who practice slash-and-burn do not attempt to improve fertility by adding fertilizers such as animal manure to the soil. They instead rely on the soil to replenish it-self over time.

When the yield from one site drops below acceptable levels, farmers then clear another piece of land, burn the brush and other vegetation, and cultivate that site while leaving their previous field to lie fallow and its natural vegetation to return.

This cycle will be repeated over and over, with some sites being allowed to lie fallow indefinitely, while others may be revisited and farmed again in five, ten, or twenty years.

Farmers who practice slash-and-burn do not always move their dwelling places as they cultivate different fields. In some geographic regions, farmers live in a central village and farm cooperatively, with fields being alternately allowed to remain fallow and farmed, making a gradual circuit around the central village.

In other cases, the village itself may move as new fields are cultivated. Anthropologists studying indigenous peoples in Amazonia, for example, discovered that village garden sites were on a hundred-year cycle. Villagers farmed cooperatively to clear a garden site. That garden would be used for about five years; then a new site was cleared.

When the fields in use became an inconvenient distance from the village—about once every twenty years—the entire village would move to be closer to the new fields. Over a period of approximately one hundred years, a village would make a circle through the forest, eventually ending up close to where it had been located long before any of the present villagers had been born.

In more temperate climates, farmers often owned and lived on the land on which they practiced swidden agriculture. Farmers in Finland, for example, would clear a portion of their land, burn the covering vegetation, grow grains for several years, and then allow that land to remain fallow for five to twenty years.

The individual farmer rotated cultivation around the land in a fashion similar to that practiced by whole villages in other areas but did so as an individual rather than as part of a communal society.

Although slash-and-burn is frequently denounced as a cause of environmental degradation in tropical areas, the problemwith it is not the practice itself but the length of the cycle.

If the cycle of shifting cultivation is long enough, forests will grow back, the soil will regain its fertility, and minimal adverse effects will occur. In some regions, a piece of land may require as little as five years to regain its maximum fertility; in others, it may take one hundred years.

Problems arise when growing populations put pressure on traditional farmers to return to fallow land too soon. Crops are smaller than needed, leading to a vicious cycle in which the next strip of land is also farmed too soon, and each site yields less and less. As a result,more and more land must be cleared.

Nomadism

Nomadic peoples have no permanent homes. They earn their living by raising herd animals, such as sheep, horses, or other cattle, and they spend their lives following their herds from pasture to pasture with the seasons, going wherever there is sufficient food for their animals.

Most nomadic animals tend to be hardy breeds of goats, sheep, or cattle that can withstand hardship and live on marginal lands. Traditional nomads rely on natural pasturage to support their herds and grow no grains or hay for themselves. If a drought occurs or a traditional pasturing site is unavailable, they can lose most of their herds to starvation.

In many nomadic societies, the herd animal is almost the entire basis for sustaining the people. The animals are slaughtered for food, clothing is woven from the fibers of their hair, and cheese and yogurt may be made from milk.

The animals may also be used for sustenance without being slaughtered. Nomads in Mongolia, for example, occasionally drink horses’ blood, removing only a cup or two at a time from the animal.

In mountainous regions, nomads often spend the summers high up on mountain meadows, returning to lower altitudes in the autumn when snow begins to fall. In desert regions, they move from oasis to oasis, going to the places where sufficient natural water exists to allow brush and grass to grow, allowing their animals to graze for a few days, weeks, or months, then moving on.

In some cases, the pressure to move on comes not from the depletion of food for the animals but from the depletion of a water source, such as a spring or well. At many desert oases, a natural water seep or spring provides only enough water to support a nomadic group for a few days at a time.

In addition to true nomads—people who never live in one place permanently—a number of cultures have practiced seminomadic farming: The temperate months of the year, spring through fall, are spent following the herds on a long loop, sometimes hundreds of miles long, through traditional grazing areas, then the winter is spent in a permanent village.

Nomadism has been practiced for millennia, but there is strong pressure from several sources to eliminate it. Pressures generated by industrialized society are increasingly threatening the traditional cultures of nomadic societies, such as the Bedouin of the Arabian Peninsula. Traditional grazing areas are being fenced off or developed for other purposes.

Environmentalists are also concerned about the ecological damage caused by nomadism. Nomads generally measure their wealth by the number of animals they own and will try to develop their herds to as large a size as possible, well beyond the numbers required for simple sustainability. The herd animals eat increasingly large amounts of vegetation, which then has no opportunity to regenerate. Desertification may occur as a result.

Nomadism based on herding goats and sheep, for example, has been blamed for the expansion of the Sahara Desert in Africa. For this reason, many environmental policy makers have been attempting to persuade nomads to give up their traditional life-style and become sedentary farmers.

Agriculture: World Food Supplies

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Soil types, topography, climate, socioeconomics, dietary preferences, stages in agricultural development, and governmental policies combine to give a distinctive personality to regional agricultural characteristics and, hence, food supplies in various areas of the world.

All living things need food to live, grow, work, and survive. Almost all foods that humans consume come from plants and animals. Not all of earth’s people eat the same foods, however. The types, combinations, and amounts of food consumed by different peoples depend upon historic, socioeconomic, and environmental factors.

History of Food Consumption

Early in human history, people ate what they could gather or scavenge. Later, people ate what they could plant and harvest and the products of animals they could domesticate. Modern people eat what they can grow, raise, or purchase.

Their diets or food composition is determined by income, local customs, religion or food biases, and advertising. There is a global food market, and many people can select what they want to eat and when they eat it according to the prices they can pay and what is available.

Historically, in places where food was plentiful, accessible, and inexpensive, humans devoted less time to basic survival needs and more time to activities that led to human progress and enjoyment of leisure.

Despite a modern global food system, instant telecommunications, the United Nations, and food surpluses in some places, however, the problem of providing food for everyone on earth has not been solved.

In 1996 leaders from 186 countries gathered in Rome and agreed to reduce by half the number of hungry people in the world by the year 2015. United Nations data for 1998 revealed that more than 790 million people in the developing parts of the world did not have enough food to eat. This is more people than the total population of North America and Europe at that time.

The number of undernourished people has been decreasing since 1990. Still, at the current pace of hunger reduction in the world, 600 million people will suffer from "acute food insecurity" and go to sleep hungry in 2015. Despite efforts being made to feed the world, outbreaks of food deficiencies, mass starvation, and famine are a certainty in the twenty-first century.

World Food Source Regions

Agriculture and related primary food production activities, such as fishing, hunting, and gathering, continue to employ more than one-third of the world’s labor force. Agriculture’s relative importance in the world economic system has declined with urbanization and industrialization, but it still plays a vital role in human survival and general economic growth.

Demands on agriculture in the twenty-first century include supplying food to an increasing world population of nonfood producers as well as producing food and nonfood crude materials for industry.

Soil types, topography, weather, climate, socio-economic history, location, population pressures, dietary preferences, stages in modern agricultural development, and governmental policies combine to give a distinctive personality to regional agricultural characteristics.

Two of the most productive food-producing regions of the world are North America and Europe. Countries in these regions export large amounts of food to other parts of the world.

North America is one of the primary food-producing and food-exporting continents. After 1940 food output generally increased as cultivated zacreage declined. Progress in improving the quantity and quality of food production is related to mechanization, chemicalization, improved breeding, and hybridization. Food output is limited more by market demands than by production obstacles.

Western Europe, although a basic food-deficit area, is a major producer and exporter of high-quality foodstuffs. After 1946 its agriculture became more profit-driven. Europe’s agricultural labor force grew smaller, its agriculture became more mechanized, its farm sizes increased, and capital investment per acre increased.

Foods from Plants

Most basic staple foods come froma small number of plants and animals. Ranked by tonnage produced, the most important food plants throughout the world are wheats, corn, rice, potatoes, cassava, barley, soybeans, sorghums and millets, beans, peas and chickpeas, and peanuts. Wheat and rice are the most important plant foods.

More than one-third of the world’s cultivated land is planted with these two crops. Wheat is the dominant food staple in North America, Western and Eastern Europe, northern China, the Middle East, and North Africa. Rice is the dominant food staple in southern and eastern Asia.

Corn, used primarily as animal food in developed nations, is a staple food in Latin America and southeast Africa. Potatoes are a basic food in the highlands of South America and in Central and Eastern Europe.

Cassava is a tropical starch-producing root crop of special dietary importance in portions of lowland South America, the west coast countries of Africa, and sections of South Asia. Barley is an important component of diets in North African, Middle Eastern, and Eastern European countries.

Soybeans are an integral part of the diets of those who live in eastern, southeastern, and southern Asia. Sorghums and millets are staple subsistence foods in the savanna regions of Africa and South Asia, while peanuts are a facet of dietary mixes in tropical Africa, Southeast Asia, and South America.

The World’s Growing Population

The problem of feeding the world is compounded by the fact that population was increasing at a rate of nearly 80 million persons per year at the end of the twentieth century. That rate of increase is roughly equivalent to adding a country the size of Germany to the world every year. Compounding the problem of feeding the world are population redistribution patterns and changing food consumption standards.

By 2001, the world had exceeded the six billion mark, and the world population was projected to reach approximately ten billion people by 2050—four billion people more than were on the earth in 2000. Most of the increase in world population was expected to occur within the developing nations.

Urbanization

Along with an increase in population in developing nations is massive urbanization. City dwellers are food consumers, not food producers. The exodus of young men and women from rural areas has given rise to a new series of megacities, most of which are in developing countries. By the year 2015, twenty-six cities in the world are expected to have populations of ten million people or more.

When rural dwellers move to cities, they tend to change their dietary composition and food-consumption patterns. Qualitative changes in dietary consumption standards are positive, for the most part, and are a result of educational efforts of modern nutritional scientists working in developing countries. During the last four decades of the twentieth century, a tremendous shift took place in overall dietary habits.

Dietary changes and consumption trends have contributed to a decrease in child mortality, an increase in longevity, and a greater resistance to disease. This globalization of people’s diets has resulted in increased demands for higher quality, greater quantity, and more nutritious basic foods.

Perspectives

Humanity is entering a time of volatility in food production and distribution. The world will produce enough food to meet the demands of those who can afford to buy food. In many countries, however, food production is unlikely to keep pace with increases in the demand for food by growing populations.

The food gap—the difference between production and demand—could more than double in the first three decades of the twenty-first century. Such a development would increase the dependence of developing countries on food imports. About 90 percent of the rate of increase in aggregate food demand in the early twenty-first century is expected to be the result of population increases.

Factors that could lead to larger fluctuations in food availability include weather variations, such as those induced by El Niño and climatic change, the growing scarcity of water, civil strife and political instability, and declining food aid.

Agronomy

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Agronomy is a group of applied science disciplines concerned with land and soil management and crop production. Agronomists’ areas of interest range from soil chemistry to soil-plant relationships to land reclamation.

The word "agronomy" derives from the ancient Greek agros (field) and nemein (manage) and therefore literally means "field management". The American Society of Agronomy defines agronomy as "the theory and practice of crop production and soil management". There are many specialties within the study of agronomy.

Agronomic Specialties

Agronomy is the family of disciplines investigating the production of crops supplying food, forage, and fiber for human and animal use. It studies the stewardship of the soil upon which those crops are grown. Agronomy covers all aspects of the agricultural environment, from agroclimatology to soil-plant relationships.

It includes crop science, soil science, weed science, and biometry (the statistics of living things) as well as crop, soil, pasture, and range management; turfgrass; agronomic modeling; and crop, forage, and pasture production andutilization.

Within each area are subdisciplines. For example, within soil science are traditional disciplines such as soil fertility, soil chemistry, soil physics, soil microbiology, soil taxonomy and classification, and pedogenesis, the science of how soils form.

Newer disciplines within soil science include such studies as bioremediation, or the study of how living organisms can be used to clean up toxic wastes in the environment, and land reclamation, the study of how to reconstruct landscapes disturbed by human activities, such as surface mining.

Scientific Goals

Chief amongdetrimental human activities is poor fieldmanagement, which leads to reduced productivity and reduced environmental quality. Historical examples abound; one is that of the 1930’s Dust Bowl in the United States.

In the early 1900’s much of the American Southern Plains, which had been natural grassland, was converted to wheatland. Planting and plowing methods of the time did not enable wheat to protect the ground against winds. Additionally, overgrazing of livestock had destroyed what grassland remained by the 1930’s.

The soil eroded, drought conditions which would last for most of the decade set in, and a series of wind and dust storms whipped through the region. An estimated 50 million acres of land were destroyed before soil conservation measures, implemented under the administration of Franklin Roosevelt, began to improve the situation.

It is the role of agronomy tomanage soil and crop resources as effectively as possible so that the twin goals of productivity and environmental quality are preserved.

Agronomy treats the agricultural environment as humankind’s greatest natural resource: It is the source of food, clothing, and building materials. The agricultural environment purifies the air humans and other animals breathe and the water they drink.

Agronomists, whatever their specific field, seek to utilize soil and plant resources to benefit society. Crop breeders, for example, use the genetic diversity of wild varieties of domesticated plants to obtain the information needed to breed plants for greater productivity or pest resistance.

Soil scientists study landscapes to determine how best to manage soil resources. Integrating agricultural practices with the environment maintains soil fertility and keeps soil in place so that erosion does not reduce the quality of the surrounding environment.

Algae

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Algae comprise a diverse group of (with few exceptions) photosynthetic oxygen-producing organisms, ranging in size from microscopic single cells to gigantic seaweeds.

The study of algae is known as phycology (in Greek, phycos means "algae"). Currently, most authors place eukaryotic algae in the kingdom Protista (domain Eukarya) and prokaryotic algae in the domain Bacteria.

In the past algae were considered to be lower plants because some forms look like plants. As in plants, the primary photosynthetic pigment in algae is chlorophyll a, and oxygen is produced during photosynthesis.

What Are Algae?

Algae can be found nearly everywhere on earth: oceans, rivers, lakes, in the snow of mountaintops, on forest and desert soils, on rocks, on plants and animals (such as within the hollow hair of the polar bear), or even on other algae. They are involved in diverse interactions with other organisms, including symbiosis, parasitism, and epiphytism.

Lichens are symbiotic associations between algae (blue-green algae, or cyanobacteria) and fungi. Atmospheric nitrogen-fixing cyanobacteria occur in symbiotic associations with plants such as bryophytes, water ferns, gymnosperms (such as cycads), and the angiosperms.

The aquatic fern Azolla, commonly used as a biofertilizer in rice fields in Asian countries, harbors the symbiotic cyanobacterium Anabaena azollae. Gunnera, the only flowering plant to house symbiotic cyanobacterium Nostoc, is widely distributed in the tropics.

Symbiotic dinoflagellates known as zooxanthellae livewithin the tissues of corals. Corals get their colors and obtain energy from their photosynthetic symbionts. About 15 percent of red algae occur as parasites of other red algae. Parasitic algae may even transfer nuclei into host cells and transform them.

After transformation, the reproductive cells of the host algae carry the parasite’s genes. Various algae live on the surfaces of plants and other algae as epiphytes. Sometimes algae can be found in strange places—the pink color of flamingos originates, for example, comes froma pigment in the algae consumed by these birds.

Algal Structure and Properties

Algal cells are bounded by a cell wall. Algal cells are either prokaryotic or eukaryotic. All prokaryotic algae belong to Cyanophyta (cyanobacteria) and lack both a nucleus and complex membrane-bound organelles, such as chloroplasts and mitochondria.

Photosynthesis occurs in cyanobacteria in thylakoid membranes similar to those of plants. However, there is no double membrane surrounding the thylakoids of cyanobacteria.

All other algal groups are eukaryotic. Eukaryotic algae differ from cyanobacteria in that they possess chloroplasts and flagella with associated structures and in their cellwall composition. According to the endosymbiont hypothesis, some eukaryotic algae (red and green algae) obtained their chloroplasts by acquiring symbiotic prokaryotic cyanobacteria. This is known as primary endosymbiosis.

Other eukaryotic algae probably obtained their chloroplasts by taking up eukaryotic endosymbiotic algae, a process known as secondary endosymbiosis. The existence of secondary endosymbiosis is indicated by the occurrence of more than two membranes around the chloroplasts of some algae, such as haptophytes, euglenophytes, dinoflagellates, and cryptomonads.

Pigments found in algae include chlorophylls, phycobilins, and carotenoids. All algae contain chlorophyll a. Accessory pigments vary among different algal groups.

Photoautotrophy is the principal mode of nutrition in algae; in other words, they are "self-feeders", using light energy and a photosynthetic apparatus to produce their own food (organic carbon) from carbon dioxide and water. The majority of algal groups contain heterotrophic species, which obtain their organic food molecules by consuming other organisms.

Numerous algae are mixotrophs; that is, they use different modes of nutrition (such as autotrophy and heterotrophy), depending on the availability of resources. The molecules used as food reserves differ among and are characteristic for algal groups. Food reserve molecules are polymers of glucose with different links between monomers.

Many algae are capable of movement. Movement is accomplished by means of flagellar action and by extrusion of mucilage. There are also peristaltic and amoeba-like algal movement. Within algal cells, movement of the cytoplasm, plastids, and nucleus also occurs.

Advantages conferred by mobility include achieving optimal light conditions for photosynthesis, avoiding damage caused by excess light, and obtaining inorganic nutrients.

Algal Reproduction and Life Cycles

Algae may reproduce either asexually or sexually. Asexual reproduction among algae includes production of unicellular spores that germinate without fusing with other cells, fragmentation of filamentous forms, and cell division by splitting.

In sexual reproduction, parent cells release gametes, which then fuse to form a zygote. Zygotes may either develop into new filaments or produce haploid spores by meiotic division.

Algae exhibit different types of life cycles. Some algal life cycles are characterized by an alteration of generations similar to that of plants. Two phases occur: sporophyte (usually diploid) and gametophyte (usually haploid).

The sporophyte produces haploid spores through meiosis, and the haploid gametophyte produces male or female gametes by mitosis. Gametophyte and sporophyte may be structurally identical or dissimilar, depending on the algal group.

Roles of Algae

Algae have played significant roles in the earth’s ecosystems since the origin of cyanobacteria (also known as blue-green algae)more than three billion years ago. Early cyanobacteriawere responsible for the development of significant amounts of free oxygen in the atmosphere, which then made aerobic respiration possible.

More than 70 percent of all photosynthetic activity on earth is carried out by phytoplankton—floating microscopic algae—rather than plants. Phytoplankton recharge the atmosphere with oxygen and simultaneously absorb carbon dioxide, helping to support the complex web of aquatic biota.

Algae are also very important in the global cycling of other elements, such as carbon, nitrogen, phosphorus, and silicon. Several algal groups—such as cyanobacteria, green algae, red algae, and the haptophyte algae—are able to generate calcium carbonate.

Sedimented algae are the major contributors to deep-sea carbonate deposits (sand), which cover about half of the world’s ocean floor. Calcified coralline red algae contribute to coral reefs in tropical waters. Silica sediments in oceans (sand) are based on abundant growth of another algal group, the diatoms, which contain silica in their cell walls.

Some algae (cyanobacteria) are able to fix atmospheric nitrogen and convert it to ammonia. Ammonia, in turn, can be a nitrogen source for plants and animals. On the other hand, high levels of nitrogen and phosphorus in rivers and lakes owing to pollution can cause the rapid and uncontrollable growth of algae, known as algal blooms.

A bloom of algae is a threat to human and marine health, both directly and indirectly. It clogs fishes’ gills, interferes with water filters, and ruins recreation sites. More than 50 percent of algal blooms produce toxins.

Cases of human respiratory, skin, and gastrointestinal disorders associated with algal toxins have been reported. Certain blooms of algae are called red tides. The water appears to be red or brown because of the color of algal bodies, mainly dinoflagellates that contain the pigment xanthophyll.

Technological Applications

Algae have been used as food, medicine, and fertilizer for centuries. The earliest known reference to the use of algae as food occurs in Chinese poetic literature dated about 600 b.c.e. More recently, algae have begun to play important roles in certain biotechnological processes.

Several algae, including reds, browns, greens, and cyanobacteria, are used for food in Pacific and Asian countries, especially Japan. The annual harvest of the red alga Porphyra worldwide is worth several billion dollars. Porphyra (Japanese nori, Chinese zicai) is used as a wrapper for sushi or may be eaten alone. Another edible alga with a high iodine content is the brown alga Laminaria (Japanese kombu). The cyanobacterium Spirulina, with a protein level of 50 to 70 percent, was cultivated for centuries by indigenous Central Americans at Lake Texcoco near modern-day Mexico City for use as human food.

Several gelling agents are produced from red and brown algae. Agar from red algae is used as a medium for culturing microorganisms including algae, as a food gel, and in pharmaceutical capsules. Red algal carrageenan is used in toothpaste, cosmetics, and food such as ice creamand chocolate milk.

Alginates from brown algae have extensive applications in the cosmetics, soap, and detergent industries. Sources of alginates are Laminaria, some Fucus species, and the giant kelp Macrocystis,which can grow to more than 60 meters long. Algae are also used as feed in the culture of commercially important fish and shrimp.

Mass cultivation of algae (microalgae)—in open ponds and photobioreactors for production of fuels (such as biomass) and biochemicals (such as carotenoids, amino acids, and carbohydrates) and for water purification—is a rapidly developing area based on the use of solar energy as energy source. The green alga Dunaliella is used in the industrial production of carotene. In wastewater treatment plants, algae are used to remove nutrients and heavy metals and to add oxygen to the water.

Algae are used worldwide as indicators (biomonitors) of water quality, helping to detect the presence of toxic compounds in water samples. Several fast-growing algae are used, including the green alga Selenastrum capricornutum.

Many algae are widely employed as research tools because they are easy to culture and manipulate. Danish biologist Joachim Hammerling’s experiments with the green alga Acetabularia identified the nucleus as the likely storage site of hereditary information.

Diversity

Taxonomists believe that there are between thirty-six thousand and tenmillion species of algae. Molecular comparisons using nucleotide sequences in ribosomal RNA (ribonucleic acid) suggest that algae do not fall within a single group linked by a common ancestor but that they evolved independently.

The algae are divided into ninemajor phyla, which differ in their photosynthetic pigments, food reserves, cell structure, and reproduction. These groups include euglenoids, cryptomonads, dinoflagellates, haptophytes, and red algae.

Phylum Euglenophyta contains mostly unicellular formswith one or two flagella. Only one-third of this group possess chlorophyll-containing chloroplasts. Other euglenoids are strictly heterotrophic.

The phylum contains more than nine hundred, mostly freshwater, species. The food reserve is the carbohydrate paramylon, a polymer of glucose. Euglenophytes have chlorophyll a and b as well as carotenoids as their photosynthetic pigments. There is no cell wall.

Cells have several small chloroplasts; each is surrounded by three membranes. A close relative of euglenophytes is the protozoan Trypanosoma, which causes the human disease African sleeping sickness. Reproduction in the euglenophytes occurs by division of cells. Sexual reproduction is unknown.

Phylum Cryptophyta includes unicellular biflagellates. In addition to chlorophyll a, chloroplasts can contain chlorophyll c, carotenoids, and phycobilins. The carotenoid pigment alloxanthin is unique to Cryptophyta. Four membranes surround each chloroplast.

Chloroplast endoplasmic reticulum borders the chloroplasts. The principal food reserve is starch. Instead of a typical cell wall, a periplast composed of protein plates occurs beneath the cell membrane. There are about two hundred freshwater and marine species. Reproduction is primarily asexual.

Members of the phylum Dinophyta, or dinoflagellates, have unicellular forms with two different flagella. There are between two thousand and four thousand marine species and about two hundred freshwater forms. Many have chlorophylls a and c as well as the unique carotenoid peridinin. Some members of Dinophyta have fucoxanthin.

Chloroplasts have three closely associated membranes. The primary food reserve is starch, but lipids are also important storage molecules. A dinoflagellate cell is not surrounded by a cell wall but has a theca (a sort of armor) made of cellulose. Dinoflagellates can reproduce asexually and sexually.

Phylum Haptophyta includes primarily marine unicellular biflagellated algae. A haptophyte cell also has a flagellum-like haptonema, used to capture food. There are about three hundred species. The photosynthetic pigments include chlorophyll a and accessory pigments chlorophyll c and the carotenoid fucoxanthin.

Each chloroplast has four membranes. The food reserve is chrysolaminarin, which is a polymer of glucose. Several layers of scales, or coccoliths, composed primarily of calcium carbonate may cover the haptophyte cell. Asexual and sexual reproduction is widespread.

Phylum Rhodophyta, or the red algae, has between four thousand and six thousand species. Red algae lack any flagellated stages. The photosynthetic pigments include chlorophyll a as well as accessory phycobilins and carotenoids. Two membranes surround each chloroplast.

The food reserve is a floridean starch. A red algal cell is encircled by a wall composed of cellulose. Asexual and sexual re production, as well as alteration of generations, are widespread among Rhodophyta. A triphasic life cycle is unique for this group of algae.

Allelopathy

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Allelopathy refers to all the biochemical interactions, both beneficial and harmful, among all types of plants, including microorganisms.

For an allelopathic interaction to occur, chemicals must be released into the environment by one plant that will affect the growth of another. In this way allelopathy differs from competition, which involves removal of some factor from the environment that is shared with other plants. Allelopathy was recognized as early as Theophrastus (300 b.c.e.), who pointed out that chick pea plants destroy weeds growing around them.

Methods of Action

Avariety of different allelochemicals are produced by plants, usu- ally as secondary metabolites that do not have a specific function in the growth and development of the host plant but that do affect the growth of other plants. Originally plant physiologists thought these secondary products were simply metabolic wastes which plants had to store because they do not have an excretory system as animals do. Their various functions are now beginning to be understood.

One class of allelochemicals, coumarins, block or slow cell division in the affected plant, particularly in root cells. In thisway growth of competing plants is inhibited, and seed germination can be prevented. Several kinds of allelochemicals, including flavonoids, phenolics, and tannins, suppress or alter hormone production or activity in competing plants.

Other chemicals, including terpenes and certain antibiotics, alter membrane permeability of host cells, making them either leaky or impermeable. In some cases, membrane uptake can be enhanced, particularly for micronutrients in low concentration in the soil. Finally, a variety of allelochemicals have both positive and negative effects on metabolic activity of the affected plant.

Allelopathy in Agriculture

Most of the negative effects of weeds on crop plants have been attributed to competition; however, experiments using weed extracts have demonstrated that many weeds produce allelochemicals. Similarly, some crop plants are allelopathic to others and themselves, including wheat, corn, and rice. In these cases the residues of one year’s crop can interferewith crop growth in subsequent years.

This is increasingly important for farmers to consider who are incorporating low-tillage methods to reduce soil erosion. To minimize these effects, some of the traditional techniques of cover cropping, companion cropping, and crop rotation must be employed. Known allelopaths are also beginning to be used as biological control agents to manage invasive and weedy plant species.

Allelopathy in Nature

Several tree species, including black walnut, black locust, and various pines, are known to produce allelochemicals that inhibit the growth of understory species. In some cases this is a result of drip from the foliage or leachate from fallen leaves and fruit. In other cases, roots secrete allelochemicals that kill seedlings of other plants. Bracken fern (Pteridium aquilinum) is known to affect the growth of many other plants.

Alternative Grains

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Alternative grains refers to alternatives to high-yield grain crops, the harvest of which has led to severe soil erosion and increased use of fertilizers and pesticides.

More than one-half of the calories consumed daily by humans comes from grains.Most of these grains are produced by plants of the grass family, Poaceae. Major cereal plants domesticated centuries ago include rice (Oryza sativa), wheat (Triticum aestivum), and corn (Zea mays). Other important grain crops, also plants of the grass family, include barley (originating in Asia), millet and sorghum (originating in Africa), and oats and rye (originating in Europe).

Grain Genetics

Since the early twentieth century, the scientific principles of genetics have been applied to improvements of crop plants. Some notable improvements occurred between 1940 and 1970. As a result of irrigation, improved genetic varieties, and the use of large amounts of fertilizers and pesticides, yields of major crops greatly increased.

Norman Borlaug received aNobel Prize in 1970 for his contributions to these developments, which came to be called the Green Revolution. However, it soon became apparent that the Green Revolution was not the boon first envisioned.

For maximum yield, large-scale farming involving huge capital investment is required. Also, environmentalists became concerned over the erosion and other environmental damage caused by the use of large amounts of fertilizers and pesticides.

Minor Cereals

Various alternatives have been proposed. For grain crops, several approaches offer promise, including more widespread use of minor cereals, especially those tolerant of unfavorable growing conditions; development of new cereal plants by hybridization or other genetic manipulation; and use of pseudocereals, nongrass crop plants that produce fruits (grains) similar to those of cereal plants.

Most sorghum (Sorghum bicolor) grown in the United States is used for silage or molasses. In Africa and India, various grain sorghums are grown in regions where rainfall is too low for most other grain crops.Well adapted to hot, dry climates, their grains are used to make a pancakelike bread.

Millet refers to several grasses that are also useful cereal plants because they tolerate droughtwell. In Africa the most important are pearl millet (Pennisetum glaucum) and finger millet (Eleusine coracana). Grains of both species can be stored for long periods and are used to make bread and other foods.

Other, perhaps less important, grain plants also called millet include foxtail millet (Setaria italica), native to India but nowgrown in China; prosomillet (Panicum milaeceum), native to China but grown in Russia and central Asia; sanwa millet (Echinochloa frumentacea), cultivated in East Asia; and teff (Eragrostis teff), an important food and for age plant of Ethiopia. Such grain sorghums and millets have the potential to growin areas with hot, dry climates far beyond the regions where they are now being utilized.

In a distinct category is wild rice (Zizania aquatica). Native to the Great Lakes region of the United States and Canada, it has been, and still is, harvested by American Indians. Like the common but unrelated rice (Oryza sativa), wild rice grows in flooded fields. Attempts to cultivate it since the 1950’s have been somewhat successful as the result of the development of nonshattering varieties.

However, it remains an expensive, gourmet item. Two cereal plants have promise because of the high-protein content of their grains. Wild oat (Avena sterilis) is a disease-resistant plant with large grains. Job’s tears (Coix lachryma-jobi), native to Asia, is now planted throughout the tropics. Research on these and related species continues.

Although all important cereal plants have been improved by genetic techniques, the most notable new alternative grain plant is triticale (Triticosecale). The first human-made cereal, it is the result of crossing wheat with rye. The sterile hybrid from such a cross was made fertile by doubling its chromosomes. Thus, triticale varieties produce viable seeds.

Triticale combines the superior traits of each of its parents: the cold tolerance of rye and the higher yield of wheat. The protein content of triticale compares favorably with that of wheat, and its quality, as measured by lysine content, is higher. However, flour made from triticale is not suitable for making bread unless mixed with wheat flour.

Pseudocereals

Pseudocereals are plants that are not of the grass family but produce nutritious, hard, grainlike fruits that can be stored, processed, and prepared for food much like grains. They belong to several plant families. Many grow under conditions not suitable for the major cereal crops. Buckwheat (Fagopyrum esculentum), of the Polygonaceae family, probably originated in China.

It tolerates cool conditions and is adapted to short growing seasons, thus permitting it to be grown in the temperate regions of North America and Europe. In the United States, it is often associated with pancakes but is used in larger quantities for livestock feed. In Eastern Europe, the milled grain is used for soups.

Quinoa (Chenopodium quinoa) of the goosefoot family, Chenopodiaceae, has been cultivated by Indians of the Andes Mountains for centuries. The leafy annual produces grainlike fruits (actually achenes) with a high protein content and exceptional quality, high in lysine and other essential amino acids.

After its bitter saponins have been removed, it can be cooked and eaten like rice or made into a flour. Quinoa has been cultivated in the Rocky Mountains since the 1980’s and has become a gourmet food in the United States.

Most amaranths (Amaranthus) plants are New World weeds. They belong to the amaranth family, Amaranthaceae. A few species were used by Aztecs and other North American peoples, but amaranth use was banned by the Spanish.

Since the late 1970’s, plant breeders have targeted several species for improvement. The results are highly nutritious grains, rich in lysine, that are suitable for making flour. Research in Pennsylvania and California has resulted in improved varieties.

Anaerobes and Heterotrophs

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The first organisms to evolve on the earth are thought to have been heterotrophs and anerobes. Heterotrophs are organisms that cannot produce their own food but must fill their energy requirements by consuming organic molecules produced by other processes or organisms. Anaerobes are organisms that do not require free oxygen gas in order to survive; for some anaerobes, free oxygen may be poisonous.

Heterotrophs include many familiar organisms (such as animals) whose existence is tied to primary producers, those organisms that create energy-storing molecules, such as photosynthesizing plants. Anaerobes also are common, though less apparent. Typically, they are microscopic organisms restricted to living in a few surface environments where oxygen is absent.

It may seem strange, then, that these organisms were perhaps the first organisms to have evolved on the earth. Yet the combination of the heterotrophic lifestyle and the anaerobic life requirement is consistent with what is known about the conditions of the early earth’s surface environment.

The earliest anaerobic heterotrophs laid the biochemical foundations for the evolution of photosynthesis, free oxygen in the atmosphere, and the rise of complex organisms. All those events had the adverse impact of limiting the range of environments available to the anaerobes.

The world’s first organism evolved in what has been called a "prebiotic soup" of energy-rich organic molecules. Heterotrophic organisms would exploit this environment by absorbing the molecules. A continuing supply of energy-rich molecules depended on the absence of free oxygen in the early atmosphere and the functioning of the abiotic synthesis.

Fermentation

The energy-richmolecules of the soupwere converted to energy by a series of biochemical reactions. One of the simplest, and therefore perhaps one of the oldest, types of energy conversion reactions is anaerobic fermentation.

During anaerobic fermentation, an energy-rich molecule, such as the simple sugar glucose, is dismantled to release energy and waste by-products. Several lines of evidence suggest that this form of energy conversion was utilized by the early heterotrophs.

One indicator that fermentation is a very ancient biochemical process is that the reaction used to release energy from the glucose molecule is very common among modern organisms. The ability to utilize the fermentation reaction is evident in the anaerobic reaction of yeast using sugar and releasing ethyl alcohol.

Although it is not the primary energy-releasing reaction for most organisms, fermentation’s widespread availability suggests that it is very old and perhaps inherited from an early, simpler ancestor.

The fermentation reaction is not very efficient. For example, it releases two units of energy for every glucose molecule, whereas oxidation of the same glucose molecule releases more than thirty energy units.

Such an inefficient reaction for energy release could not be tolerated by an advanced organism with many energy demands. Alternatively, single-celled heterotrophs surrounded by, and absorbing, energy-rich molecules such as glucose (which is unlikely to decompose in an anoxic environment) do not expend much energy in gathering their food.

The Earliest Organisms

Thus, two very different types of evidence—that which points to an anoxic early atmosphere and evidence for the ancient ancestry of the glucose fermentation reaction—suggest that the earliest organism was a single-celled heterotroph that absorbed energy-rich molecules from the surrounding anaerobic environment.

Modern analogues for such an organism exist. Single-celled bacteria, called obligate anaerobes, exist in a few anoxic environments today.

It is likely that the modern obligate anaerobes have not changed significantly (especially in their morphology, or shape and size) from their Precambrian ancestors. Given this much information, paleontologists know that their search for early Precambrian fossils, the petrified remains of organisms, is not easy.

The process of fossilization—that is, the preservation of the shape of an organism in rock—is best at preserving the details of hard body parts. Hard skeletons and shells or their impressions are easier to preserve than are soft body parts. In the case of early Precambrian fossils, the most likely organisms (bacteria) not only are small but also contain no hard body parts.

Despite these barriers to preservation and despite the very poorly preserved Precambrian rock record, some early Precambrian fossil remains have been found and described. The fossils are usually found preserved in rock called chert, which probably began as a gelatinous material. Microscopic remains of organisms embedded in this gelatin were delicately preserved when the chert lost some of its water and solidified.

The oldest fossil remains identified have been found in cherts from southern Africa. These cherts, part of what is called the Fig Tree Formation, are more than three billion years old. The fossils consist of the wispy, spherical remains of what may have been a type of alga and the rod-shaped remains of a possible heterotrophic bacterium.

Anaerobic Photosynthesis

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Anaerobic photosynthesis, also known as anoxygenic photosynthesis, is the process by which certain bacteria use light energy to create organic compounds but do not produce oxygen. Anaerobes are those bacteria that cannot use oxygen to generate energy.

The photosynthetic process in all plants and algae, as well as in specific types of bacteria, involves the reduction of carbon dioxide to carbohydrate and the removal of electrons from water, resulting in the release of oxygen.

This process is known as oxygenic or aerobic photosynthesis. Water is oxidized by a multi-subunit protein located in the photosynthetic membrane. This is a molecular protein feature shared among more than 500,000 species of plants on earth.

While this is a common feature among nearly every form of plant life on earth, some photosynthetic bacteria can use light energy to extract electrons from molecules other than water. These bacteria are of ancient origin and are believed to have evolved before aerobic photosynthetic organisms.

These anaerobic photosynthetic organisms occur in the domain Bacteria. Anaerobic photosynthetic bacteria, also known as anoxygenic photosynthetic bacteria, differ from aerobic organisms in that each species of these bacteria has only one type of reaction center.

In some photosynthetic bacteria the reaction center involves the oxidation of water and the reduction of the aromatic molecule plastoquinone. In other species it involves the oxidation of plastocyanin and the reduction of ferredoxin protein.

Photosynthetic bacteria are typically aquatic microorganisms inhabiting marine and freshwater environments, including wet and muddy soils, stagnant ponds, sulfur springs, and still lakes. They are classified into five groups based on pigment composition, metabolic requirements, and membrane structure: green bacteria, purple sulfur bacteria, purple nonsulfur bacteria, heliobacteria, and halophilic archaebacteria.

Some of these organisms are strict anaerobes; that is, they can grow only in the complete absence of oxygen. They cannot use water as a substrate, and they do not produce oxygen during photosynthesis. Facultative anaerobes, on the other hand, can grow either in the presence or in the absence of oxygen.

Green bacteria include two families, the Chloroflexaceae and the Chlorobiaceae. The Chlorobiaceae are strict anaerobes that grow by utilizing sulfide, thiosulfate, or organic hydrogen as an electron source.

Chloroflexaceae are facultative aerobes which use reduced carbon compounds as electron donors. Purple sulfur bacteria uses an inorganic sulfur compound, such as hydrogen sulfide, as a photosynthetic electron donor.

Purple nonsulfur bacteria depend on the availability of simple organic compounds such as alcohols and acids as electron donors, but they can also use hydrogen gas. Purple sulfur bacteria must fix carbon dioxide to live, whereas nonsulfur bacteria can grow aerobically in the dark by respiration on an organic carbon source.

Heliobacteria are anaerobic photosynthetic bacteria that contain a special type of bacteriochlorophyll, BChl g, that works as both antenna and reaction center pigment. Halobacteria are very unusual. They cannot grow in low salt concentrations (or their cell walls collapse).

Typically, they are heterotrophs with an aerobic electron-transport chain, but they can also respire anaerobically, with nitrate or sulfur. In the absence of suitable electron acceptors they can ferment carbohydrates.

Halobacteria, when exposed to light in the absence of oxygen, can synthesize a purple membrane containing a single photosensitive protein called bacteriorhodopsin which, when illuminated, begins cyclic bleaching and regeneration, extruding protons from the cell. This light-stimulated proton pump operates without electron transport.

The mechanism by which halobacteria convert light is fundamentally different from that of higher organisms because there is no oxidation/reduction chemistry, and halobacteria cannot use carbon dioxide as their carbon source. As a result, some scientists do not consider halobacteria as being photosynthetic.

Process

The common features to both aerobic and anaerobic photosynthesis have been known since the mid-twentieth century:

Green plants:
CO2 + 2H2O + light → (CH2O) + O2 + H2O
Green sulfur bacteria:
CO2 + 2S + H2O + light → (CH2O) + 2S + H2O

In each case, inorganic carbon (CO2) is fixed into organic carbon (CH2O), the source of reductant is hydrogen in either water or hydrogen sulfide, and the chemical energy required for this activity is derived from light energy. The sulfur produced anaerobically is analogous to the oxygen produced by the oxygenic photosynthesis of green plants.

Photochemical processes in photosynthetic bacteria require three major components: an antenna of light-harvesting pigments, a reaction center within an intra-cytoplasmic membrane containing at least one bacteriochlorophyll, and an electron transport chain.

All photosynthetic bacteria can transform light energy into a trans membrane proton gradient used for the generation of adenosine triphosphate (ATP) and production of oxidase, but none of the anaerobic photosynthetic bacteria are capable of extracting electrons from water, so they do not evolve oxygen.

Many species can only survive in low-oxygen environments. To provide the necessary electrons for carbon dioxide reduction, anoxygenic photosynthetic bacteria must oxidize inorganic or organic molecules from their immediate environment.

Despite basic differences, the principles of energy transductions are the same in anaerobic and aerobic photsynthesis. Anaerobic photosynthetic bacteria depend on bacteriochlorophyll, a group of molecules similar to chlorophyll, that absorbs in the infrared spectrum between 700 and 1,000 nanometers. The antenna systems in these bacteria consist of bacteriochlorophyll and carotenoids, serving a reaction center where primary charge separation occurs.

Electron carriers include quinone and cytochrome bc complex. Electron transfer is coupled to the generation of electrochemical potential that drives phosphorylation by ATP synthase, and the energy required for the reduction of carbon dioxide is provided by ATP and dehydrogenase.

Angiosperm Cells and Tissues

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Some cell types and tissues which are not found in any other groups of plants occur in angiosperms (flowering plants).

Angiosperms are a group of plants with seeds that develop within an ovary and reproductive organs in flowers. They are commonly referred to as flowering plants and represent the most successful group of plants on earth, with approximately 235,000 species.

Various cell types and tissues, many of which are not found in any other groups of plants, occur in angiosperms. These cells and tissues perform varied functions, which are very efficient compared to their counterparts in other plants. These include dermal, vascular (xylem and phloem), and ground tissues (such as parenchyma, collenchyma, and sclerenchyma).

The growth of plants is carried on by a group of cells at their tips. These groups of cells are referred to as apicalmeristems, which are composed of initials and theirmost recent derivatives. The initials are the main source of body cells in plants,while the derivatives become any of the cells and tissues in the plant body.

The apical meristems of both the shoot and the root show continued cell division, with cells enlarging, elongating, and differentiating in regular, distinctly organized patterns. Apical meristems bring about the increase in the length of the stems and roots and are responsible in the formation of the primary plant body.

The shoot apical meristem may continually initiate the aerial components of the plant ormay enter a state of periodic quiescence. In some plants, the shoot apical meristem transforms into a floral or inflorescence meristem that eventually terminates in a single flower or clusters of flowers, respectively.

The root apical meristem is enclosed by a thimble-shaped root cap that hastens the penetration of roots between soil particles.Unlike the shoot apicalmeristem, the root apical meristem forms no appendages. In fact, the site of lateral root initiation is far removed from it.

Shoot Apex

The shoot apical meristem is typically dome- shaped but flattened, and concave outlines also exist. The outline is not constant but changes in response to plastochron (the time interval between the initiation of one leaf primordium and the next). At least three models describe the shoot apical meristems. Although each of these is based on one or two unique criteria, they also have a few overlapping features.

Cell Lineage Analysis

This model holds that three clonally related lay- ers of cells characterize the shoot apical meristem. These layers can be more than one cell layer thick. L1 is the outermost layer and gives rise to the epidermis, L2 is the middle layer and gives rise to the vascular tissues and cortex, and L3 is the inner most layer and gives rise to the pith.

This model was based on studies using periclinal chimeras (organs or parts of tissues of diverse genetic constitution), where one of the cell layers was genetically altered using drugs that inhibit separation of chromosomes.

Tunica-Corpus Concept

This model is based on microscopic analysis of constituent cells. It says that the shoot apical meristem is made up of two groups of cells. The tunica, a group of cells that form one or two stratified layers, undergoes anticlinal divisions only and gives rise to the epidermis.

Partly enclosed by the tunica is the corpus, a group of loosely arranged cells that divide in various planes and give rise to the vascular and ground tissues. The tunica maintains its individuality by surface growth, whereas the corpus adds bulk by increase in volume.

Cytohistological Zonation

This model recognizes various definable zones in the shoot apical meristem. Three zone boundaries are distinguished by cell size: staining quality, degree of vacuolation, and frequency of cell division.

The central (mother cell) zone represents a conspicuous group of enlarged and isodiametric cells that undergo infrequent cell division, possess prominent nuclei, and are often highly vacuolated. The flanking peripheral zone is derived from, and partly surrounds, the central zone. Cells of this zone are smaller, are mitotically active, and have dense cytoplasms.

They give rise to the epidermis, vascular tissues, and cortex. The rib zone is located at the base of the central and peripheral zones. This zone is directly formed from the central zone, produces longitudinal files of cells by periclinal divisions, and gives rise to the pith.

Root Apex

The organization of the root apical meristem is different from that of the shoot apical meristem. Root apicalmeristems are commonly interpreted as having either a close or open type of organization. In a close type of organization, the dermal, vascular, and ground tissues each have their own set of initials.

This organization shows a clear boundary between root cap and other tissues of the root apex. In an open type of organization, all of the root tissues share a group of initials, and therefore the boundary of the root cap is indistinguishable from the other tissues of the root apex.

Developmental Processes

The cells produced by apicalmeristems undergo several key developmental processes, which include growth, differentiation, and morphogenesis. Although each of these can be separated individually, they overlap in highly complex fashion.

Growth refers to the quantitative increase in a cell’s volume or mass due to enlargement and multiplication. Differentiation is the qualitative change in the form and function of organelles, cells, tissues, and organs, resulting in the establishment of new structures and functions.

From an anatomical point of view, cell differentiation is related to changes in cell size and shape, modifications of the wall, and changes in staining characteristics of nucleus or cytoplasm, as well as the degree of vacuolation and the ultimate loss of the protoplast in some cases. Morphogenesis is the visible manifestation of all of the changes, brought about by growth and differentiation, as expressed in the overall morphology of the plant.

Dermal Tissues

The primary plant body is composed of three basic tissues: dermal, vascular, and ground tissues. The dermal tissue (or epidermis) is made up of several cell types and is involved in a variety of functions, including retention and absorption of water and minerals, protection against herbivores, and control of gas exchange. Each of these functions is attributable to one or more of the unique features of the epidermis.

Most epidermal cells are flat and tightly packed, forming a single layer around stems, leaves, and other organs. The outer walls of epidermal cells are equipped with a waterproof layer made up of a fatty material called cutin. The tightly packed and cutinized epidermis protects the plants from desiccation by keeping moisture in.

Epidermal cells lack chloroplasts and are transparent. It is the underlying cells that give leaves and stems their green color. However, the vacuoles of some epidermal cells occasionally contain pigments and are responsible in the coloration of flowers and colored parts of variegated leaves.

Stomata are specialized structures that form part of the epidermis of leaves, stems, flowers, and fruits. They are involved in regulating the intake of carbon dioxide for photosynthesis as well as the release of oxygen. Trichomes are single-celled or multicellular out growths of epidermal cells that are involved in deterring herbivores and restricting transpiration.

Root hairs are also outgrowths of epidermal cells that are specialized for absorbing water and minerals from soil. They occur near the tip of the root and function to increase its absorptive surface area several-thousand fold.

Vascular Tissues

Vascular tissues are of two types: xylem and phloem. Xylem occurs throughout the plant body, and the type that differentiates directly from the apical meristem is called primary xylem. (Secondary xylem is formed from the vascular cambium.)

Primary xylem is formed as stems and roots elongate. The two kinds of conducting cells in xylem are tracheids and vessels, or vessel elements. Both are dead at maturity and have thick, lignified secondary cell walls. Tracheids are long, slender cells with tapered, overlapping ends.

They are the only water-conducting cells in most gymnosperms (an evolutionary line of plants that includes conifers).Water moves upward in roots and stems from tracheid to tracheid through thin areas in their cellwalls called pits. With only a few exceptions, all angiosperms contain vessel elements and tracheids.

Vessel elements are short, wide in diameter, and connected end to end. Their end walls are partly or wholly dissolved, forming long hollow vessels through which water moves. All these features of vessels enable them to transport water more rapidly than tracheids.

Phloem transports dissolved organic materials throughout the plant. The conducting cells of the phloem are called sieve elements, which are devoid of nuclei but otherwise have intact cytoplasm. They also have thin areas along their cell walls called sieve areas that are perforated. Solutes move from sieve element to sieve element through these pores.

Ground Tissues

The three types of ground tissue are parenchyma, collenchyma, and sclerenchyma. Parenchyma cells are the most abundant and versatile cells in plants. These cells are isodiametric, are alive at maturity, are highly vacuolated, and have a primary cell wall. Parenchyma functions as food- and water-storage tissue as well as sites of metabolism in plants. Chlorenchyma cells are chloroplast-containing parenchyma specialized for photosynthesis.

Collenchyma cells are relatively long, with unevenly thickened primary walls. They support growing regions of shoots and are common in petioles, elongating stems and expanding leaves. Collenchyma cells are well adapted for support because their cell walls are able to stretch. They often form in strands or a cylinder just beneath the epidermis; such location maximizes support, as would a rod locatedin the center of a stemor petiole (leaf base).

Sclerenchyma cells are rigid; produce thick, non-stretchable secondary walls; and are usually dead atmaturity. They occur in, support, and strengthen mature regions of plants, including stems, roots, and leaves. There are two types of sclerenchyma cells: sclereids and fibers.

Sclereids are relatively short and variable in shape and usually occur in small groups. Fibers are long and slender and occur in strands or bundles. Sclereids are found in the roots, leaves, and stems.

They produce the gritty texture of pears and mostly make up the tough core of apples as well as the seed coats of peanuts and walnuts. Fibers are often associated with vascular tissues and, compared to sclereids, are typically elongated cells that vary in length from a few millimeters to more than half a meter long.

Angiosperm evolution

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Angiosperms (flowering plants) appeared about 130 million years ago and today dominate the plant world, with approximately 235,000 species.

In early Devonian-age rocks, approximately 363- 409 million years old, fossils of simple vascular and nonvascular plants can be seen. Ferns, lycopods, horsetails, and early gymnosperms became prominent during the Carboniferous period (approximately 290-363 million years ago).

The gymnosperms were the dominant flora during the Age of Dinosaurs, the Mesozoic era (65-245million years ago). More than 130 million years ago, from the Jurassic period to early in the Cretaceous period, the first flowering plants, or angiosperms (phylum Anthophyta), arose. Over the following 40 million years, angiosperms became the world’s dominant plants.

The angiosperms show high species diversity, and they occupy almost every habitat on earth, from deserts to high mountain peaks and from freshwater ecosystems to marine estuaries. Angiosperms range in size from eucalyptus trees well over 100 meters (328 feet) tall with trunks nearly 20 meters (66 feet) in circumference to duckweed, simple floating plants barely 1 millimeter (0.003 inch) long.

Special Characteristics

Some of the defining characteristics of angio- sperms involve their physical appearance or morphology and internal anatomy: the presence of flowers and fruits containing seeds, stamens with two pairs of pollen sacs, a microgametophyte (the male, haploid stage of the life cycle contained in the pollen) with three nuclei, a megagametophyte (the female, haploid stage of the life cycle enclosed in the ovary) with eight nuclei, companion cells, and sieve tubes in the phloem (vascular tissue important in the transport of organic molecules).

Some of these characteristics involve life-cycle features, such as double fertilization, that are distinct from almost all other members of the plant kingdom. (Double fertilization is also known in the genera Ephedra and Gnetum, members of the gymnosperms.)

Because angiosperms possess so many unique features, plant taxonomists have long believed that angiosperms originated from a single common ancestor. Because the first flowers and pollen grains appear in fossils from the early Cretaceous period, up to about 130 million years ago, it is probable that angiosperms actually arose more than 130 million years ago.

As the findings of paleobotanists (botanists who study plants in the fossil record) have been combined with more recent knowledge from evolutionary genetics and biochemistry, a clearer picture of angiosperm evolution has emerged.

Proposed Ancestors

Because gymnosperms (the other large group of seed plants) have long been considered ancestral to the angiosperms, researchers have attempted to develop models for the evolution of the ovule-bearing structures of flowering plants from the similar, naked ovule-bearing structures of gymnosperms.

Some lines of evidence indicate that groups of extinct cycad-like gymnosperms known as the Bennettitales and the gnetophytes, amodern division of the gymnospermswhich showup in the fossil record about 225 million years ago, are the seed plantsmost closely related to angiosperms.

All three groups, the Bennettitales, the gnetophytes, and the angiosperms, share, or shared, superficially similar flowerlike reproductive structures. The strobili, or cones, of some gnetophytes closely resemble flowers, and the xylem (vascular tissue specialized for transporting water) of some gnetophytes is similar to the xylem found in angiosperms.

Seed Ferns

Other lines of evidence suggest that a group of plants called the seed ferns, or pteridosperms, might represent the ancestors of the angiosperms. The seed ferns, which predate the angiosperms by many millions of years, had seed-bearing cupules and specialized organs that produced pollen.Many plant taxonomists believe that the seed-bearing cupules in some groups of seed ferns could have evolved into the carpels of flowers.

Earliest Flowers

Most paleobotanists assume that the first flowerswere small, simple, and green in color and by modern standards were rather unattractive. Their petals and sepals were probably not clearly differentiated.

In November of 1998, Ge Sun and David Dilcher and their colleagues published their discovery of the oldest angiosperm fossil to date, estimated to be at least 122 million years old and possibly as old as 145million years. Either age qualifies it as the oldest.

The fossils were discovered in China, and the fruits show the characteristic enclosed ovule (a carpel) that is distinctive to angiosperms. It was given the scientific name Archaefructus liaoningensis. Given its great age, this find implies that angiosperms may have arisen as early as the Jurassic period, more than 145 million years ago.

Other early flowers produced pollen with a single aperture, or opening, a trait that the monocot branch of the angiosperms shares with cycads and ginkgos. Plant taxonomists believe that pollen with a single opening is an ancestral feature that some plants have kept as they evolved. The pollen of eudicots,with its three apertures, is thought to be a derived feature (that is, a later evolutionary development).

Recent studies of angiosperm evolution, using data fromdeoxyribonucleic acid (DNA) sequences, have led to the proposal that an obscure shrub from the South Pacific island of New Caledonia, called Amborella trichopoda, represents what is left of the ancestral sister group (a related organism that branched off before the evolution of another group of organisms) to all the angiosperms.

As a sister group to all the angiosperms, it is considered to be themost primitive (in an evolutionary sense) of the angiosperms and therefore should resemble what the ancestor to the angiosperms was like. It does possess some of the expected primitive traits for a primitive angiosperm, such as small, greenish-yellow flowers and a lack of vessels for conducting water from the ground to the leaves.

Angiosperm Classification

Approximately 97 percent of angiosperm species are classified as either Monocotyledones (monocots), with approximately 65,000 species, or Eudicotyledones (eudicots), with about 165,000 species. The monocots include such familiar plants as the grasses, lilies, irises, orchids, cattails, and palms. The more diverse eudicots include most of the familiar trees and shrubs (other than the conifers) and many of the herbaceous plants.

The remaining 3 percent of angiosperms are called the magnoliids, a group of plants considered to have primitive features, among them pollen with a single aperture. Many magnoliids also feature oil cells with ether-containing oils providing the characteristic scents of laurel and pepper, for example. The magnoliids are typically divided into the woody magnoliids and paleoherbs.

Woody magnoliids have large, often showy, bisexual flowers with multiple free parts.Magnolia trees and tulip trees (both in Magnoliaceae, or the magnolia family) are examples of this group. The paleoherbs have small, often unisexual flowers and usually just a few flower parts. Modern paleoherbs include the pepper family (Piperaceae), the birth-wort family (Aristolochiaceae), and the water lily family (Nymphaeaceae).

Recent studies of angiosperm evolution, using data from DNA sequences, have also sharpened the understanding of some of the relationships among monocots, eudicots, and magnoliids. If these groups are displayed as an evolutionary tree (or phylogenetic tree), the magnoliids are polyphyletic (that is, they do not share a single common ancestor).

The magnoliids branch off near the base of the tree on several different branches. The monocots are monophyletic (that is, they share a single common ancestor) and form a separate branch from among the magnoliid branches. The eudicots branch off last and represent the most diverse and evolutionarily complex group.

Geographic Origins

As hotly debated, perhaps, as exactly which group of plants were ancestral to the angiosperms is the question of where the angiosperms first evolved. Some botanists believe that angiosperms first developed in theNorthernHemisphere; others look at the Southern Hemisphere.

At the time angiosperms are proposed to have evolved, the continents were not arranged the way they are now. At that time, all of the world’s major landmasses were grouped into a supercontinent called Pangaea.

The southern part of this continent is referred to as Gondwanaland, and the northern part is called Laurasia. Based on what is known about late Cretaceous angiosperms and their habitats, some scientists suggest that the westernmost, semiarid regions of Gondwanaland may be the place where angiosperms first evolved.

As Pangaea broke up, the separate continents moved in different positions, resulting in new configurations. India collided with Asia, raising the Himalaya Mountains and the Tibetan Plateau. Antarctica slipped over the South Pole, and Australia became isolated. These plate movements created new climatic regimes, opening up new niches that were rapidly exploited by the angiosperms.

Diversification and Spread

Regardless of their geographic origins, by about ninety million years ago the flowering plants were well on their way to dominating the world’s flora. The early angiosperms were well adapted to drought and cold.

Adaptations that conferred resistance to these conditions included strong leaves, efficient water-conducting cells, and tough, resistant seed coats. Some woody flowering plants evolved the ability to lose their leaves, called the deciduous habit.

This characteristic allows the shutdown of metabolism during adverse environmental conditions, such as during seasonal droughts or winter weather. Because of greater climate instability during the past fifty million years or so compared to earlier times, the above-mentioned traits were important in allowing the flowering plants to adapt to different and often harsher climatic conditions.

Pollination

A major innovation that likely led to some of the great diversity seen in angiosperms was pollination by insects or other animals. As plants adapted to the various available pollinators, the pollinators also adapted to the plants, sometimes in very specific ways. Many pollination systems include specialized colors or markings, flower shapes, and flower scents.

This process of evolving "together" is called coevolution. Coevolution has also occurred between plants and their predators. Evolution of chemical compounds to deter herbivory have, in turn, led to adaptations in many animal groups to circumvent the toxicity of the chemical compounds.

Angiosperm Life Cycle

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The word "angiosperm" comes from the Greek words for "vessel" and "seed" and translates roughly as "enclosed seed". In part, angiosperms (the flowering plants, phylumAnthophyta) are defined by the fact that their seeds are enclosed by an ovule. The life cycle of an angiospermis defined by the formation of the seed and its development to a full-grown plant which, in turn, produces seeds.

Angiosperms are vascular plants with flowers that produce seeds enclosed in an ovule—a fact that is recognized as the angiospermy condition.

Reproductive Flower Parts

In general, angiosperms have a floral axis with four floral parts, two of which are fertile. At the receptacle, or tip, of the axis there is an ovule-bearing leaf structure known as the carpel. The ovule or ovules can be found inside the pistil. Three portions compose the pistil: the ovary, the style, and the stigma, where the pollen usually germinates.

The mature ovule consists of the placenta, the integu- ments that are modified leaves that cover the entrance to the embryo sac, the micropyle, and the chalaza. These latter two parts of the ovule complement each other in their positions and functions.

While the micropyle receives and guides the pollen tube, the chalaza relates to the vascular supply of the ovule, nutrition, and support. The stamens, which are often composed of the filament and sporangia sacs that make up the anther, surround the pistil. Stamens carry the male gametes, and the pistil carries the female gamete needed for sexual reproduction.

It is believed that the great diversity and adaptability of the angiosperms is related to the presence of a unique reproductive cycle. This cycle consists of an alternation of generations and the production of a pair of spores on two types of sporophylls: microspores (which become male gametophytes) and megaspores (which become female gametophytes).

Male Gamete Development

The angiosperm reproductive cycle begins with the process of microsporogenesis, or microspore formation. The stamen consists of a filament and the anther, also known as the microsporangium. Inmost of the cases, the anther consists of four pollen sacs, or locules.

Within each locule, the archesporial cell develops through mitosis and extends as a row of cells throughout the entire length of the young anther. These mitotic cell divisions generate the anther wall, which is made up of several cell layers, the outermost of which transforms itself into the epidermis. The layer of cells belowthe epidermis is known as the endothecium.

During anther development, the endothecial cells acquire thickenings whose function is related to anther opening and pollen release. The innermost layer of the anther wall is the tapetum,whose primary function correlates with the nourishment of the young pollen and the deposition of the exine, a coating of the pollen grain.

As development proceeds, the sporogenous cells located below the tapetum transform into microsporocytes. These microsporocytes will undergo meiosis, and tetrads (units of four) of microspores will form.

Shortly after their formation, the tetrads separate into individual microspores. Each microspore is haploid, and often it will enlarge and separate from the tetrad, becoming sculptured by the deposition of sporopollenin and other substances that will turn into the ornamented surface of the pollen grain.

The second phase of pollen development is known asmicrogametogenesis. Themicrospore is the first cell of the gametophytic generation, the cell that generates themature pollen. The single-nucleus microspore develops into the male gametophyte before the pollen is released.

This developmental process occurs through two or three unequalmitotic divisions of the nucleus and subsequent cytokinesis (cell separation). The two daughter nuclei and cells differ in size and in form.

The larger cell represents the tube cell and nucleus,while the smaller cell represents the generative cell and nucleus. At maturity, the grain can be shred in two or three nucleate conditions. When the anther opens, the mature male gametophytes or pollen grains will be disseminated and ready for germination.

Female Gamete Development

The ovule (female sex organ) consists of two opposite ends: the micropyle, where the integuments come together, and a more distant end, where the ovular tissue is more massive. This part is also known as the chalaza, and it lies directly opposed to the micropyle.

The mature ovule is composed of three layers: the outer integument; the inner integument; and, underneath the integuments, the nucellus. During ovular development, one cell lying below the nucellar epidermis changes into a primary archesporial; this will divide to form the primary parietal cell and primary sporogenous cell.

The primary sporogenous cell functions as the megaspore mother cell, which divides meiotically, originating four haploid megaspores. In the majority of angiosperms, three of the megaspores will degenerate, and only the chalazal one will develop into the megagametophyte (embryo sac).

After the completion of the embryo sac stage, a series of cellular events occurs, ending with the formation of the mature embryo sac, ready for fertilization by the male gametes. The chalazal megaspore enlarges and undergoes threemitoses, giving rise to eight haploid cells. The mature megagametophyte consists of two groups of four cells located at both ends of the embryo sac.

The result is three antipodals at the chalazal end: the egg apparatus (consisting of the egg and two synergids at the micropylar end) and the polar nuclei. These two cells, present at both ends, usually fuse before pollination, and during fertilization they form the primary endosperm nucleus.

Pollination

The plant reproductive structures are now ready for the union of male and female gametes or fertilization, which eventually will produce a seed with a viable embryo and cotyledons. Before that step takes place, however, the pollen must be transferred from the anther to the stigma. Biotic agents (such as birds, insects, or mammals) or abiotic agents (such as wind or water) can accomplish this transfer process, known as pollination.

After landing on the stigma, pollen tubes will emerge through the grain apertures if the environ- ment is high in humidity. Successful germination of the pollen in the stigma requires nutrients. In most plants, growth of the pollen tube lasts between twelve and forty-eight hours, frompollen germination to fertilization.

Pollen germination starts with pollen-tube initiation, elongation, and penetration of the stigmatic tissue. During this period intense metabolic activity takes place, for the tube must synthesize membrane material and cell wall for growth and expansion. Simultaneously, at its tip the tube carries the vegetative cell nucleus, fol- lowed by the germinative cell.

Angiosperms have evolved complex breeding systems that ensure theywill be pollinated by their own species. Today it is recognized that two pollination syndromes exist: self-pollination and cross-pollination. In self-breeding species, the pollen comes fromthe anther of the same flower.

In cross-pollination (or outcrossing) species, the pollen comes from the anthers of a different flower or even a different plant of the same species. In these plants, incompatibility in the stigma guarantees that only pollen from other flowers will germinate.

Fertilization

The union of one sperm with the egg is known as fertilization. However, several developmental processes in the vegetative and germinative cells prepare the two sperms for a process known as double fertilization. A mitotic division of the germinative cell generates the spermcells. This process that can take place on the growing pollen tube or inside the pollen grain.

In a growing pollen tube, the vegetative nucleus disintegrates and the sperm cells will take the lead and enter the embryo sac for successful fertilization. Usually, the interactions between the pollen grain and the pistil ensure that the sperm cells will often reach the micropyle of the ovule.

Once the spermreach themicropyle, the growth of other tubes stops. In the embryo sac (female gametophyte), four cells are located at themicropylar side.Of those four, the first pair that the spermcells will encounter are the synergids.

One of these is always bigger than the other and carries the filiform apparatus, a structure resembling hairs that degenerates after pollination and before fertilization. The synergids act as chemical attractants to the pollen tube, which penetrates the synergids via the filiform apparatus and then releases the two sperm cells.

One of the sperm cells will fuse with the egg, producing the zygote; the other sperm cell will fuse with the primary endosperm nucleus, generating the endosperm. The remaining cells of the female gametophyte are the antipodals; they usually degenerate after fertilization has taken place.

Seed and Fruit Formation

Once fertilization has occurred, the ovulewill go through a series ofmetabolic steps ending with the formation of the seed and the fruit. The recently created zygote transforms into amulticellular and com- plex embryo that has two well-defined polar ends: the radicle, or primary root, and the embryonic apical meristem with the first leaves.

After successive mitosis, the mature endosperm usually grows close to the embryo and may provide nutrients needed for germination. The integuments will undergo further transformation, replication, and elongation and will become the seed coat—of variable texture, consistency, and colors, depending on the type of plant.

In general, after pollination or during fertilization, the ovary undergoes a series of physiological changes regulated by synchronized hormonal and genetic alterations that will modify the size of the parenchyma cells and its sugar and organic acids contents.

This process turns the ovary into fruit—in many cases familiar as the edible fruits familiar in human diets. The fruit provides nourishment for the seed until it ripens and drops to the ground, where the next stage in the life cycle begins.

Germination, Seedling Development, and Maturation

Seeds are released from the fruit in a large variety of ways that have evolved to ensure the survival of species. Whether ingested by mammals and passed through their feces to the ground, borne by wind on feathery "wings", or simply falling from rotting fruit that has abscissed and dropped from the plant, the seed must next undergo a process called germination, in which the embryo enclosed in the seed begins its growth. The embryo develops a hypocotyl (root axis) and a fleshy part known as the cotyledon; inmonocots there is one cotyledon, in dicots, two.

Germination requires certain conditions, such as the softening of the seed coat, moisture, and adequate warmth, to occur. During germination, the hypocotyl begins growing downward to become the root; the cotyledon(s) will develop into the shoot, stems, and leaves.

The process of germination results in the sprouting through the ground’s surface of the seedling, which will develop into the mature plant with flowers. The cycle then begins again.

Angiosperm Plant Formation

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Angiosperms are flowering plants. Their formation entails development from embryo to seed, through germination to seedling, and finally to mature plant.

The life cycle of angiosperms (flowering plants) involves an alternation of generations between a dominant sporophytic (spore-producing) phase and a reduced gametophytic (gamete-producing) phase. The first cell of the sporophyte is the fertilized egg, or zygote, which undergoes repeated divisions, growth, and differentiation to form an embryo enclosed in the ovule.

After fertilization, the ovule is transformed into the seed, which germinates into a seedling. The seedling becomes the adult plant; the plant produces flowers inwhich the sperm and egg—representing, respectively, the male and female gametophytic generations—are formed. Fertilization occurs, and seeds are produced to continue the life cycle.

Dicot Embryo Formation

In most angiosperms, embryo development, or embryogenesis, is initiated with a division of the fertilized egg into a small apical cell and a large basal cell, forming a two-celled proembryo. The apical cell generates the embryo proper, and the basal cell forms a filamentous suspensor that anchors the embryo.

Two weeds, Capsella bursa-pastoris (shepherd’s purse) and Arabidopsis thaliana (mouse ear cress or wall cress), both belonging to the Brassicaceae family, have attained prominence as textbook examples of embryogenesis in typical dicots (plants with two cotyledons, or seed leaves; a monocot has one seed leaf).

In these plants, the apical cell of the proembryo divides by two successive longitudinal walls to forma quadrant that is immediately partitioned by transversewalls into an octant, composed of an upper and lower tier of four cells each.

The fates of the two tiers are already fixed in the octant embryo, as the upper tier forms the shoot apex and much of the cotyledons. The lower tier, in addition to providing derivatives to the remaining part of cotyledons, generates the hypocotyl, the radicle, and the root apex.

However, the central region of the root cap, known as the columella, and the quiescent center of the root apical meristem are derived from the terminal cell of the suspensor closest to the embryo, known as the hypophysis. The apicobasal pattern of the future seedling plant is established in the octant embryo.

Aseries of divisions separating eight peripheral cells from a core of eight inner cells heralds histogenesis in the embryo. The result is the formation of a sixteen-celled, globular embryo, in which the peripheral cells form the protoderm (precursor cells of the embryonic epidermis), and the inner cells differentiate into the procambium and ground meristem (precursors of the vascular tissues and ground tissues, respectively) of the mature embryo. This initiates the formation of radial-pattern elements made up of concentric tissue layers in the basal part of the embryo.

The globular stage of the embryo is completed by approximately three additional rounds of divisions, mostly in the inner core of cells. The suspens or attains its genetically permissible number of six to nine cells by this stage. Gradually the cells begin to lose connection from one another and from the embryo and disintegrate.

Emerging from the globular stage, the embryo expands laterally by cell divisions to formthe cotyledons and becomes heart-shaped. The heart-shaped stage is followed by the torpedo-shaped stage, in which elongation of the cotyledons and hypocotyl, as well as extension of the vascular tissues, occurs.

The basic body plan of a shoot-root axis becomes unmistakably clear at this stage, with the establishment of the shoot apical meristem in the depression between the cotyledons and the organization of a root apex by incorporation of derivatives of the hypophysis at the opposite end of the embryo.

During further growth, the cotyledons bend toward the hypocotyl (bent cotyledon or walking-stick shaped stage), and the embryo is phased into the mature stage. A mature embryo of Arabidopsis has fifteen thousand to twenty thousand cells and, under favorable conditions of growth, develops in about nine days fromthe time of fertilization to the mature embryo stage.

Sensitive genetic screens have led to the isolation of Arabidopsis mutants defective in apicobasal and radial patterning of embryos. Characterization of the mutant genes and their protein products has unraveled to some extent the molecular components of the embryonic pattern-forming system in this plant.

Monocot Embryo Formation

The early divisions of the zygote in monocots follow the same pattern as in dicots. However, in the Poaceae (grasses) family, which includes wheat and the other cereals, the sequence and orientation of later divisions in the proembryo are irregular, resulting in highly complex mature embryos. The main feature of the cereal embryo is the development of an absorptive organ known as the scutellum (considered equivalent to the single cotyledon).

Other organs of the embryo for which there are no counterparts in the dicot embryo are a sheath like tissue covering the root (coleorhiza), a tissue that covers the shoot (coleoptile), and an internode known as the mesocotyl. On one side of the coleorhiza there is also a small, flaplike out growth called the epiblast.

Embryo Maturation to Seed

As the embryo matures, the ovule progressively desiccates to become the seed enclosed within the ovary. Concomitantly, the integuments of the ovule harden to form the protective seed coat. Within the ovule itself, the primary endosperm nucleus formed after double fertilization begins to divide, ahead of the zygote, to produce the endosperm charged with nutrient substances. In seeds ofmany plants, including Arabidopsis, Capsella, bean, and pea, the endosperm is utilized by the developing embryo.

In other plants, especially the cereals, the bulk of the seed (grain) is made up of the endosperm surrounding the small embryo. The mature embryo enclosed in the seed consists of an axis bearing the radicle (embryonic root) at one end and the plumule (the embryonic shoot consisting of the shoot apex and one or two leaves) at the other end, and one (in monocots) or two (in dicots) cotyledons.

The part of the embryo axis above the point of attachment of the cotyledon(s) is known as the epicotyl, whereas the part below the attachment point connecting to the radicle is called the hypocotyl.

Seed Germination

The dry seed enclosing the mature embryo may not germinate immediately; if it does not, it enters a period of quiescence or dormancy. Quiescent seeds germinate when provided with the appropriate conditions for growth, such as water, a favorable temperature, and the normal composition of the atmosphere.

Dormant seeds germinate only when some additional hormonal, environmental, metabolic, or physical conditions are met. In almost all seeds, the first part of the embryo to emerge during germination is the radicle. It forces it way through the soil and forms the primary root of the seedling. However, the manner in which the shoot emerges and develops varies considerably in different seeds.

In the epigeous type of germination (for example, in beans), emergence of the radicle is followed by the elongation of the hypocotyl, which arches above the soil surface as a hook. As the hook straightens, it pulls out the cotyledons and plumule above the soil surface. In the hypogeous type of germination (in peas, for example) the cotyledons enclosed within the seed coat remain in the soil during germination.

It is the epicotyl that arches above the soil surface, and as the hook straightens out, it carries the plumule along with it to the surface of the soil. In the monocot, such as the onion, after emergence of the radicle the single cotyledon arches above-ground and subsequently straightens.

Members of the Poaceae display a type of germination in which, following the outgrowth of the radicle, the coleoptile enclosing the plumule grows out of the grain and appears above the soil. The seedling leaves force theirway, breaking the coleoptile, and appear outside as the first photosynthetic organs.

The growth of the coleoptile during germination of grains is facilitated by the elongation of the mesocotyl. These various types of germination ensure an efficient use of food materials stored in the embryo or in the endosperm for the growth of the seedling until it becomes autotrophic.

Embryo to Adult Plant

Although the question as to whether the seedling will become a gigantic tree or a small, herbaceous plant is determined by its genetic blueprint, certain common postgermination growth and developmental episodesmark the development of the seedling into an adult plant.

In dicots, continued growth of the primary root produces an extensively branched root system consisting of secondary roots or lateral roots. In monocots, the primary root disintegrates shortly after it is formed, and so the root system is constituted of numerous adventitious roots which arise from the base of the stem.

Although the cotyledons retain their photosynthetic capacity for some time after germination of the seed, the seedling becomes completely autotrophic as the shoot apex produces new leaves and branches arise in the axils of leaves.

These activities are coordinated by the division of cells in the root and shoot apical meristems and the differentiation of cells into specialized tissues and organs. The shoot and root apical meristems, considered analogous to the stem cells of animals, remain active throughout the life of the plant and, hence, are known as indeterminate meristems.

Angiosperms

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The name "angiosperms" has long been used by botanists to refer to the flowering plants, a group of approximately 235,000 species. All angiosperms are members of the phylum Anthophyta.

The name "angiosperm" is actually derived from two Greekwords, angeion,meaning "vessel" or "container", and sperma, meaning "seed". The name was given in reference to the fact that the seeds of all flowering plants develop from ovules that are enclosed in a structure called a carpel.

This characteristic sets the angiosperms apart from all other plants, which either do not have seeds or have seeds that are not developed in structures resembling a carpel. Although the name angiosperm is used widely, plant taxonomists and many botanists typically refer to them by the more formal name Anthophyta, the phylum that contains the flowering plants.

Unique Features of Angiosperms

In addition to possessing enclosed seeds, Anthophyta differs from other plant phyla in a number of ways. The most obvious distinguishing feature is the flower, a complex structure containing the reproductive parts of the plant. The reproductive structures in other plants are much less complex and showy. The angiosperm life cycle differs from that of almost all other plants.

The sporophyte is the dominant, diploid stage and is the more visible form of the plant, with the leaves, stems, roots, and flowers. The haploid gametophyte is confined to life inside the ovary or anther of the flower, unlike the typically free-living gametophytes of most other plants.

Fertilization is also unique in angiosperms. Many angiosperms rely on insects or other animals to transfer pollen from one flower to another. Pollen grains produce two haploid sperm that travel through a pollen tube from the stigma into the ovary of the flower and into one of the embryo sacs.

Within the embryo sac one of the sperm fertilizes the egg, which will lead to formation of the diploid embryo, and the other sperm fuses with two or more polar nuclei to form the endosperm, which will nourish the embryo and young seedling. This process is often referred to as double fertilization. Other, less obvious features set Anthophyta apart as well, including a unique vascular anatomy, pollen structure, and various biochemical characteristics.

Size and Geographic Diversity

There are approximately 235,000 species of flowering plants, and they are found in almost all terrestrial habitats, with the exception of extremely high elevations and some polar regions. As mall proportion of flowering plants are aquatic (that is, found in freshwater habitats), and an even smaller number aremarine (found in saltwater habitats).

The greatest species richness is in tropical regions, especially tropical rain forests, and species richness steadily decreases at increasing latitudes north and south of the equator.

Angiosperms have been so successful in terres- trial ecosystems that they represent the majority of the herbs and shrubs and many of the trees as well. The diversity of growth forms is tremendous, represented by such diverse families as Poaceae (grasses and bamboos), which have greatly reduced and modified flowers; Cactaceae (cactuses), which have spines instead of leaves and very showy flowers; and Lemnaceae (duckweed), which has a highly reduced plant body sometimes comprising a single leaf with no true roots or stem and the smallest flowers of any angiosperm.

Other families include Asteraceae (sunflower or aster family), with reduced disc and ray flowers crowded together into inflorescences called heads; Salicaceae (willow family), a widespread, water-loving family of trees and shrubs with reduced flowers arranged in catkins; and Orchidaceae (orchid family), with some of the showiest and most intricate flowers of all, which have extremely numerous and minute seeds.

Economic Importance

Economically, angiosperms have made a profound impact. Essentially all of the world’s food crops, from rice, wheat, and corn to other fruits and vegetables, are derived from flowering plants. In fact, it is almost impossible to think of more than a handful of foods or food ingredients from plants that are not flowering plants.

The same is true of ornamental plants. Although a few gymnosperms (such as conifers) and ferns are common as ornamentals, most of the remaining plants, even many valued for their foliage rather than their blooms, are flowering plants.

The only area where angiosperms do not dominate economically is in forest products, where conifers account for a significantly larger proportion of the harvest, but even there, hardwoods predominate for certain applications.

Medicine has also reaped many benefits from angiosperms. In fact, it was primarily the herbalists, fromtheMiddle Ages to the Scientific Revolution, who expanded humankind’s understanding of flowering plants. Knowledge of flowering plants for food andmedicine amongmany indigenous peoples has always been wide spread.

Modern medicine has capitalized onmuch of this knowledge and has even expanded the search for new medicines. Flowering plants have been the original source of many precursors to modern medicines, including aspirin (willows, Salix), quinine (Cinchona species), and digitalin and digoxin (Digitalis species).

Lifestyle Diversity

Alongwith the diversity in structure comes a diversity in lifestyles. Most angiosperms are free-living, that is, receiving their primary energy and carbon from photosynthesis and their nutrients from the soil.

A few groups of plants receive their energy or nutrients in other ways. Some are saprophytes, which receive their energy and carbon from decaying organic material in the soil and their nutrients from other soil components, much like other plants.

Some of the best-known saprophytes are in Ericaceae (heath family). Their most distinctive feature is that they are either white or some shade of pink or red and are never green. Monotropa uniflora (Indian pipes), for example, is a ghostly white and has no chlorophyll.

Parasitism is an alternative for some angio- sperms. One well-known parasite is the mistletoe (Loranthaceae), popular as a Christmas decoration, which is a branch parasite on trees. Many types of mistletoe have green foliage and therefore receive some of their energy fromphotosynthesis, but their primary nourishment comes from the host tree.

Some species have foliage that is brown or yellow and do not photosynthesize much at all. The seeds ofmistletoe are spread fromtree to treewhen birds eat their berries and defecate the seeds on the branch of another tree. Probably the most unusual parasite is Rafflesia, from Malaysia and Sumatra.

It parasitizes species of Tetrastigma, a vine that grows on the forest floor and has no stems or leaves of its own. When it blooms it has the largest flowers in the world, and it is often called the corpse flower because it has a very strong odor, like that of rotting flesh.

Other parasites receive varying proportions of their energy and nutrients fromtheir host and conventional means, and when the contributions are nearly equal they are referred to as hemiparasites. Hemiparasites are common in Castilleja (paint- brushes), and many species invade the roots of other plants to obtain part of their nutritional needs.

Aunique approach to obtaining nutrients is rep- resented by insectivorous plants, commonly known as carnivorous plants These plants use a variety of adaptations for trapping and absorbing nutrients from insects.

Sundews (Droseraceae) have special glands on their leaves that excrete a sticky fluid that traps insects like flypaper. Pitcher plants (Nepenthaceae and Sarraceniaceae) have special tubular leaves that resemble cups or pitchers.

The inside of the leaves fill with water near the base, and the lip and inside surface of the pitcher are slippery. Once an insect gets inside, it slips into the water at the bottom. Venus’s flytrap (Dionaea, also in Droseraceae) is evenmore intricate,with leaves spe- cially modified with traps that spring shut when an insect lands or walks on them.

There is even an aquatic carnivore, the bladderwort (Utricularia), which has saclike leaves with small openings that can close after a small aquatic insect or crustacean is sucked in.Although insectivorous plants do obtain some of their nutrients from insects, they also obtain nutrients from the soil or, in the case of bladderworts, surrounding water.

Angiosperm Classification

Traditionally Anthophyta has either been considered as a single class Angiospermae or Magnoliopsida, with two subclasses, or has been divided into two classes, Eudicotyledones, or Magnoliopsida, and Monocotyledones, or Liliopsida. The second of these two options is more commonly accepted by contemporary plant taxonomists, and the two classes are often referred to by the common names dicotyledons or dicots and monocotyledons or monocots, respectively.

The monocot/dicot dichotomy has long been considered a major evolutionary split in the angiosperms. The two classes a differ fromeach other in a number of ways. Monocots generally have bladelike leaves with parallel venation, whereas dicots more typically have pinnate or palmate venation. Monocots have fibrous root systems without taproots; dicots typically have taproots.

The flower parts in monocots occur typically in threes, whereas they occur most often in fours and fives in dicots. Monocots lack cambial secondary growth,which is common in dicots. Monocots have scattered vascular bundles in their stems, as opposed to the more orderly arrangement seen in dicot stems.

It has long been proposed that the monocots branched off fromthe dicots very early in the evolution of the angiosperms, but until recently it was difficult to sort out the probable events and the resulting classification system that would be needed to reflect them.

With the advent of molecular tools, such as deoxyribonucleic acid (DNA) sequencing, the study of early angiosperm evolution is getting much more attention. It has now become clear that, if the classification system is to reflect evolutionary history, Anthophyta must be divided intomore than just two classes.

Currently there is no agreement on how many other classes there should be, but Monocotyledones and Eudicotyledones will retain most of the taxa. This new approach to the classification of Anthophyta has also resulted in changing the common name of the "dicots" to "eudicots", meaning "true dicots".

Many of the remaining taxa not included in the monocots or eudicots are now often referred to as magnoliids and are considered to represent taxonomic groups that have branched off fromthe early angiosperms before the monocot/eudicot split. Some of these groups include the orders Magnoliales (which includes Magnoliaceae, long considered as having many primitive characteristics), Winterales, and Laurales.

The placement of a few taxa, such as Ceratophyllaceae and Chloranthaceae,is particularly controversial. With continued analyses of DNA sequences it is hoped that a clearer picture of the relationships among the magnoliids and related taxa will be obtained and a more phylogenetically based classification system can be devised.

Animal-plant Interactions

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The ways in which certain animals and plants interact have evolved in some cases to make them interdependent for nutrition, respiration, reproduction, or other aspects of survival.

Ecology represents the organized body of knowledge that deals with the relationships between living organisms and their nonliving environments. Increasingly, the realm of ecology involves a systematic analysis of plant-animal interactions through the considerations of nutrient flow in food chains and food webs, exchange of such important gases as oxygen and carbon dioxide between plants and animals, and strategies of mutual survival between plant and animal species through the processes of pollination and seed dispersal.

A major example of animal-plant interactions involve the continual processes of photosynthesis and cellular respiration. Green plants are classified as ecological producers, having the unique ability, by photosynthesis, to take carbon dioxide and incorporate it into organic molecules.

Animals are classified as consumers, taking the products of photosynthesis and chemically breaking them down at the cellular level to produce energy for life activities. Carbon dioxide is a waste product of this process.

Mutualism

Mutualism is an ecological interaction in which two different species of organisms beneficially reside together in close association, usually revolving around nutritional needs.

One such example is a small aquatic flatworm that absorbs microscopic green algae into its tissues. The benefit to the animal is one of added food supply. The mutual adaptation is so complete that the flatworm does not actively feed as an adult.

The algae, in turn, receive adequate supplies of nitrogen and carbon dioxide and are literally transported throughout tidal flats in marine habitats as the flatworm migrates, thus exposing the algae to increased sunlight. This type of mutualism, which verges on parasitism, is called symbiosis.

Coevolution

Coevolution is an evolutionary process wherein two organisms interact so closely that they evolve together in response to shared or antagonistic selection pressure. A classic example of coevolution involves the yucca plant and a species of small, white moth (Tegitecula).

The female moth collects pollen grains from the stamen of one flower on the plant and transports these pollen loads to the pistil of another flower, thereby ensuring cross-pollination and fertilization. During this process, the moth will lay her own fertilized eggs in the flowers’ undeveloped seed pods.

The developing moth larvae have a secure residence for growth and a steady food supply. These larvae will rarely consume all the developing seeds; thus, both species (plant and animal) benefit.

Although this example represents a mutually positive relationship between plants and animals, other interactions are more antagonistic. Predatorprey relationships between plants and animals are common. Insects and larger herbivores consume large amounts of plant material. In response to this selection pressure, many plants have evolved secondary metabolites that make their tissues unpalatable, distasteful, or even poisonous. In response, herbivores have evolved ways to neutralize these plant defenses.

Mimicry and Nonsymbiotic Mutualism

Inmimicry, an animal or plant has evolved structures or behavior patterns that allow it tomimic either its surroundings or another organism as a defensive or offensive strategy.

Certain types of insects, such as the leaf hopper, walking stick, praying mantis, and katydid (a type of grasshopper), often duplicate plant structures in environments ranging from tropical rain forests to northern coniferous forests. Mimicry of their plant hosts affords these insects protection from their own predators as well as camouflage that enables them to capture their own prey readily.

Certain species of ambush bugs and crab spiders have evolved coloration patterns that allow them to hide within flower heads of such common plants as goldenrod, enabling them to ambush the insects that visit these flowers.

In nonsymbiotic mutualism, plants and animals coevolve morphological structures and behavior patterns bywhich they benefit each other but without living physically together.

This type of mutualism can be demonstrated in the often unusual shapes, patterns, and colorations that more advanced flowering plants have developed to attract various insects, birds, andmammals for pollination and seed dispersal purposes. Accessory structures, called fruits, form around seeds and are usually tasty and brightly marked to attract animals for seed dispersal.

Although the fruits themselves become biological bribes for animals to consume, often the seeds within these fruits are not easily digested and thus pass through the animals’ digestive tracts unharmed, sometimes great distances from the parent plant. Some seeds must pass through the digestive plant of an animal to stimulate germination.

Other types of seed dispersal mechanisms involve the evolution of hooks, barbs, and sticky substances on seeds that enable them to be easily transported by animal fur, feet, feathers, or beaks. Such strategies of dispersal reduce competition be- tween the parent plant and its offspring.

Pollinators

Because structural specialization increases the possibility that a flower’s pollenwill be transferred to a plant of the same species, many plants have evolved a vast array of scents, colors, and nutritional products to attract pollinators. Not only does pollen include the plant’s spermcells; it also represents a food reward.

Another source of animal nutrition is a substance called nectar, a sugar-rich fluid produced in specialized structures called nectaries within the flower or on adjacent stems and leaves.

Assorted waxes and oils are also produced by plants to ensure plant-animal interactions. As species of bees, flies, wasps, butterflies, and hawk-moths are attracted to flower heads for these nutritional rewards, they unwittingly become agents of pollination by transferring pollen from stamens to pistils.

Some flowers have evolved distinctive, unpleasant odors reminiscent of rotting flesh or feces, thereby attracting carrion beetles and flesh flies in search of places to reproduce and deposit their own fertilized eggs.

As these animals copulate, they often become agents of pollination for the plant itself. Some tropical plants, such as orchids, even mimic a female bee, wasp, or beetle, so that the insect’s male counterpart will attempt to mate with them, thereby encouraging precise pollination.

Among birds, hummingbirds are the best examples of plant pollinators. Various types of flowers with bright, red colors, tubular shapes, and strong, sweet odors have evolved in tropical and temperate regions to take advantage of hummingbirds’ long beaks and tongues as an aid to pollination.

Because most mammals, such as small rodents and bats, do not detect colors as well as bees and butterflies do, some flowers instead focus upon the production of strong, fermenting, or fruitlike odors and abundant pollen rich in protein. In certain environments, bats and mice that are primarily nocturnal have replaced day-flying insects and birds as pollinators.

Antarctica Flora

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The harsh climate of Antarctica makes it one of the most inhospitable places on the earth, allowing only a relatively small number of organisms to live there. Permanent terrestrial (land) animals and plants are few and small. There are no trees, shrubs, or vertebrate land animals. Native organisms are hardy, yet the ecosystem is fragile and easily disturbed by human activity, pollution, global warming, and ozone layer depletion.

The Antarctic continent has never had a native or permanent population of humans. In 1998 the United States, Russia, Belgium, Australia, and several other countries signed one of an ongoing series of treaties to preserve Antarctica. The continent is used for peaceful international endeavors such as scientific research and ecotourism.

Terrestrial Flora

There are only two types of flowering plants in Antarctica, a grass and a small pearlwort (Deschampsia antarctica). These are restricted to the more temperate Antarctic Peninsula. Antarctic hairgrass (Colobanthus quitensis) forms dense mats and grows fairly rapidly in the austral summer (December, January, and February). At the end of summer, the hairgrass’s nutrients move underground, and the leaves die. Pearlwort forms cushion-shaped clusters and grows only 0.08 to 0.25 inch (2 to 6 millimeters) per year.

Numerous species of primitive plants, such as lichens, mosses, fungi, algae, and diatoms, live in Antarctica. Lichens are made up of an alga and a fungus in a symbiotic (interdependent) relationship. They can use water in the form of vapor, liq uid, snow, or ice.

Lichens grow as little as 0.04 inch (1 millimeter) every one hundred years, and some patches may be more than five thousand years old. Mosses are not as hardy as lichens and also grow slowly; a boot print in a moss carpet may be visible for years. Fungi are found in the more temperate peninsula, and most are microscopic.

Algae grow in Antarctic lakes, runoff near bird colonies, moist soil, and snow fields. During the summer, algae form spectacular red, yellow, or green patches on the snow. Bacteria are found in lakes, melt water, and soils. As elsewhere on the earth, bacteria play a role in decomposition. Because of the extreme conditions, they are not always as efficient in Antarctica as they are in warmer climates, and carcasses may lie preserved for hundreds of years.

Aquatic Plants

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Aquatic plants are any "true" plants, members of the kingdom Plantae, that are able to thrive and complete their life cycle while in water, on the surface of water, or on hydric soils.

Hydric soils developwhen the ground is flooded or ponded long enough during the growing season to become anaerobic (depleted of oxygen) in the rooting zone. These soils include organic (peats and mucks) and inorganic (mineral) sediments.

Aquatic plants grow in fresh, brackish, and salt water but are most common in fresh water. Their habitats include flowing waters (rivers, streams, brooks), standing waters (lakes, ponds), and wetlands (bogs, fens, marshes, swamps), which are categorized as riverine, lacustrine, and palustrine communities, respectively.

Wetland plants are sometimes referred to as helophytes. Marshes are dominated (that is, more than half covered) by herbaceous species and swamps by woody species. Bog plants are aquatics that grow in acidic organic soils. Fen plants occur in alkaline organic soils.

Aquatic plants (also known as hydrophytes, macrophytes, and water plants) occur throughout the plant kingdom. The term "macrophyte" distinguishes them from microscopic aquatic algae, which are not true plants. Aquatic plants have evolved repeatedly, having more than 250 independent origins by some estimates.

They occur occasionally in spore-producing plants such as ferns, liverworts, lycopods, and mosses but are relatively rare among nonflowering seed plants (gymnosperms), with bald cypress (Taxodium) a notable exception.

Flowering plants (angiosperms) contain the greatest hydrophyte diversity, with more species proportionally in monocotyledons than in dicotyledons. Nevertheless, fewer than 2 percent of flowering plant species are aquatic.

Life-Forms

Regardless of their taxonomic affinities, aquatic plants are often classified ecologically by their lifeforms. Categories include the following:

Floating (acropleustophytes), with stems and leaves floating completely on the water surface and stems not rooted in the bottom, such as duckweed (Lemna) and water hyacinths (Eichhornia).
Emergent (hyperhydrates), with stems and leaves extending mainly above the water surface and stems rooted in the bottom, such as cattails (Typha) and reeds.
Phragmites (planmergents or ephydrates), floating-leaved, with some or all leaves floating on the water surface and stems rooted in the bottom, such as floating-leaved pondweed (Potamogeton natans), water chestnut (Trapa natans), and water lily (Nymphaea).
Submersed (hyphydrates), with stems and leaves completely under water and stems rooted in the bottom, such as Eurasian water milfoil (Myriophyllum spicatum) and wild celery (Vallisneria).
Suspended (mesopleustophytes), with stems and leaves completely under water and stems not rooted in the bottom, such as the bladderwort (Utricularia vulgaris) and the coontail (Ceratophyllum).

Benthophyte and pleustophyte are used respectively to differentiate between forms that are either rooted in the substrate or unrooted. Species with elongate, leafy stems are termed vittate or caulescent (such as coontail).

Those with leaves clustered in a basal rosette are rosulate (such as wild celery), and those not clearly differentiated into stems and leaves are thalloid (such as duckweed). Species whose floating or emergent leaves differ morphologically from their submersed leaves are heterophyllous (such as floating-leaved pondweed).

Adaptations

Water plants are anatomically and structurally reduced. Watermeal (Wolffia), the world’s smallest angiosperm, contains plants only 0.4 millimeter long. Submersed species often lack water conducting tissue (xylem), mechanical tissue (sclerenchyma), and cuticle.

Some lack roots entirely. Support and floatation of underwater stems are accommodated by buoyant tissue (aerenchyma) and extensive air spaces (lacunae) which also transport oxygen throughout the plant.

Submersed plants usually possess either highly dissected (compound) or thin, ribbonlike leaves. Some leaves become fenestrate, that is, lacking tissue between the veins. Such leaf shapes increase surface area-to-volume ratios for more efficient nutrient uptake and to reduce damage from water currents.

Floating leaves are normally flat and circular, with stomata on their upper surfaces. They may reach 2.5 meters in diameter (such as Victoria). For stability, the stalks (petioles) of most floating leaves are positioned centrally by emargination of the base, as in the water lily, or are peltate by complete fusion of leaf lobes, as in the water shield (Brasenia) and Victoria. Physiological adaptations enable aquatic plants to tolerate deleterious effects of anaerobic hydric soils.

Reproduction

Most aquatic plants are perennials that reproduce vegetatively (asexually). Species survive winters or other unfavorable periods as intact plants, by dying back to dormant stem apices, by means of modified stems (rhizomes, stolons, tubers), or by use of specialized dormant structures (hibernacula) in the sediment.

"Winter buds" are a kind of hibernaculum; buds are insulated by normal foliage leaves on shortened internodes. They usually remain attached to the plant. Turions are specialized hibernacula that produce modified, morphologically distinct leaves to protect the enclosed buds.

Turions always detach from the plant and function as propagules for dispersal. Water plants also disperse vegetatively by fragmentation of stems, which are characteristically brittle, due to the lack of mechanical tissue. Detached stems can establish themselves by production of adventitious roots.

The few aquatic plants that are annuals produce seeds as their dormant stage. Some aquatic annuals also multiply vegetatively by fragmentation during the growing season. Generally, sexual reproduction is rare in submersed species, more common in floating-leaved species, and quite common in emergent species (and annuals).

Pollination in water plants is facilitated by insects (entomophily), wind (anemophily), and water (hydrophily). Most aquatics are insect-pollinated; about one-third of them are wind-pollinated.

Less than 5 percent of aquatic species are hydrophilous, with pollen transported on the water surface (ephydrophily) or under the water surface (hyphydrophily). Most marine angiosperms (seagrasses) are hydrophilous.

Seeds, fruits, and vegetative propagules are dispersed locally by water currents and more widely by waterfowl. Waterfowl transport propagules in plumage, in mud adhering to their feet, and by excretion of seeds consumed as food. Many water plants are distributed broadly, with some species achieving worldwide distributions.

Uses

Aquatic plants are important economically. Foods include rice (Oryza sativa), which sustains more human life than any other plant on earth. Aquatic plants are important horticulturally as aquarium and water-garden ornamentals.

Some aquatic plants, such as the water hyacinth, are invasive weeds that interfere with shipping, irrigation, or recreation and cost millions of dollars to eradicate. The beauty of many water plants, especially water lilies, has inspired art and religion since ancient times.

Archaea

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The domain Archaea represents a diverse group of prokaryotes originally found in environments once considered to be hostile to life, now known to be widely distributed in nature.

The cycling of plant nutrients, such as carbon, nitrogen, and sulfur, requires the activity of microorganisms that convert these elements to forms readily available to plants. These microorganisms, which are generally found in both soil and water, include both prokaryotic organisms of the domain Bacteria and the domain of prokaryotes called Archaea, which play significant roles in nutrient cycling.

Along with Eukarya, to which protists, fungi, plants, and animals belong, the Archaea formone of the three domains of life. The Archaea are related to both Bacteria and Eukarya and, in some respects, appear to bemore closely related to Eukarya.

Biochemical and genetic studies, including information obtained from whole genome sequencing, suggest that Archaea may be closely related to an ancestor that gave rise to both Bacteria and Eukarya. Thus, Archaea may provide some insight into the processes that resulted in the evolution of higher life-forms, including plants and animals.

A Third Domain

For more than fifty years, biologists categorized living organisms into two groups based on their cellular organization and complexity: prokaryotes (originally all classified in kingdom Monera), the single-celled organisms whose chromosomes are not compartmentalized inside a nucleus (which include the domainBacteria), and eukaryotes, consisting of all other organisms, whose cells contain a nucleus. In the late 1970’s studies on a unique group of microorganisms led investigators to question the accepted classification of prokaryotes.

Originally called Archaebacteria by molecular biologist Carl Woese and his colleagues in 1977, these microorganisms were isolated from environments characterized by extremes in heat, acidity, pressure, or salinity, and many were found to be able to utilize sulfur and molecular hydrogen as part of their growth process.

Like all prokaryotes, Archaea do not have a nucleus. However, in their biochemistry and the structure and composition of their molecular machinery, they are as different from bacteria as they are from eukaryotes.

Woese and his colleagues analyzed and compared specific molecules of ribonucleic acid (RNA) present within the ribosome in all organisms, called ribosomal RNA (rRNA). Their findings suggested that all extant life is composed of three distinct groups of organisms: the eukaryotes, or domain Eukarya, which includes plants and animals, and two different prokaryotes, domains Bacteria and Archaea.

In 1990 Woese and others recommended the replacement of the simple prokaryote/ eukaryote view of life with a new tripartite scheme based on three domains: the Bacteria, Archaea, and Eukarya. Since 1990 the three-domain classification has been the subject of considerable debate, and as a consequence, both old and new terminology are used in scientific and popular literature.

Characteristics

Generally, the size and shape of Archaea are similar to those of Bacteria. They are single-celled microscopic organisms that, in some cases, are motile (capable of self-movement) and may be found in chains or clusters.

Archaea multiply in the same manner as bacteria: via binary fission, budding, or fragmentation. Like Bacteria, archaeal chromosomes are circular, indicating the absence of breaks or discontinuities, and many genes are organized in the same fashion as those found in Bacteria.

On the other hand, the specific chemical composition of Archaea plasma membranes and cell walls is unique to the Archaea and is quite different from the composition of these structures typically found in either Bacteria or Eukarya. In fact, the distinctive ether-linked isoprenoid lipids that compose the external membranes of Archaea are a hallmark of these microorganisms.

Another unique characteristic of Archaea is the composition of the molecular genetic machinery, which is amosaic of the components found in Bacteria and Eukarya. For example, the ribosomes (which are responsible for protein synthesis) of Archaea resemble the ribosomes of Bacteria in shape and composition and are distinct from the ribosomes of Eukarya.

On the other hand, the enzyme utilized by Archaea in the production of RNA, namely RNA polymerase, is quite different from the enzyme found in Bacteria. In Bacteria, RNA polymerase molecules are composed of four major proteins, while in the Archaea, RNA polymerase molecules consist of more than ten proteins and are surprisingly similar to the enzyme found in Eukarya.

In fact, archaeal RNA polymerase is so similar to the eukaryotic enzyme that combining certain proteins from both archaeal and eukaryotic sources results in a functional enzyme, a manipulation that is not possible with any bacterial RNA polymerases.

Among species of the Archaea, there is a variety of metabolic processes that differ greatly from the better-known metabolic routes of Bacteria and Eukarya. Many of the archaeal pathways used to convert food sources to energy and building blocks for growth involve enzymes having biological activities not found in any other biological systems.

In some cases, the enzymes require the involvement of rare metals, such as tungsten. While a requirement for metals in the activity of many bacterial and eukaryotic enzymes is ubiquitous, the use of tungsten appears to be unique to Archaea.

Diversity

A fascinating feature of Archaea is that they are found in niches that support the growth of few other organisms. These include highly reduced (oxygen-free) environments or very high-temperature environments found near hot springs or undersea hydrothermal vents as well as sites that are sulfur-rich and highly acidic.

Archaea are also found in highly saline marine environments and hypersaline lakes where the salinity is as much as ten times that in seawater. Based on the comparison of ribosomal RNA sequences as well as physiological and metabolic characteristics, the Archaea have been divided into three subdomains: Euryarchaeota, Crenarchaeota, and Korarchaeota.

The Euryarchaeota includes members of the methanogenic (methane-producing) and halophilic (salt-requiring) Archaea as well as many that grow at very high temperature, the thermophilic and extremely thermophilic, or hyperthermophilic, Archaea.

Representatives of hyperthermophilic Archaea are found in the Crenarchaeota, which also includes cold-dwelling Archaea that have been isolated in association with certain marine sponges. The Korarchaeota also includes hyperthermophilic Archaea, although these were not isolated or characterized as of 2001, but whose presence in hot spring and deep-sea samples has been identified by molecular biological techniques.

Methanogenic Archaea

Methane-producing Archaea are found in strictly anaerobic environments. They have no tolerance for oxygen: Trace amounts are inhibitory for growth, and too much is lethal. These Archaea obtain energy for growth by a process called methanogenesis, which results in the conversion of carbon dioxide to methane gas.

Methane production requires several enzymes that use coenzymes unique to methanogenic Archaea. The production of methane is of great importance to carbon cycling inmany anaerobic environments, and microorganisms that produce this gas have been known for centuries.

In 1776 the scientist Alessandro Volta demonstrated that air generated from sediments rich in decaying vegetation, such as those present in bogs, streams, and lakes, could be ignited. It is now known that methanogenic Archaea are responsible for generating this "marsh gas".

Because methanogens require an oxygen-free environment for growth, they are found onlywhere carbon dioxide and hydrogen are available and oxygen has been excluded. Thus, methanogens thrive in stagnant water, natural wetlands, paddy fields, and in the rumen of cattle and other ruminants as well as in the intestinal tracts of animals and the hindguts of cellulose-digesting insects, such as termites.

Methanogens are also found in hot springs and the deep ocean and are major components of the anaerobic process in waste treatment facilities. It has been estimated that production of methane by themethanogenicArchaeamay account for almost 90 percent of the total methane released into the atmosphere each year.

In addition to playing a role in carbon cycling, several methanogenic Archaea are also involved in nitrogen cycling, as they are able to convert molecular nitrogen into organic nitrogen via nitrogen fixation, a process that is shared by only a few prokaryotes.

Thermophilic Archaea

Thermophilic Archaea live in environments ranging in temperature from55 degrees Celsius (131 degrees Fahrenheit) to 80 degreesCelsius (176 degrees Fahrenheit). Hyperthermophilic Archaea grow at temperatures near or greater than the boiling point ofwater and as high as 113 degrees Celsius (235 degrees Fahrenheit).

These Archaea have been isolated from hot sulfur springs, sulfur-laden mud at the base of volcanoes, and near very hot deep-sea hydrothermal vents where super heated water is emitted at very high temperatures under pressure.

Species that can use oxygen, as well as those that have no tolerance for oxygen, are known. Many of the anaerobic representatives obtain energy for growth by the metabolism of elemental sulfur.

In addition, many are found in environments that are extremely acidic, including those that are members of Thermoplasmatales. This group is noted for its ability to growat a pHof 2.0 and below(on a scale where pH 7.0 is neutral), which is equivalent to the acid in car batteries.

Arepresentative is Thermoplasma, which does not possess a cell wall but has a chemically unique structure composed of a lipid-polysaccharide (tetraether lipid with mannose and glucose units) that is distinctly different from the unusual ether-linked lipids found in the membrane components of typical Archaea.

Halophilic Archaea

The salt-dependent halophilic Archaea require extremely high concentrations of salt for survival, and some grow readily in saturated brine, where the salt concentration reaches 32 percent (in seawater it is approximately 3.5 percent) and where very alkaline conditions are not uncommon. Halophilic Archaea are found in salty habitats along ocean borders and inland waters such as the Dead Sea and the Great Salt Lake.

The reddish-purple color observed in salt evaporation ponds is due to production of red- and orange-colored carotenoids and other pigments associated with the massive growth of halophilic Archaea.

Some halophilic Archaea are capable of harvesting light to provide energy for growth by a mechanism that does not involve chlorophyll pigments. Light harvesting by these halophilicArchaea is done by a membrane-bound protein called bacteriorhodopsin that is equivalent to the mammalian eye pigment rhodopsin in both function and structure.

Bacteriorhodopsin contains retinal, a purple carotenoid like molecule used for light trapping. Interestingly, retinal is produced via a pathway that contains many of the same enzymes used for the production of lycopene by tomatoes during ripening.

Window to the Past

The extreme conditions in which Archaea are found suggests that these organisms have adapted to environments thought to exist during early life on earth, three billion to four billion years ago. Thus, the Archaea might be considered as a window into the past, and they may shed light on the processes involved in evolution as well as their relationships with Bacteria and Eukarya.

In order to survive in their unique environments, Archaea possess molecules that withstand heat or cold, acids, salt, and in some cases, pressure—characteristics that are tailor-made for specific applications inmolecular biology and biotechnology.

Uses

A number of important applications have been developed as a consequence of studying the Archaea. These include the identification of heat stable enzymes for analyses used in genetic fingerprinting and cancer detection (certain polymerase chain reaction enzymes), the use of halophilic pigments for holographic applications, optical signal processing and photoelectric devices, and methanogenesis as an alternative fuel source.

Arctic Tundra

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The Arctic tundra is a biome representing the northernmost limit of plant growth on earth. Arctic tundra has a circumpolar distribution in the Northern Hemisphere, extending from the ice cap southward to the forested taiga of North America, Europe, and Asia. Tundra is also found on islands within the Arctic Ocean and along coastal Greenland.

The term "tundra" was derived from the Finnish word for a treeless or barren landscape. The Arctic tundra biome is located within one of the harshest climates on earth for plant growth, with winter temperatures averaging –34 degrees Celsius (–30 degrees Fahrenheit).

The climate is comparatively dry, with annual precipitation of 150 to 250 millimeters (6 to 10 inches). Locked in snow or frozen within soil, the majority of moisture is not available for plant use.

In addition to surviving extreme temperatures and dry conditions, plants must adapt to seasonal variation in available sunlight; winter nights, for example, last twenty-four hours. The tundra’s growing season is very short, extending over only about sixty days.

Continuous sunlight during warmer months, July and August, contributes to the productivity of tundra plant communities that can yield 227-454 kilograms (500-1,000 pounds) of vegetation per acre. This biomass serves as an important food source for caribou, musk ox, and migratory waterfowl.

Tundra vegetation is made up of herbaceous plants (grasses, forbs, and sedges), mosses, lichens, and shrubs that grow close to the ground, where temperatures are highest. By providing an insulating layer, snowfall is advantageous for tundra plants during cold winter months.

Herbaceous Plants

Rushlike tundra sedges belong to the flowering plant family Cyperacaeae. Common to the tundra, cottongrass is really a sedge within the genus Eriophorum. Perennial forbs are broadleaf plants that survive winter months as bulbs that are protected below the ground level.

During warm months the plants begin to grow rapidly and will develop flowers and seeds when temperatures climb above 10 degrees Celsius (50 degrees Fahrenheit).

Lichens and Low-Growing Shrubs

Acting as a single organism, pioneering lichens growing on rock surfaces represent a symbiotic relationship between fungi and algae. The fungi anchor to the rock, absorbing water directly into their cells, while the algae occupy this moist area, creating food through photosynthesis that is shared with the fungi.

Tundra lichens are found in fruiticose (stalklike), crustose (crustlike), or foliose (leaflike) forms. The heath (Ericaceae) family includes several species of shrub, many of which have tough, evergreen leaves.

Examples include rhododendron, cranberry, blueberry, and Labrador tea. Another heath, the alpine azalea (Loiseleuria procumbens), forms a mat or cushion where several plants clump tightly together.

Adaptations

In many ways tundra vegetation must adapt to many of the same environmental conditions as grasslands or deserts, such as little precipitation, strong winds, and extreme temperature variations. As a result of the brief growing season, plant reproduction in the tundra must take place rapidly.

Other adaptations include compact plant size that protects from cold temperatures, hairy stems that help retain heat, and dark-colored leaves that absorb sunlight.

Some plants have hollow stems that require fewer nutrients to grow. A unique adaptation made by the Arctic poppy (Papaver radicatum) and mountain aven (Dryas integrifolia) allows them to orient their flowers to track the sun’s movement across the sky, maximizing solar radiation received.

Although sunlight is usually beneficial to plant growth, some plants such as Arctic algae must implement protective measures to avoid damage from ultraviolet radiation. The green alga Ulva rigida, also called sea lettuce, produces amino acids and carotenoid pigments that absorb harmful radiation.

Cushion plants grow in tight but low-profile clumps, forming windbreaks that protect them from the cold, and may trap airborne dust and soil used as a source of nutrients.

Many tundra plants are capable of carrying out photosynthesis under relatively low light intensities. With short growing seasons, some plants reproduce by budding and division instead of by the creation of seeds. Plants may also store nutrients in rhizomes, underground stems that survive after root systems die.

Edaphic Influences

Soils of the tundra are principally thin soils (inceptisols). Contributing to the lack of soil development are cold temperatures that inhibit the growth of soil-producing organisms such as bacteria. The tundra’s treeless plain may be interrupted by patterned ground made up of stone polygons, soil circles, or soil stripes.

These unusual features are formed by the thrusting action of repeated freezing and thawing in soil that overlies rock or permanently frozen ground called permafrost. Impenetrable permafrost that inhibits root system development, rather than cold temperatures, is thought to be responsible for the lack of tree growth in the tundra.

Warmer summer temperatures lead to a thaw in permafrost that extends only about a meter below the surface. Ponds and boggy areas form in places where soil above the permafrost melts and cannot move downward, creating a source of moisture for plants.

Environmental Concerns

As a result of growing under harsh conditions, tundra plants are slow to recover from disturbances. Vehicles can destroy tundra plants. Other concerns include oil spillage, damage caused by pipeline construction, and other impacts tied to petroleum production.

Ascomycetes

2011-12-09T20:51:00.000-08:00

The ascomycetes are fungi (phylum Ascomycota or Ascomycotina) that produce sexual spores in a specialized cell called an ascus. These diverse fungi, with more than thirty thousand species, can be found in almost every ecosystem worldwide. One of the most famous members of the ascomycetes is the truffle.

Ascomycetes, one of the four phyla of the fungus kingdom, by definition possess an ascus, a single cell inside of which sexual spores are produced.

The reproductive process has been well documented and occurs when the dikaryotic mycelium (the mass of hyphae forming the body) undergoes changes that precede the formation of the ascus. Dikaryotic is the genetic state in which two haploid nuclei are present in the cell. One nucleus is donated by each parent.

The first change occurs when the end cell of a hyphal strand begins to form a small bend. The cell divides into three cells; the outer two cells are haploid, and the middle cell is dikaryotic. The middle cell then elongates, and the nuclei migrate into its center.

The two haploid nuclei then fuse to form a single diploid nucleus, which undergoes mitosis and meiosis to form eight haploid nuclei. Cell walls form around the nuclei producing eight haploid ascospores. The ascospores are then liberated from the ascus.

The ascus wall determines the kind of dispersal of the spores. Some asci have a thin, single-layer wall, which breaks down to liberate spores. Unitunicate asci have a multilayer cell wall with a pore at the end of the ascus. Spore release is active through the pore.

Bitunicate asci have multilayer cell walls, and release of spores is by the separation of the layers of the cell wall, with the inner layer inflating to several times its normal size and then lifting off of the ascus, allowing the spores to be released.

The spores are released into the environment, where they germinate and produce haploid hyphae. The haploid hyphae fuse with compatible haploid hyphae, forming dikaryotic hyphae, and the process begins to repeat itself.

There are five different ways in which asci are formed in nature. First, asci can be produced by exposure to the environment, as are the asci of yeasts or the ascus of the peach leaf-curl pathogen Taphrina deformans. With these fungi, the ascospores are released by the breakdown of the ascus wall.

The other four ways of production of asci all take place inside structures made from mycelium, called ascocarps. These structures range from totally closed to open, like a cup. The totally closed ascocarps are called cleistothecia. Within these, the asci are scattered, and the spores are released by breakdown and decomposition of the fungal tissue.

Ascomycetes as Pathogens

Some members of the ascomycetes are very important plant and animal pathogens, causing serious diseases. One of the more impressive plant pathogens is the ergot fungus (Claviceps purpurea). This fungus colonizes the ovaries of grains, such as rye (Secale cereale).

It produces a mass of mycelium, called a sclerotium, which is hard and has a density similar to that of a seed. Because of this, the sclerotia are often found in the threshed grain. Sclerotia contain an accumulation of alkaloids and other secondary metabolites.

When the sclerotia are ground into flower and baked into bread, many of these secondary metabolites are passed into the bread. During the Middle Ages this fungus was responsible for a human disease called St. Anthony’s Fire. Today, this fungus is used for the natural production of a coagulant which is used in medicine.

Another group of plant pathogens are the powdery mildews. There are several hundred species of these fungi, which produce a powdery spore mass on the outer surfaces of plant leaves. If a leaf is infected before it has expanded, it will remain small and puckered andmay drop from the plant.

The powdery mildews are superficial and send hyphae through the leaf cuticle into the epidermis. The fungus then grows over the surface of leaf, giving it a powdery appearance. During the winter, the fungus produces cleistothecia on the surface of the leaf. Powdery mildew can occur on most plant species and can be very damaging to crop and ornamental plants.

Economic Uses

Truffle is a generic term for fungi that form mycorrhizae (a symbiotic association) with the roots of various trees. The fungus grows into the roots and helps the plant tolerate stress, providing the plant with increased absorption of phosphorus from the soil.

In return, the plant gives the fungus metabolites that it needs for growth. These fungi then produce fruiting bodies, either in the soil or upon the surface of the soil. The truffles are produced in the soil up to a depth of 1 foot.

Truffles can be located in the soil using a trained sowor dog that is able to sniff out the volatile chemicals that are produced. However, it is important to note that edible fungi such as truffles can often be mistaken for highly toxic fungi and should never be gathered or eaten without expert identification.

Truffles are used as condiments and are able to impart unique aromas and sensations to food. They are shaped like small balls, varying in size from that of a pea to that of a golf ball. Truffles are only produced in nature and therefore fetch high prices.

The price of truffles depends on the species, size, and freshness. Prices of the famous French Black Périgord truffle (Tuber melanosporum) can reach into thousands of dollars per kilogram (2.2 pounds). In the United States, the Oregon white truffle is quite appealing and can be found in the Pacific North-west.

Morels (Morchella) are another choice edible ascomycete. Shaped like a little hat sitting upon a stalk, they are brown in color and have an appealing aroma. They add flavor to any food and are a favorite of many wild animals.

Scientific Uses

Some members of the ascomycetes are used for genetic studies. Such is the case for Neurospora, a common fungus found growing on soil and organic matter and one of the first organisms to be found in an area after a fire. A swith all ascomycetes, there are two compatible mating types, which makes for easy genetic study.

Using spore characteristics of shape, color, and texture, it is possible to see how mitosis and meiosis occur in the ascus by determining the placement of the spores. The fungus is readily mutated, which further enhances genetic study.

Asian Agriculture

2011-12-09T09:18:00.000-08:00

Land constraints and growing population and urbanization throughout Asia underscore the need for environmentally sound technologies to sustain agricultural growth.

The first agricultural revolution occurred in Asia and involved the domestication of plants and animals. It is believed that vegeculture first developed in Southeast Asia more than eleven thousand years ago. In vegeculture, a part of a plant—other than the seed—is planted for reproduction.

The first plants domesticated in Southeast Asia were taro, yam, banana, and palm. Seed agriculture, now the most common type of agriculture, uses seeds for plant reproduction. It originated in the Middle East about nine thousand years ago, in the basins of the two major rivers of present-day Iraq, the Tigris and the Euphrates.

Wheat and barley were probably the first crops cultivated there. Although many plants were domesticated simultaneously in different parts of the world, rice, oats, millet, sugarcane, cabbage, beans, eggplant, and onions were domesticated originally in Asia.

Asia supports about 60 percent of the global population on only about 23 percent of the world’s agricultural land. As a result, Asian agriculture is far more intensive than on any other continent. Despite the population pressure on arable land, Asia has made remarkable progress in agricultural productivity.

Between 1966 and 1995, wheat production grew 5.5 percent annually, and rice production 2.2 percent. In Asia as a whole, food production has out-paced the growth of population. In most Asian countries, particularly in the low-income countries of South Asia, per-capita food availability has risen.

Agrarian Structure

Most people in Asia are farmers, owning an average of about 2.5 acres (1 hectare) of land per family. Topographic and climatic conditions, to a large extent, determine farm size. Agricultural potential is limited in Nepal, for example, because of the Himalaya Mountains, and in Saudi Arabia because of the Arabian Desert. In these countries, average farm size is larger relative to countries like Bangladesh, which contains a vast, fertile floodplain and receives abundant rainfall.

Another feature of Asian agrarian structure is the inequitable distribution of farmland. For example, in India more than 25 percent of cultivated land is owned by less than 5 percent of farming families.

Farm holdings in most Asian countries are highly fragmented, and tenancy is widespread. Fragmentation of farms inhibits agricultural mechanization, and land consolidation efforts have had limited success in most Asian countries.

Most Asian farmers are subsistence farmers, cultivating crops for family consumption. Almost all farm operations are done manually or with the help of draft animals. Exceptions are found in Japan, South Korea, and Taiwan, where small-scale equipment similar to garden tractors is widely used.

Only recently have Asian farmers started to use chemical fertilizer; for water, they largely depend on rain. As a result, yields are low, which compels farmers to cultivate the land intensively. Double-cropping is the norm; some farmers grow three crops a year.

Therefore, only a small fraction of the arable land in humid regions of Asia remains fallow. Farming is labor-intensive, and the extended family is the main source of labor. This helps to explain why family size is generally large in agrarian countries of Asia.

Rice and Wheat

The coastal areas and inland river valleys of East, Southeast, and South Asia are the agricultural cores of the continent. More than half of the crop area of these regions is used to cultivate food crops such as rice and wheat. Rice is the principal food crop of all Asian countries located east and north of India and for the people of southern and eastern India.

Rice is the staple food of more than half of the world’s population, and 90 percent of it is grown in coastal and deltaic plains and in the river valleys of Monsoon Asia. This region encompasses a broad geographic area characterized by a distinctive climate, stretching from Japan in the east, through Indonesia in the south, and west to Pakistan. Rice farming there is practiced mostly at the subsistence level, using traditional methods.

Wheat is the primary food crop of northern and western India and all Asian countries located west of India. People of the wheat-producing region consume rice as a secondary staple. Eighteen of the twenty-five top rice-producing countries of the world are in Asia. India has the largest area devoted to rice cultivation of the world’s countries, but China is first in total production. Indonesia and Bangladesh rank third and fourth in world rice production.

With the exceptions of Japan, South Korea, China, and Taiwan, yields of all crops—particularly rice and wheat—are low in Asia compared to world standards. Although crop yields increased significantly after 1970, typical yields in Asia remained low for several reasons: Fertilizer use and the area under irrigation are among the lowest in the world.

Also, most Asian farmers practice traditional farming methods, where high yields are a typical. Another major obstacle to increasing crop yields is the preponderance of small farms. Because small farms do not have access to assured irrigation and cannot afford modern agricultural inputs, their average yield is generally much lower than that of medium and large farms.

Slash-and-Burn Agriculture

In the tropical rain forests of Southeast Asia, the mountainous and hilly parts of South Asia, and in southern China, a type of primitive agriculture known as shifting cultivation or slash-and-burn agriculture is practiced. Shifting cultivators plant different crops, such as rice, corn, millet, yams, sugarcane, oilseeds, potatoes, taro, vegetables, and cotton, on one site.

These farmers must abandon their fields and establish new ones every few years. As a result, a large area of land is required to support a small population. The land devoted to shifting cultivation is declining at a rapid rate worldwide because of the demand for forest resources for other uses.

Dry Agriculture

Farmers in the colder, drier parts ofAsia (northeastern China, northern Japan, southeastern East Asia, northeastern Southeast Asia, and the western half of South Asia) and the river valleys of the Middle East practice a system of intensive subsistence agriculture called peasant grain-and-livestock farming, or dry agriculture. The dominant grain crops are wheat, barley, sorghum, millet, oats, and corn, while cotton, tobacco, and sugarcane are grown as cash crops.

In arid areas, such as the Middle Eastern river valleys, irrigation helps support dry farming. Traditional water-lifting devices, such as the shaduf (a counterweighted, lever-mounted bucket), and the naria (waterwheel), permit limited double-cropping in the dry season near the rivers of the Middle East.

In the arid and semiarid parts of South Asia and the Middle East, and in the dry and cold western two-thirds of East Asia, nomadic herders graze cattle, sheep, goats, and camels. Nomadic herders move from place to place with their livestock in search of forage. As in other places, nomadic herding is declining in Asia.

Mediterranean Agriculture

A distinctive type of subsistence agriculture, called Mediterranean agriculture, is practiced along the Mediterranean coast of the Middle East and in the northern part of Turkey that borders the Black Sea. Traditional Mediterranean agriculture is based on wheat and barley cultivation in the rainy winter season. Farmers of this region also cultivate vine and tree crops, such as grapes, olives, and figs, and raise small livestock.

Export Crops

Plantation crops such as tea, rubber, coconuts, and coffee are grown in Asia. Tea is indigenous to China, which is the world’s largest producer, followed by India, Sri Lanka, and Bangladesh. Tea is Sri Lanka’s largest export crop, accounting for about one-third of annual exports by value.

Tea is grown in the central highlands of Sri Lanka and in the hilly regions of northeastern India and Bangladesh. Rubber is grown in Malaysia and Indonesia—which account for about 75 percent of total world production—and Cambodia, India, and Sri Lanka. Malaysia, Indonesia, and Sri Lanka are world-leading exporters of coconuts and coconut products.

Green Revolution

A dramatic growth in food production in Asia began with the Green Revolution in the late 1960’s, particularly for wheat and rice. Cultivation of the new varieties of rice and wheat caused an impressive increase in the use of fertilizer and the expansion of irrigation, particularly the exploitation of groundwater through tube wells. With proper and timely application of fertilizers and water, yields of wheat can be tripled, and yields of rice can be doubled.

Critics of the Green Revolution have concentrated on the negative impacts of increased use of fertilizer and pesticides, which causes surface water pollution.With high-yield seeds, three crops a year can be cultivated.

Adopting this practice has two consequences: It causes overuse of land, a major source of soil degradation, and it leads to increasing monocultures of rice and wheat, reducing the genetic diversity of food crops.

Without the Green Revolution, feeding current Asian populations at prevailing nutritional standards would have been impossible. New agricultural practices enabled Asia to avoid the famine that was widely predicted in the 1970’s. The new rice and wheat varieties also have stimulated agricultural employment, because more people are needed to cultivate, harvest, and handle the increased production.

Throughout Asia, agricultural growth and the increase in food production were somewhat slower in the 1990’s than in the 1980’s. The opportunity for bringing more land under cultivation has largely been exhausted. Therefore, any increase in crop outputwill have to come largely from an increase in yields.

Rice and wheat yields are still relatively low in many Asian countries, primarily because of low use of modern agricultural inputs. For example, the use of chemical fertilizer in South Asian countries has not reached the levels of neighboring regions.

Demand for fruits, vegetables, meat, fish, milk, and eggs is likely to grow with the increased urbanization and industrialization of Asia. This will reduce the demand for cereal crops. In Japan, South Korea, and Taiwan, consumption of rice has already begun to decline. Increased crop production is required to feed the growing populations of most Asian countries.

While increasing agricultural production, Asian policy makers must also promote environmentally sound technologies and implement effective land reforms to address the problems of inequality and poverty caused by landlessness. Better crop management and better management of irrigation water are also needed to sustain agricultural growth in Asian countries.

Forestry

Forests of significant economic importance are found primarily in northeastern East Asia and Southeast Asia. The softwood forests of northeastern East Asia cover most of Japan and parts of North Korea. Trees grown there are used for construction lumber and to produce pulp for paper. Tropical hardwood forests cover all Southeast Asian countries and the south central part of China, several places in India, and the northern part of Iran.

Trees grown in those forests are used primarily for fuel wood and charcoal, although an increasing quantity of special-quality woods are cut for export as lumber. Nearly 80 percent of the world’s hardwood log exports in the early 1990’s came from Malaysia. Cambodia, Malaysia, Indonesia, the Philippines, Thailand, and Myanmar export large quantities of forestry products.

Overexploitation of hardwoods and conversion of forest lands for other uses have become serious concerns. Rates of forest conversion are most rapid in continental Southeast Asia, averaging about 1.5 percent a year. Deforestation has important local, regional, and global consequences, ranging from increased soil and land degradation to greater food insecurity, escalating carbon emissions, and loss of biodiversity.

Small-scale, poor farmers clearing land for agriculture to meet food needs and the gathering of wood to be used for cooking account for roughly two-thirds of the deforestation in Southeast Asia.Commercial logging and urban expansion account for most of the remaining deforestation.

Asian Flora

2011-12-09T05:42:00.000-08:00

Asia has the richest flora of the earth’s seven continents. Because Asia is the largest continent, it is not surprising that 100,000 different kinds of plants grow within its various climate zones, which range from tropical to Arctic.

Asian plants, which include ferns, gymnosperms, and flowering vascular plants, make up 40 percent of the earth’s plant species. The endemic plant species come from more than forty plant families and fifteen hundred genera.

Asia is divided into five major vegetation regions based on the richness and types of each region’s flora: tropical rain forests in Southeast Asia, temperate mixed forests in East Asia, tropical rain/ dry forests in South Asia, desert and steppe in Central and West Asia, and taiga and tundra in North Asia.

Tropical Rain Forests

The Asian regions richest in flora, tropical rain forests, are found in the island nations of Southeast Asia, which extend from Kinabalu in the north to Java in the south, and from New Guinea in the east to Sumatra in thewest. In this vast archipelago, the longest island chain between Asia and Australia, are thirty-five thousand to forty thousand vascular plant species.

Tropical rain forests grow there year-round because of the region’s warm temperatures and plentiful rainfall. The forests contain great varieties of tall trees, some towering 148 feet (45meters) high.Within any 1-square-mile area, one can see as many as one hundred tree species with no single species dominant.

The rain forests have mostly broad-leafed evergreens,with some palm trees and tree ferns. The upper most branches of the trees form canopies that cover and protect the earth below. Because little sunlight penetrates the dense canopies, few shrubs or herbs grow in the rain forests. Instead, many vines, lianas, epiphytes, and parasites are twined on tree branches and trunks. Mangroves fringe the tropical rain forests along the coasts.

Temperate Mixed Forests

Second in floral richness, East Asia’s temperate mixed forests contain thirty thousand to thirty- five thousand plant species. This region ranges from Japan in the east to the Himalayan nations (Bhutan, Sikkim, and Nepal) in the west, and from Russia’s Amur River Valley in the north to China’s Hainan Island in the south.

East Asia’s temperate weather is similar to the climate of eastern North America, with hot summers and cool winters. From south to north and from the east coasts to lower elevations in mountainous areas in the west, the vegetation changes from evergreen to deciduous broad-leafed forests, with dense shrubs, bamboo, and herbs in different layers beneath the forest canopy.

The major tree species are of the magnolia, oak, tea, laurel, spurge, azalea, and maple families. Herbs include members of the primrose, gentian, pea, carrot, foxglove, composite, buttercup, and rose families.

The Himalayan range is the point where the regions of South Asia, East Asia, and Central and West Asia join. From the Qinghai-Tibet Plateau in southwest China to the lower areas of the Himalayas, elevation usually is between about 5,000 and 13,000 feet (1,500 and 4,000 meters).

Mountains with deep valleys showcase complex, multiple vegetation types—from mixed forests and dense shrubs to alpine meadows in mountain plains. Many primary seed plants (gymnosperms and flowering plants), grow there.

Untouched native vegetation in East Asia is usually found only in mountainous or remote areas.On mountains at high elevations, the points where the temperatures are so cold that trees cannot grow form what is called the tree line.

Near the tree line, only plants related to coniferous and alpine species grow. Above about 13,000 feet (4,000 meters) in high mountain areas, no vegetation grows. Instead, snow caps or icebergs exist year-round.

Tropical Rain/Dry Mixed Forests

The third-richest region, tropical rain/dry forests, is found in SouthAsia, which reaches from the Philippines in the east to Pakistan in the west, and from the Himalayas in the north to Thailand in the south. Twenty-five thousand to thirty thousand species of plants grow there.

This region has both tropical rain forests and tropical seasonal dry forests. The tropical rain forest is mainly found in the region’s lowlands and the seasonal dry forests in the highlands or mountainous areas. More often, these two types of forests are combined.

The tropical seasonal dry forests usually grow in a climate with wet and dry seasons or under a somewhat cooler climate than the tropical rain forests. The canopy, formed from primarily deciduous broad-leafed species, is much thinner than the canopy in the tropical rain forest, so more sunlight reaches plants below.

Many different plant species live together, forming tropical jungles. Tall, thick-trunked trees, colorful orchids, ferns, dense mosses, and twined vines and lianas dominate this vast region.

The major components of these kinds of forests are members of the dipterocarpas, sweetsop, laurel, piper, fig, dissotis, akee, gardenia, periwinkle, milkweed, African violet, palm, and aroid families. In central and southern India and in some areas of Pakistan there are tropical grasslands, called the savanna. Because of the savanna’s hot, dry weather, mainly coarse grasses grow there.

Desert and Steppe

The desert and steppe region in Central and West Asia has twenty to twenty-five thousand species of plants. This region stretches from north and northwest China and Mongolia in the east to Turkey in the west, and from Kazakhstan in the north to the Arabian Peninsula in the south. This region’s vegetation changes from semi desert or desert to the temperate grassland called the steppe.

Central and West Asia contains the largest desert-steppe landscape in the Northern Hemisphere. Few plant species grow in the steppe and nearly none in the desert. The herbs and fewwoody plants that grow in these dry areas are members of the grass, pink, mustard, pea, saxifrage, stonecrops, lignum vitae, forget-me-not, and lily families. Because the desert environment is so dry, plant species must be able to survive in the arid weather for long periods of time.

Central and West Asia—with its steppe between the desert in the south and coniferous forests in the north—forms one of the world’s largest foraging areas, providing food resources for both wild and domestic animals, such as camels, sheep, goats, cows, and horses.

Taiga and Tundra

The poorest region in floral richness, with only about five thousand vascular plant species, is North Asia. This region is primarily Siberia, the eastern part of Russia, reaching from the Ural Mountains in the west to the Bering Strait in the east and fromthe Arctic Circle in the north to Mongolia and Kazakhstan in the south.

The region’s weather is temperate, with short, mild summers and long, cold winters. The predominant vegetation in North Asia is coniferous (boreal) forest. This region, called the taiga, contains mainly pine, spruce, fir, larch, and some species in the birch, aspen, and willow families.

Because the trees there are straight and tall, the taiga provides timber for Russia’s forestry industry. Small, perennial herbs and a few types of shrubs grow in the taiga’s swamps or marshes. Farther north is the cooler Arctic area called the tundra. Plants that grow in tundra are resistant to the cold climate.

During the summer they complete their life cycle quickly, before winter comes. Tundra plant species are members of such common families as composites, peas, grasses, and reeds. Far beyond the tundra is Arctic ice.

Asia’s native plant species provide shelter and food for animals. For example, arrow bamboo and umbrella bamboo, found in the forests of central to southwest China, are the main food of the giant panda. Many plants in Asia also provide food, ornaments, or medicine for humans.

Food Crops

Rice is the main food for humans in Asia, especially in the tropics. In temperate Asia, wheat—one of the world’s main food sources—joins rice as a primary food source. Various beans and peas provide plant protein in the human diet and are eaten with vegetables and grains.

Asia has many tropical fruit plants, such as the mango, banana, litchi, citrus fruits, and breadfruit. Pears, apples, grapes, peaches, and strawberries are temperate fruits. The kiwi, one of the most nutrient-rich fruits, is cultivated in New Zealand but originally came from central China. The Chinese not only eat kiwi but also make kiwi wine.

Palm dates are another important fruit in West and Southwest Asia (the Arabian Peninsula). Vegetables grown in Asia include various cabbages, lettuce, onions, garlic, celery, carrots, soybeans, cucumbers, and squash. Ginger originally came from Asia.

Soybean oil is the major cooking oil in Asia. Although soybeans are native to Asia, they have become the biggest crop grown in the United States. According to the U.S. Department of Agriculture, in 1999 U.S. farmers harvested 73.3 million acres of soybeans, 2.3 million acres more than corn and 18.6 million acres more than wheat.

Another oil plant, the sunflower, is grown in temperate Asia. In tropical Asia, people use mustard oil, palm oil, cotton oil, and peanut oil. In Central and West Asia, the most popular oil is olive oil. Many other foods people enjoy throughout the world are native Asian plants, for example, tea and coconuts. Black pepper and sugarcane also are grown in tropical Asia.

Ornamental and Medicinal Plants

Many of Asia’s plant species have great ornamental value. Azaleas, dogwood, primroses, camellias, peonies, roses, lotus, daisies, cherries, and begonias are frequently planted in gardens. Ornamental conifers from Asia include pines, spruces, cedars, junipers, umbrella pines, and yews.

Thousands of wildflowers originating in Asia include poppies, snapdragons, slippers, columbine, trillium, marigolds, buttercups, gentian, lilies, bluebells, and violets. Europeans who explored Asia centuries ago brought ornamental plants back to their home countries.

As a result, many of these plants are now grown throughout the world. The world’s largest flower, rafflesia, grows in the tropical rain forests of Sumatra. In full bloom, the flower’s diameter is about 3 feet (1 meter).

Plants make up a large part of traditional Chinese medicine, which has been practiced for thousands of years. Today, some of these plants are used in alternative medicine in the West. They include ephedra, eucommia, cinnamon, ginseng, sanqi, and ginkgo.

Scientific Value

Botanists view the region ranging from central China to the Himalayas to the northern part of South Asia as a key area for research into the origin of flowering plants. Native plant species in Asia are numerous; botanists also study Asian plants that are relics of ancient times, from millions of years ago, as well as fossils.

Ancient species include such gymnosperms as the dawn redwood of central China. Dawn redwood is similar to California’s redwood and giant sequoia. Another fossil-like tree is East Asia’s ginkgo. This species not only has great ornamental value but also has great commercial value as an alternative medicine.

Ginkgo trees are also dust- resistant, which makes them a favorite in urban landscaping and the ornamental industry. Other Asian plants such as the magnolia and its allied families may represent the most primitive flowering plants.

Introduced Plants

Asian flora today also includes introduced plant species from other parts of the world that play important roles in people’s lives. For example, the rubber tree of South America is cultivated in tropical Asia.

This tree produces raw material for the natural rubber industry, in which Asia is the largest producer in the world. Cacao, a tree species that provides the basis of chocolate, was introduced from tropical America. Corn, one of the most common crops in Asia, was introduced from America several thousand years ago.

Several vegetables, including the tomato, potato, eggplant, green pepper, hot pepper, and chili (all from the nightshade family), were introduced to Asia long ago. Peanuts, originally from Brazil, are also cultivated in Asia.

Coffee, an increasingly popular beverage in Asia, came originally from Africa. An introduced fruit is the pineapple, which came from tropical America but now is popular in tropical Asia. The sweet potato, from Central America, is also cultivated in Asia. Tobacco is another crop introduced to and cultivated in Asia. It originally came from tropical America, but its yield in China has made that nation a leading producer.

Impact of Human Activity

Asia’s highly diversified flora have contributed positively to the daily lives of people around the world, but the demands of a rapidly growing population are a constant threat. Deforestation, overgrazing, and urbanization have become major reasons for heavy losses of Asian flora, especially in South and East Asia.

In China alone, eight key plant species were added to the first-class protection list in the Red Book of 1992 (equivalent to the U.S. endangered species list). Among the mare the Chinese silver fir, dawn redwood, and ginseng.

These plants only grow in several isolated locations and are rare in their original range. As natural vegetation is cut for farming, grazing, or simply for cooking and heating fuel, fewer plants remain. Although scientists from around the world have worked on this problem for decades, the situation has not improved.

Australian Agriculture

2011-12-07T06:24:00.000-08:00

Agriculture is an important part of Australia’s economy. Australia’s exports were overwhelmingly agricultural products until the 1960’s, when mining and manufacturing grew in importance.

Agriculture occupies 60 percent of the land area of Australia, but much of this is used for open-range cattle grazing, especially in huge areas of the states of Queensland and Western Australia. Only 5 percent of Australia’s agricultural land is used for growing crops.

Western Australia and New South Wales have the largest areas of cropland. The limited area suitable for growing commercial crops is limited mainly by climate, because Australia is the world’s driest continent.

Annual rainfall of about 20 inches (500 millimeters) is necessary to grow crops successfully without irrigation; less than half of Australia receives this amount, and the rainfall is often variable or unreliable.

Years of drought may be followed by severe flooding. High temperatures throughout most of Australia also mean high evaporation rates, so rainfall figures alone are not a good guide to the feasibility of agriculture.

Australian soils usually require the application of fertilizer to grow crops successfully. The east coast of Australia is suitable for growing sugarcane, while the cooler southern parts are suited to growing wheat.

Irrigation has opened up large areas of drier land to agriculture, especially for growing fruit, but salinization (the buildup of salt in the soil) has become a major problem in some areas, especially near the mouth of the Murray River. Major agricultural plant exports from Australia are wheat and sugar. Other important agricultural exports are fruits, cotton, rice, and flowers.

Wheat

Long the most important crop of Australia, wheat is produced in the Wheat Belt, a crescent of land just west of the Eastern Highlands, or Great Dividing Range, which extends from central Queensland through New South Wales to Victoria, as well as in the south of South Australia and southwest Western Australia.

More than 120,000 farms in Australia grow grains, and wheat is the principal crop on some 25,000 farms. The average Australian wheat farm is family-owned and has an area of 3,700 acres (1,500 hectares). Crops are rotated, usually because of low soil fertility.

Australian wheat is planted during the winter, which is much milder than winter on the prairies of North America. Harvesting begins in September in the warm state of Queensland and moves south to Victoria and Western Australia by January.

Australian wheat is high in quality and low in moisture, so it is easy to mill. Wheat crops are frequently affected by drought; another problem is Australia’s markets, because the nation competes with the United States in wheat export.

When the British first came to Australia, convicts planted wheat on a government farm in what is now inner Sydney. They had difficulty growing wheat because of poor soils, unfamiliar climate, and inexperience, causing fear of widespread hunger.

As settlement spread beyond the coastal plain and into the interior, wheat production rose dramatically. The rapid increase in population after the gold discoveries of the 1850’s also led to increased demand for wheat. Australia began exporting wheat in 1845 and is now the world’s fourth-largest exporter of wheat.

Sugar

Sugarcane is grown in a series of small regions along the tropical coast of Queensland, extending slightly across the border into northern New South Wales. A warm, wet climate is required for successful cultivation of sugarcane, so it is confined to parts of the coastal plain with good, deep soils and reliable rainfall.

Australia is the world’s third-largest exporter of sugar. Sugar is grown on more than six thousand small, individually owned farms. Until the 1960’s, cane was cut by hand. Now it is harvested mechanically and taken by light rail to a nearby mill. There are twenty-five mills in Queensland and three in New South Wales.

Fruit

Fruit growing has a long history in Australia and is strongly influenced by climatic considerations. In Queensland, tropical fruits such as bananas, pineapples, mangoes, and papaya (called paw paw in Australia) are cultivated. In the cooler south, apples, peaches, apricots, cherries, and grapes are grown.

Grapes are grown for eating and are dried as raisins, but more important is wine production, especially in the BarossaValley (South Australia), Hunter Valley (New South Wales), Margaret River area (Western Australia), and the Murrumbidgee Irrigation Area and Riverina.

John Mac arthur, the founder of the Australian wool industry, established the first commercial vineyard, in New South Wales. Later European settlers planted vineyards in Victoria and South Australia.

In the 1960’smodern plantings and production methods were introduced. Australia has 242,060 acres (98,000 hectares) of vineyards and more than one thousand wineries. Wine is an important export for Australia, with the European Union purchasing 60 percent of wine exported.

Other Agricultural Products

Cotton is grown in drier interior parts of northern New South Wales and in part of central Queensland. Cotton is usually grown in conjunction with sheep farming, on family farms. Indonesia is the major customer for Australian cotton. Rice has been grown commercially in Australia since 1924, using irrigation.

New South Wales is the main producer, where the Murrumbidgee Irrigation Area dominates rice production. Australia exports most of its rice crop and in 1999was theworld’s eighth-largest exporter of rice.

Oats are grown where the climate is too cool and too moist for wheat. In Australia, this is in the interior southeast with a small area in Western Australia. This state and New South Wales are the biggest producers of oats, which is used mainly for livestock fodder.

Other agricultural products of Australia include barley; grain sorghum; corn, called maize in Australia; vegetables, including potatoes, peas, tomatoes, and beans; oil seeds such as sunflower; soybeans; and tea and coffee in northern Queensland.

Australia is a major producer of honey, with more than eight hundred commercial apiarists. Blossoms of the eucalyptus tree produce distinctive-tasting honey, which is sold mainly to European Union countries.

Australian Flora

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Australia broke off from the supercontinent Pangaeamore than fifty million years ago, and the species of plants living at that time continued to change and adapt to conditions on the isolated island. This led to distinctive plants, differing from those of the interconnected Eurasian-African-American landmass, where new immigrant species changed the ecology.

Many species of plants in Australia are found nowhere else on earth, except where they have been introduced by humans. Such species are known as endemic species. This distinctiveness is the result of the long isolation of the Australian continent from other landmasses.

Australian vegetation is dominated by two types of plants—the eucalyptus and the acacia. There are 569 known species of eucalypts and 772 species of acacia. Nevertheless, a great deal of botanical diversity exists throughout this large continent.

Climate and Ecology

Climate is a major influence on Australian flora, and the most striking feature of the Australian environment as a whole is its aridity. Nutrient-poor soils affect the nature of Australia’s vegetation, especially in arid areas.

Half of the continent receives less than 11.8 inches (300 millimeters) of rainfall per year; small parts of Australia receive annual rainfall of 75 inches (800 millimeters). Therefore, forests cover only a small percentage of Australia.

A close correlation exists between rainfall and vegetation type throughout Australia. Small regions of tropical rain forest grow in mountainous areas of the northeast, in Queensland. In the cooler mountains of New South Wales, Victoria, and Tasmania, extensive temperate rain forests thrive.

More extensive than rain forest, however, is a more open forest known as sclerophyllous forest, which grows in the southern part of the Eastern Highlands in New South Wales and Victoria, in most of Tasmania, and in southwest Western Australia.

A huge crescent-shaped region of woodland vegetation, an open forest of trees of varying height, with an open canopy, extends throughout northern Australia, the eastern half of Queensland and the inland plains of New South Wales, and to the north of the Western Australian sclerophyllous forest.

Beyond this region, the climate is arid, and shrubs, forbs (smaller herbaceous plants), and grasses predominate. The tropical north of inland Queensland,Northern Territory, and a smaller part of Western Australia have extensive areas of grassland.

Much of Western Australia and South Australia, as well as interior parts of New South Wales and Queensland, are shrubland, where grasses and small trees grow sparsely. In the center of the continent is the desert, which has little vegetation, except along watercourses.

Eucalypts

Many people familiar with the song "Kookaburra Sits in an Old Gum Tree", may not realize that gum tree is the common Australian term for a eucalyptus tree. When the bark of a eucalyptus tree is cut, sticky drops of a transparent, reddish substance called kino ooze out.

In 1688 the explorer William Dampier noticed kino coming from trees in Western Australia and called it "gum dragon", as he thought it was the same as commercial resin. Kino is technically not gum, as it is not water-soluble.

The scientific name Eucalyptuswas chosen by the first botanist to study the dried leaves and flowers of a tree collected in Tasmania during Captain James Cook's third voyage in 1777. The French botanist chose the Greek name because he thought that the bud with its cap (operculum) made the flower "well" (eu) "covered" (kalyptos). The hard cases are commonly called gum nuts.

Themore than five hundred species of eucalypts in Australia range from tropical species in the north to alpine species in the southern mountains. Rain-fall, temperature, and soil type determine which particular eucalyptwill be found in any area. Eucalyptus trees dominate the Australian forests of the east and south, while smaller species of eucalyptus grow in the drier woodland or shrubland areas.

It is easier to mention parts of Australia where euca- lypts do not grow: the icy peaks of the Australian Alps, the interior deserts, the Nullarbor Plain, and the tropical and temperate rain forests of the East- ern Highlands.

The scientific classification of the eucalypts proved difficult to European botanists. Various experts used flowers, leaves, or other criteria in their attempts to arrange the different species into a meaningful and useful taxonomy, or classification scheme. George Bentham eventually chose the shape of the anthers—the part of the stamen that holds the pollen—together with fruit, flowers, and nuts.

A simpler classification of the eucalypts, commonly used by foresters, gardeners, and naturalists, arranges them into six groups based on their bark: gums have smooth bark, which is sometimes shed; bloodwoods have rough, flaky bark; iron barks have very hard bark with deep furrows between large pieces; stringy barks have fibrous bark that can be peeled off in long strips; peppermints have mixed but loose bark; boxes have furrowed bark, firmly attached. This system was devised in 1859 by Ferdinand von Müller, the first government botanist of the Colony of Victoria and the father of Australian botany.

Many of the native plants of Australia, along with eucalypts, show typical adaptations to the arid climate, such as deep taproots that can reach down to the water table. Another common feature is small, shiny leaves, which reduce transpiration.

Eucalyptus leaves are tough or leathery and are described as sclerophyllous. Sclerophyllous forests of eucalypts cover the wetter parts ofAustralia, the Eastern Highlands, or Great Dividing Range, and the southwest of Western Australia.

The hardwood from these forests is generally not of a quality suitable for building, so areas are cleared and the trees made into wood chips that are exported for manufacture of newsprint paper. This has been a controversial use of Australian forests, especially where the native forest has been cleared and replaced with pine plantations.

The southwest corner of Western Australia has magnificent forests featuring two exceptional species with Aboriginal names, karri and jarrah. Karri is one of the world’s tallest trees, growing to 295 feet (90 meters) tall. This excellent hardwood tree is widely used for construction.

The long, straight trunks are covered in smooth bark that is shed each year, making a colorful display of pink and gray. These forests are now protected. Jarrah grows to 120 feet (40meters) in height and is a heavy, durable timber. It was used for road construction in the nineteenth century, but now the deep red timber is prized for furniture, flooring, and paneling.

During the nineteenth century, Australia could also claim to be home to the world’s tallest trees, the mountain ash. The tallest tree, which observers claimed was 433 feet (132 meters) high and with the top broken away,was felled in 1872. The tallest accurately measured tree was 374 feet (114 meters) high.

The most widely distributed of all Australian eucalypts is the beautiful river red gum. These trees grow along riverbanks and watercourses throughout Australia, especially in inland areas; their spreading branches provide wide shade and habitat formany animals.

Koalas eat leaves from this tree. In the song "Waltzing Matilda", Australia's unofficial national anthem, aman camps "under the shade of a coolabah tree". This word might apply to any eucalypt, but it is most likely a river red gum.

In drier interior areas and in some mountain areas, there are more than one hundred smaller species of eucalypts that are known by the Aboriginal name mallee. These many-trunked shrubs have underground lignotubers, roots that storewater.

Much of this marginal country was cleared for farming, creating a situation similar to that of the 1930’s Dust Bowl in the United States. In the dry Australian summers bushfires are a great danger in themallee and in any eucalyptus forest. The volatile oils of the eucalyptus trees can lead to rapid spread in the tree crowns, jumping across human-made firebreaks.

On the other hand, several Australian trees not only can survive fires but actually require fire for their seeds to germinate. Eucalypts have been introduced to many countries, including Italy, Egypt, Ethiopia, India, China, and Brazil, and they are common in California, where they have been growing as introduced trees for 150 years.

Acacias

These plants are usually called wattles in Australia, because the early European convicts and settlers used the flexible twigs of the plant for wattling, in which twigs are woven together, making a firm foundation for a thatched roof or for walls. The walls are then covered inside and out with mud. This style of building, known as wattle-and-daub, was common throughout Australia in pioneering days.

Wattles frequently have masses of colorful flowers, usually bright yellow. One species is the national flower of Australia. Other interesting acacias include the mulga, which has an attractive wood.

Rain Forests

Although rain-forest vegetation covers only a small area of Australia, it is exceptionally varied and of great scientific interest. Neither of the two general types of rain forest found in Australia has eucalypts. Rain forests are located along the Eastern Highlands, or Great Dividing Range, where rainfall is heavy.

In small areas of tropical Queensland, where rainfall is also heavy, true tropical rain forest is found. The flora are similar to those in Indonesian and Malaysian rain forests. The tropical rain forest contains thousands of species of trees, as well as lianas, lawyer vine, and the fierce stinging tree,whose touch could kill an unwary explorer.

Toward the north of New South Wales, a kind of subtropical to temperate rain forest grows. Cool, wet Victoria and Tasmania have extensive areas of temperate rain forest, where only a few species dominate the forests.Arctic beech trees are found, aswell as sassafras and tall tree ferns.

Sclerophyllous Forest

This is the typical Australian bush, which grows close to the coast of New South Wales, Victoria, and Tasmania. The bright Australian sun streams down through the sparse crowns and narrow leaves of the eucalypts. As the climate becomes drier, farther inland from the coast, the open forest slowly changes to a shrubbier woodland vegetation.

Grasslands

Moving farther inland, to still drier regions, woodlands giveway to grasslands, where cattle are raised for beef in the tropics, and sheep are raised for wool in the temperate areas. Before Europeans came to Australia, therewere native grasslands in the interior—tropical grasslands in themonsoonal north, and temperate grasslands in the south and south-west.

Kangaroo grass and wallaby grass once grew in the temperate interior of New South Wales, but much of this has been cleared for agriculture, especially for wheat farming. Mitchell grass is another tussock grass, which grows in western Queensland and into the Northern Territory. Cattle and sheep graze extensively on this excellent native pasture.

The most common grassland type in Australia is dominated by spinifex, a spiky grass that grows in clumps in the arid interior and west. Even cattle cannot feed on spinifex grass, so this ecosystem is less threatened than most other grasslands. The northern grasslands are dotted with tall red termite mounds; those that are aligned north-south for protection from the hot sun are built by so-called magnetic termites.

Other Trees and Plants

Many people think that macadamia nuts are native to Hawaii, which produces 90 percent of the world’s crop, but in fact, the tree is native to Australia. It was discovered by Ferdinand von Müller in 1857 on an expedition in northern Australia. Müller named the tree after his Scottish friend, John Macadam. The trees were introduced to Hawaii in 1882.

Cycads are plants from an ancient species which still thrive in Australia. The Macrozamia of North Queensland is a giant fernlike plant. Similarly old are the Xanthorrhea grass-trees, which used to be called "blackboys". A single spearlike stem rises from a delicate green skirt on this fire-resistant species.

In northwest Australia, the baobab, or bottle tree, can be found. This fattrunked tree collects water in its tissue. One is said to have served as a temporary prison. The only other baobabs are found in Africa, a reminder that these continents were once joined. Bottlebrush is an Australian shrub with colorful flowers that has become popular with gardeners in many parts of the world.

Extinct and Endangered Plants

Human activities in Australia have led to the extinction of more than eighty species of plants, and the list of endangered plants contains more than two hundred species. Many nonnative species have been introduced to Australia by Europeans.

Some have become pests, such as the blackberry in Victoria, the lantana in north Queensland, and water hyacinth, found throughout the continent. There are 462 national parks in Australia, as well as other conservation areas, where native flora are protected.

Aboriginal Plant Use

The Australian Aborigines used plants as sources of food and for medicinal purposes. Food plants included nuts, seeds, berries, roots, and tubers. Nectar from flowering plants, the pithy center of tree ferns, and stems and roots of reeds were eaten.

Fibrous plants were made into string for weaving nets or making baskets. Weapons such as spears, clubs, and shields, as well as boomerangs, were made from hardwoods such as eucalyptus. The bunya pine forests of southeast Queensland were a place of great feasting when the rich bunya nuts fell.

Autoradiography

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Autoradiography produces an image formed by a substance’s own radioactivity when exposed to a photographic film. This technique is often used for investigation of biological processes.

In 1896 Antoine-Henri Becquerel was working with rocks containing uranium ore. By chance, he put one rock sample into a dark drawer on top of a box of unexposed photographic film. When the film later was developed, it showed a clear outline of the uranium rock.

Evidently, some radiation had been emitted from the rock, penetrated through the wrapping paper, and exposed the film inside. An autoradiograph, that is, an image produced by radioactivity, was visible on the film. Autoradiography, much refined, is now a valuable technique for investigating biological processes.

Hungarian chemist Georg von Hevesy pioneered the use of radioactive tracers in biological research in the 1920’s, and two developments in the 1930’s greatly expanded their use. Firstwas the discovery of induced radiation by Frédéric Joliot-Curie and Irène Joliot-Curie, which raised the exciting possibility that artificially created radioactivity could be induced in almost any element found in nature.

The second development was the invention of the cyclotron. The cyclotron beam was used to bombard various elements to produce new radioactive isotopes, including radioactive sodium, potassium, sulfur, and iron. After 1950 radioactive hydrogen and carbon also became available as tracers, allowing organic molecules such as carbohydrates and proteins to be labeled.

Macroautoradiography

Whole-body autoradiography has been widely used to trace the routes of molecules in metabolism. First, a radioactive tracer is administered to an organism by ingestion or injection. After a period of time, individual samples of tissue are removed and pressed directly against X-ray film for several days, to expose the film wherever the radioactivity has become concentrated.

The film is then developed and viewed, frequently with the aid of a microscope. This process has been used to trace the uptake of nutrients by plants from the soil into the leaves or buds. Experimentation with whole organisms is called macroautoradiography.

Microautoradiography

A refinement of this methodology, called microautoradiography, has been developed for studying subcellular structures, even those as small as individual strands of deoxyribonucleic acid (DNA). Much interesting information has been learned about the mechanisms of cell division and other processes in cell biology. The cells being studied are given a nutrient solution containing molecules that have been labeled, usuallywith radioactive tritium, carbon, or phosphorus.

After a period of incubation, some cells are transferred to a glass slide. The slide is dipped into a liquid photographic emulsion containing light-sensitive silver bromide, which clings to the slide in a thin layer.

The slide with the cells covered by emulsion is then placed into a light-tight box for several days to allow time for radioactive decay. The beta particles from tritium cause the photographic emulsion to become exposed.

The emulsion is then developed and fixed as any photographic negative would be. The developer washes the soluble silver bromide away and leaves behind the insoluble grains of silver, which show up as small black dots. A stain may be applied to show the outlines and structures within.

Finally, the cell is examined with a microscope. Autoradiographs typically show the black dots of exposed silver grains against a faint background of the surrounding cell structure. When higher magnification and resolution are desired, an electron microscope can be used.

In some studies, the radioactive nutrient is supplied to the cell for a short time interval, perhaps only a few minutes. This procedure is called pulse-labeling. Only those molecules that are being freshly synthesized in the cell during the "pulse" will incorporate radioactive atoms. Autoradiography will then show which cells were active.

When autoradiography is applied to chromosomes or other subcellular structures, the matter of resolution becomes very important. For high resolution to be obtained, the radioactive particles should have a short range within the photographic emulsion; the black dots of silver in the developed film should pinpoint the source of radioactive decay as precisely as possible.

Tritium works very well because it emits low-energy beta particles, which travel only a few millimeters in the emulsion, producing a well-localized image on the film. Radioactive carbon 14 emits higher-energy beta particles, so the silver grains in the film are more diffuse and the resolution is not as high.

Autoradiography and Electrophoresis

Autoradiography also has been very useful in biochemistry research when combined with the methodology of electrophoresis. Living cells are exposed to radioactively labeled amino acids, which are gradually absorbed into the proteins.

For electrophoresis, the cells are transferred to a gel to which a voltage has been applied. The protein molecules will diffuse along the gel and be sorted out by their relative molecular weights. A photographic emulsion then is placed over the gel.

Radioactivity from the proteins exposes the film, producing an image with black spots that show the distances that the different molecules drifted in the gel. The relative molecular weight of complex molecules that contain many thousands of atoms can be determined in this way.

One commercial catalog of radioactive materials listsmany hundreds of organic chemicals that have been labeled with radioactive tritium. The other most common radioactive isotopes used for autoradiography are carbon 14, phosphorus 32, and sulfur 35. A large inventory of labeled chemicals has become available for the continuing use of radioactive tracers in biological research.

Applications

Autoradiography has been used in biology on the macroscopic level to study the uptake of radioactive tracers by both plant leaves and animal organs. Since the 1960’s the technique has been applied to successively smaller structures, such as individual cells, chromosomes and organelles within a cell, strands of DNA, and protein molecules. It is easier to understand the microscopic applications after first looking at a large-scale example.

In one experiment, bean plants were grown in a nutrient solution containing radioactive phosphorus. The phosphorus moved from the roots to the leaves as expected, shown by an autoradiograph of a leaf pressed against photographic film.

When the bean plant is allowed to continue growing in a nonradioactive solution, autoradiography shows that radioactive phosphorus is withdrawn from older leaves and translocated to new leaves and buds. Evidently, nutrients not only travel up from the roots but also move around the plant.

In another experiment, a solution containing phosphorus was sprayed directly onto the leaf surface and was shown to migrate away from it. Redistribution of nutrients on an even larger scale takes place in deciduous trees, where as much as 90 percent of some minerals are withdrawn from leaves before they fall.

Practical Applications

In agricultural research, the effectiveness of herbicides, insecticides, and fertilizers is studied to determine which ones can increase productivity without causing serious environmental problems. Radioactive phosphorus can be used in this regard to study plant metabolism.

The uptake of iron or zinc from the soil and their circulation in a plant can be studied to ascertain the effect of soil acidity and chemical form. Sometimes the presence or absence of other elements can inhibit translocation of an essential nutrient.

New plant growth regulators may move from one plant through the soil to a nearby untreated plant. Autoradiography is an important analytical technique for observing the route of micronutrients and discoveringwhat factors can change their mobility in a plant.

The sequence of bases in DNA molecules can be decoded by using electrophoresis combined with autoradiography, and the study of DNA sequences is crucial to research in many diverse areas of biology. Although alternatives to using autoradiography in DNA sequencing are now common, autoradiography is still a standard technique used in many other aspects of molecular biology.

Bacterial Genetics

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Bacterial genetics is the study of the genetic material of bacterial DNA, which can provide valuable insights into the process of mutation because of bacteria’s rapid rate of reproduction.

Plants were the original candidates for genetic studies, which began in the late 1800’s. Studies with animals soon followed; bacteria did not become candidates for such study until the mid-1940’s, when adequate technology for handling bacteria developed. Bacteria have become extremely useful organisms for genetic studies since the early 1950’s.

Two major features of bacteria make them desirable subjects. First, bacterial cells typically divide every twenty minutes. Their rapid rate of reproduction allows a very large number of bacteria to be produced in a short time. This, in turn, provides the researcher with more opportunity to detect the "rare genetic events" of mutation or recombination.

Even more important, unlike all other organisms, bacteria have a single chromosome with a single set of genes. Thus, genetic modifications are more likely to result in immediately observable changes. In organisms that have multiple chromosomes, a change in a single gene may go undetected because its effect is masked by genes on other chromosomes.

Bacterial DNA

All bacteria have a single circular chromosome, composed of deoxyribonucleic acid (DNA). The DNA is subdivided into specific message areas known as genes, and the chromosome carries from four thousand to five thousand individual genes. For many bacteria, this constitutes the entirety of its genetic information.

A number of bacteria, however, have additional DNA in the form of plasmids. A plasmid is a small additional circular piece of DNA, independent of the chromosome,which can hold an additional twenty to one hundred genes. Plasmid-containing cells often have several plasmids.

Many researchers have described the plasmid genes as nonessential to the normal activities of bacteria. Under certain circumstances, however, those genes might provide a survival advantage to the possessor. For example, genes for antibiotic resistance are often carried on a plasmid.

Normally, antibiotics are not present in the bacteria’s environment; such resistance genes would therefore be unnecessary. If the bacteria later were to come into contact with antibiotics, however, having antibiotic- resistant genes would be to their distinct advantage.

Two major types of plasmids exist: F plasmids, or fertility plasmids, and R plasmids, or resistance plasmids. Both types can carry resistance genes. Only the F plasmids, however, are able to control the formation of a special cytoplasmic tube known as the sex pilus. Cells with the F plasmids are known as F+, or donor cells. Cells without the F plasmids are called F-, or recipient cells.

Conjugation

The plasmid is a prerequisite to one type of genetic exchange, conjugation. During conjugation, the donor cell copies its plasmids and transfers them to a recipient cell to which it has attached itself by means of a sex pilus.

The recipient cell can now take advantage of whatever additional genes it has received. If, in the process, it received an F plasmid, it has also become a potential donor cell.Whenever bacterial cells undergo cell division, any plasmids they possess are typically passed on to their progeny.

Originally it was thought that conjugation could occur only between members of the same species, but that is not always true. For example, it is now known that some strains of the bacteria responsible for causing gonorrhea, Neisseria gonorrhoeae, have received antibiotic-resistant genes from unrelated species of bacteria.

There is one other type of donor cell, the Hfr+, or high-frequency recombinant, cell. Instead of the plasmid remaining independent of the cell’s chromosome, it inserts itself into the chromosome. When that plasmid gets ready to copy itself, the chromosomal genes are the first to be copied.

Unless the donor and recipient cells are able to maintain direct contact for a fairly long period of time, which almost never occurs, the recipient cell will not receive the plasmid. It will, however, receive numerous chromosomal genes from the donor. Those genes may later be incorporated into the chromosome of the recipient, causing gene replacement.

Not all species of bacteria participate in conjugation. Some rely on transduction as a means of receiving new genetic information. This is how Staphylococcus aureus has developed resistance to many antibiotics.

Transduction

There are two types of transduction: generalized and specialized. In both cases, a donor cell becomes infected with a bacteriophage, a virus that attacks bacteria. Upon the death of that donor cell, fragments of donor DNA are transferred as the escaping bacteriophage infects another bacterium.

In generalized transduction, a bacteriophage infects a bacterial cell. Shortly after infection, the bacterial chromosome becomes fragmented, and viral components are produced. Later the viral components are assembled to form a complete virus particle.

Occasionally during this assembly process, a particle becomes contaminated with fragments of the bacterial chromosome or plasmids. After assembly is completed, the bacterial cell ruptures, allowing the escape of all virus particles.

Eventually these virus particles will invade other bacterial cells. Any cells that are invaded by contaminated bacteriophage particles are said to be transduced, because they have received DNA from another bacterium. The DNA received in this manner is strictly random.

Specialized transduction involves what is known as a latent bacteriophage. After the initial invasion of a bacterium, the bacteriophage inserts itself into a specific region of that cell’s chromosome. At some later time, the bacteriophage removes itself from the chromosome and accidentally takes a few bacterial genes located near its original insertion point.

When the bacterial cell finally begins making new bacteriophage components, it behaves as if those particular genes are part of the bacteriophage and replicates them as such. Therefore, all the newly formed bacteriophage particles will contain those bacterial genes. Transduction then occurs when these bacteriophage particles invade other bacterial cells.

Transformation

The final method of genetic transfer is transformation. An extensively utilized organism for such investigation has been Streptococcus pneumoniae. The most famous studies involved converting nondisease-causing strains of Streptococcus pneumoniae intodisease-causing strains.

Transformation also occurs in a wide variety of other bacteria. The process of transformation requires that a population of actively reproducing bacteria come into contact with DNA fragments, often from closely related dead bacteria. These DNA fragments are referred to as either naked or cell-free DNA.

Genetic Modification

A small portion of that DNA can be absorbed and utilized by the growing bacteria. These recipients can then take advantage of any usable genes that the fragments might contain, incorporating them into their chromosome in place of their own copies of these genes by the process of recombination.

Conjugation, transduction, and transformation are all mechanisms of genetic change within a bacterial population. These mechanisms allow a specific characteristic to be spread throughout the population within a few hours.

A wide number of bacterial genes have been found to be transferred by these methods, including genes that control a bacterium’s ability to cause disease, to produce toxins, and to develop resistance to antibiotics and other drugs as well as genes that control a number of other characteristics.

The purpose of these mechanisms, as far as the bacteria are concerned, is to enable the bacteria to adapt to changing environmental conditions so that their survival is ensured. Scientists, however, have found ways to adapt some of these mechanisms for human benefit.

Scientists have used the mechanisms of genetic transfer along with new technology from DNA research to perform genetic engineering on bacteria. They can use genes and specially engineered plasmids, called plasmid vectors, to make recombinant DNA in the laboratory.

Recombinant plasmids can then be used to transform bacteria such as Escherichia coli (E. coli). The bacteria will treat these recombinant plasmids just like ordinary plasmids, replicating them and, for expression vectors, expressing any genes included in them.

In this manner, bacteria can be used to produce a wide variety of products for medicine, agriculture, and industry. Genetic engineering and the products that result from it would not be possible without the knowledge of genetic transfer gained from studies of bacterial conjugation, transduction, and transformation.

Bacteriophages

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Viruses that attack bacterial cells are known as bacteriophages. Many results gained from studying bacteriophages have universal implications.

For example, the physical properties of DNA and RNA are remarkably identical in all organisms, and these are perhaps easiest to study in bacteriophage systems.

Bacteriophages, or phages for short, are viruses that parasitize bacteria. Viruses are an extra ordinarily diverse group of ultramicroscopic particles, distinct from all other organisms because of their noncellular organization.

Composed of an inert outer protein shell, or capsid, and an inner core of nucleic acid—either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA) but never both—viruses are obligate intracellular parasites, depending to a great extent on host cell functions for the production of new viral particles.

There is considerable variation in size and complexity among viruses. Some have fewer than ten genes and depend almost entirely on host functions. Others are known to contain from thirty to one hundred genes and rely more on proteins encoded by their own DNA.

Even the largest viruses are too small to be seen under the light microscope, so studies on viral structure rely heavily on observation with the transmission electron microscope.

The Study of Bacteriophages

Because scientists know more about the molecular and cell biology of the common bacterium Escherichia coli than about any other cell or organism, it is perhaps not surprising that the best-known phages are those that require E. coli as a host (coliphage).

It is not possible to observe phage growth directly (as bacterial growth can be detected by the appearance of colonies on an agar plate), but phage growth can be indirectly observed by the formation of plaques, small clear areas in an otherwise continous lawn of host bacteria growing on a solid growth mediumin a petri dish.

Reproductive Cycles

Bacteriophages can multiply by two different mechanisms, termed the lytic cycle and the lysogenic cycle. Some phages are capable only of lytic growth, while others retain the ability to reproduce by either lytic growth or entry into the lysogenic cycle. In the lytic cycle, phages first attach themselves to specific receptor sites on the host cell wall.

The phage nucleic acid (DNAor RNA) is injected inside the host, while the protein capsid of the infecting particle remains outside of the host cell at all times. Once the DNA or RNA is inside, transcription of phage genes begins, and phage-encoded proteins begin to be made.

Some of these proteins serve to inactivate and destroy the host cell DNA, ensuring that the cell’s energy resources will be directed exclusively toward the production of phage proteins and the replication of phage nucleic acid. Phage DNA or RNA replication ensues quickly and is followed by the packaging of this genetic material into the newly synthesized capsids of the progeny phage particles.

The final step is host cell lysis—the bursting of the host cell to release the completed and infective phage progeny. The number of phages released in each burst varies with growth conditions and species, but ideal conditions often result in a burst size of one hundred to two hundred per host cell.

For temperate bacteriophages, those capable of entering the lysogenic cycle, infection of the host cell only rarely causes lysis. Injection of the phage DNA into the host is followed by a brief period of messenger RNA (mRNA) synthesis, necessary to direct the production of a phage repressor protein, which inhibits the production of phage proteins involved with lytic functions.

A DNA-insertion enzyme is also made, allowing the phage DNA to be physically inserted into the DNA of the host. The cell then can continue to grow and multiply, and new copies of the phage genes are replicated every cell generation as part of the bacterial chromosome.

The host cell is said to be lysogenic, for it retains the potential to be lysed if the prophage pops out of the host DNA and enters the lytic cycle. The integrated prophage does confer a useful property on the host cell, however, for the cell will now be immune to further infection from the same phage species.

T4 Coliphage

One of the best-known lytic phages, which is often used in genetic studies, is the coliphage T4. Its protein capsid consists of three major sections—the head, the tail, and the tail fibers.

The double-stranded circular DNA molecule of T4 is packaged into the icosahedral-shaped head, and during the infection process it is forced through the hollow core of the cylindrical tail and then directly into the host cell. Contact with the cell is established and maintained throughout the infection process by the tail fibers.

Self-assembly of progeny phages occurs in at least three distinct cellular locations, as complete heads, tails, and tail fibers are first assembled separately and then pieced together in one of the last phases of the infection cycle.

Packaging of the replicated T4 DNA is an integral part of the head assembly process. Each of the three subassemblies involves a reasonably complex and highly regulated sequence of assembly steps.

For example, head assembly is known to require the activity of eighteen genes, even though only eleven different proteins are found as structural components of mature heads. Identification of the number and sequence of genes involved with each subassembly process has been facilitated by the analysis of artificial lysates from t⁸ mutants.

For those temperate phages capable of entering the lysogenic cycle, many additional strategies for genetic control and regulation have evolved. The most thoroughly studied of the temperate coliphages is phage lambda (λ).

Genes controlling phage DNA integration, excision, and recombination, and those involved with repressor functions, have been identified in phage λ as well as structural genes involved with lytic functions that are similar to those studied in T4.

Research Tool

One of the most important conclusions to be drawn from studies on bacteriophages, and viral genetics in general, is that many of the results have universal implications. For example, the physical properties of DNA and RNA are remarkably identical in all organisms, and these are perhaps easiest to study in bacteriophage systems. The experiment that provided the final proof that DNA was the genetic material was performed using a coliphage very similar to T4.

Studies on the origin of spontaneous mutations, first performed in phage, have extended to higher forms of life as well. Some of the most basic questions concerning protein-DNA interactions are best addressed in viral systems, and the principles that emerge seem to hold for all other experimental systems.

There is every reason to believe that many basic questions in cell and molecular biology will continue to be best studied in viruses such as bacteriophages, and that some of these investigations will spawn applications that can directly benefit humankind.

It is certain that advances in molecular biology that have revolutionized the understanding of cell biology and the molecular architecture of cells will continue to expand the frontiers of knowledge in the study of viral genetics. Applications in human medicine, veterinary medicine, and plant breeding are sure to follow, as scientists continue to unravel the complexities of these simplest of organisms.

Basidiomycetes

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The Basidiomycetes constitute the largest of the three classes of the Basidiomycota (basidiosporic fungi), a very large class of about fourteen thousand species of the most diverse terrestrial fungi.

The largest fungi belong in the Basidiomycetes class, as do some of the most unusual. All members of Basidiomycetes produce a basidium from hyphal cells and not from spores. (The basidiumis a cell produced at the end of a dikaryotic hypha.)

The basidium will produce either two or four spores as the result of meiosis. The basidium may be either a nonseptate cell, with two or four sterigmata (the basidiospore is produced on the end of the sterigma) at the apex, or it may be septate.

The septa can be either horizontal or vertical. When observed from the apex, the vertical septa will produce a crosslike pattern. In either case, septate basidia will have one sterigma per cell. Basidiospores are thin-walled and may be released either actively or passively.

Basidiocarps

The basidiocarp is the fruiting body of the fungus. The fungus grows as a dikaryotic mycelium through the substrate. When the fungus has acquired sufficient energy, and environmental conditions are adequate, the fungus will produce a basidiocarp. The basidiocarp often appears overnight and may reach a meter in height.

Some basidiocarps are tiny, less than a centimeter in height. The basidiocarpmay look like a mushroom or may have the appearance of a golf ball or any variation in between. The basidiocarp may be edible or deadly poisonous. It often serves as food for wild animals and insects.

Classification

The Basidiomycetes are divided into two groups based on septa in the sterigma. Those that have a septate sterigma are classified in Phragmobasidiomycetidae, while those without septa in the sterigma are classified in Holobasidiomycetidae. Phragmobasidiomycetidae is a small group that includes some smaller fungi whose basidiocarps often have a gelatinous appearance.

Holobasidiomycetidae is a large group of fungi and is easily divided into two major groups based on the release of the basidiospore. The hymenomycetes release spores actively, while the gasteromycetes release spores passively.

Hymenomycetes

The hymenomycetes are the most familiar fleshy fungi. These are the ones that resemble the common mushroom and can be seen when the weather is warm and damp. These fungi may have gills or pores on the underside of the pileus (the cap).

The gills or pores are lined with a layer of basidia that produce spores. Some of these fungi are produced on the sides of trees or fallen wood and may be a centric. Many of them appear to arise from the ground. Colors of these fungi can be found anywhere in the rainbow, but most appear in earth tones.

Some of the major orders within the hymenomycetes are the Agaricales and the Boletales. The Agaricales contain the fungi that produce gills on the underside of the pileus. The Boletales contains the fungi that produce pores on the underside of the pileus.

For the most part, the Agaricales are fleshy fungi that are supple at maturity and last for no more than a week or two in nature. The Boletales are also fleshy and can be confused with other fungi that have pores and are hard. Some of these hard fungi are common parasites of trees; the "shelves" that they produce can grow for years.

Gasteromycetes

The gasteromycetes are a much more diverse group of fungi. The fungi in this group are often associated with soil or decomposing organic matter, although some may be mycorrhizal (in a symbiotic association with plant roots). There is tremendous diversity in this group, and many scientists believe that this group is artificial (not based on evolutionary relationships).

Some of themore interesting fungi of this group are shaped like balls that lie on the soil. Members of Lycoperdales and Sclerodermatales produce ball-like basidiocarps that form at the soil line. The size can range from that of a small marble up to that of a soccer ball.

These are called puffballs, as they release spores in small clouds when kicked. In nature, the upper layers of the basidiocarp crack, and spores are released as drops of water, hitting the outer layer of the basidiocarp. Many of these are edible when properly identified.

Other interesting fungi are found in the order Nidulariales. These are called the bird's nest fungi, as the basidiocarp resembles a small bowl containing two or three small "eggs", which contain the basidia. When a droplet of water lands in the "nest" the "eggs" are thrown upward and outward.

As they are released, a small thread is pulled behind, and the thread sticks onto some part of a plant, such as a blade of grass. The "egg" will then degrade and release the spores to be disseminated in the wind.

Among themost bizarre fungi are the stinkhorns. These fungi produce basidiocarps from structures that look like chicken eggs. The elongate structure produces the basidia on the end in a mass of slimy, smelly mucus. Flies are attracted to the smell, land on the mucus, and fly away. The basidiospores adhere to their feet and drop off, thereby disseminating the fungus.

Basidioporic Fungi

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Basidiosporic fungi (also known as the Basidiomycota or Basidiomycotina) are fungi that produce sexual spores on a specialized cell called a basidium.

The basidiosporic fungi are the most diverse phylum of the fungi world, with more than 22,300 species described. Some of the fungi in this phylum are microscopic, while the larger members of this group produce fruiting structures that are basketball-sized and weigh in excess of 10 pounds.

This phylum contains fungi that fall into three classes: mushroom, rusts, and smuts—and range widely in appearance, from the common mushroom to weblike fungi with an odor that can be detected at several feet.

Taxonomy

The basidiosporic fungi are divided into three classes: Basidiomycetes (mushrooms); Teliomycetes (rusts); and Ustomycetes (smuts). The Basidiomycetes are the higher basidiosporic fungi, which are normally fleshy. They produce true basidiocarps, and the only spore formed is the basidiospore.

The other two classes both have more than one spore form and do not have extensive mycelium. The Teliomycetes are commonly called rusts and are serious biotrophic parasites of plants. The rusts are able to complete their life cycle only in the presence of living plant host tissue.

The Ustomycetes are commonly called smuts and are mostly minor pathogens of plants, especially monocots. Some smuts have been cultured in axenic culture, where they form a "yeastlike" phase. The yeastlike phase has no true mycelium but rather individual cells.

Basidium

The basidium is a single cell on which basidiospores are produced externally. The basidium forms either as the terminal cell of a dikaryotic mycelium or from a resting spore that initially is dikaryotic.

The dikaryotic mycelium or spore contains two haploid nuclei, one donated by each of the parent strains. As the basidium begins to form, the two nuclei migrate into the center of the cell and fuse, forming a diploid nucleus.

This nucleus then undergoes meiosis, forming four haploidnuclei. As this is occurring, the cell wall of the basidium begins to produce little extensions called sterigmata, upon which the basidiospores will form. The tips of the sterigmata then inflate, and one nucleus migrates into each forming basidiospore.

The basidiospore is haploid and has a very thin cell wall. The spore is normally transmitted in air currents. Upon germination, the basidiospore produces a haploid mycelium which will fuse with a compatible hyphae, producing a dikaryotic mycelium.

Spore release from the basidiumcan be either active or passive. Passive release occurs when the junction of the sterigma and basidiospore separates, releasing the spore. Active release is more specialized. When the basidiospore is forming, a small segment of the spore wall at the junction with the sterigma loosens and fills with either gas or liquid.

At the time of release, the fluid or gas escapes, propelling the basidiospore away from the basidium. The distance traveled is not great, just enough to make sure that the basidiospore is able to enter into air currents for dissemination.

Hyphal Structure

The hyphae of the Basidiomycetes are septate and have special modifications at the septa. When a cell divides, a cross wall forms between the two daughter cells. With the dikaryotic hyphae of the Basidiomycetes, as the cell divides, the nuclei migrate toward the apex of the hyphae.

The nuclei then undergo mitosis, with one of the nuclei migrating into a small outgrowth of the hyphae and the other migrating backward. Septa form, creating a new dikaryotic cell near the apex and two haploid cells, one in line and the other as the outgrowth.

The outgrowth then turns and fuses with the haploid cell, and the nucleus migrates back to form a dikaryotic cell. The outgrowth remains visible with a microscope and is called a clamp connection.

The reproductive structure of the Ustomycetes is called a sorus. The sorus is a mass of dikaryotic spores that are normally dark brown or black in color. The sorus is formed in meristematic regions of the plants. The spores are called probasidia, because they form basidia when they germinate.

With the Teliomycetes, there are up to five distinct spore forms. The basidiospore lands on a susceptible plant and germinates, producing a haploid mycelium that infects the plant. The infection results in the formation of a haploid spermagonium that produces both spermatia and receptive hyphae.

When a compatible spermatia and receptive hypha combine, a dikaryotic hypha is produced, which initiates formation of an aecium. The aecium produces dikaryotic spores that are transmitted by air currents and infect another plant.

The resultant infection produces a subcuticular or subepidermalmass of thin-walled spores. These dikaryotic spores are called urediniospores and are formed in the uredinium. The urediniospores are blown by air currents and produce reinfection of the same species of plant.

At the end of the growing season, infections by urediniospores will result in the formation of a subcuticular or subepidermal mass of thick-walled spores called teliospores which are formed in the telium. These spores are initially dikaryotic but then become diploid and finally germinate by formation of the basidium.

Basidiocarps

The basidiocarp is the fruiting body of the higher Basidiomycetes. This structure is multicellular and composed of hyphae. The basidiocarp resembles the familiar image of the mushroom. The mushroom consists of a stalk (stipe) which has a cap (pileus)on top.

The stipe can be as tall as a meter (40 inches), and the pileus as long as ameter in diameter. Alternatively, both parts could be less than a centimeter in size. The pileus has pores or gills on the underside, where the basidia are produced. The layer of basidia is called a hymenium or "fertile layer."

Other kinds of basidiocarpsmay be found in nature. Some are totally enclosed and remain on the ground, looking much like a golf ball. These are called puffballs. As the puffball matures, the other layers begin to crack at the apex.

When drops of rain fall, the force of the impact causes spores to puff out of the opening. Another kind of puffball is the earthstar. In these unique fungi, the outer layers pull away from central part of the puffball and form a starlike pattern on the ground.

Ecological Importance

The basidiosporic fungi all play important roles in ecosystems. The rusts and the smuts are impor- tant plant pathogens, capable of great destruction of crops. These fungi have been known for thou- sands of years and are some of the most devastating fungi around.

The mushrooms are part of the natural cycle of decay. They are found on the ground or on wood and are the later stages of decay of organic matter. Some mushrooms are found on living plants, where they can be serious pathogens. Others are edible and are excellent sources of digestible protein. Still others are toxic or poisonous and can be fatal when eaten.

Stinkhorns and the bird’s nest fungi are unique basidiosporic fungi. The stinkhorns are basidiocarps that form on the soil and produce the basidia in a mass of putrid cells.The stench from the cells draws flies, which walk over the spores and then disseminate them. These can be found in wooded areas and can be detected by smell at distances of up to several meters.

The bird’s nest fungi look like small birds’ nests. The outer part of the basidiocarp resembles a small nest, up to an inch in diameter. On the inside, several small puffball-like structures can be found, with basidia on the inside. These look like small eggs.

When a drop of water enters the nest, the force thrusts the “egg” upward and extends a small cord from the back. The small cord catches hold of a plant and suspends the egg in the air. As the egg dries, it turns into a powdery mass, which is blown about by the wind.