Plant Life: March 2011

Genetically Modified Foods

Genetically modified

Applications of genetic engineering in agriculture and the food industry could increase world food supplies, reduce environmental problems associated with food production, and enhance the nutritional values of certain foods.

However, these benefits are countered by food-safety concerns, the potential for ecosystem disruption, and fears of unforeseen consequences resulting from altering natural selection. Humans rely on plants and animals as food sources and have long used microbes to produce foods such as cheese, bread, and fermented beverages.

Conventional techniques such as cross-hybridization, production of mutants, and selective breeding have resulted in new varieties of crop plants or improved livestock with altered genetics. However, these methods are relatively slow and labor-intensive, are generally limited to intraspecies crosses, and involve a great deal of trial and error.

Gregor Mendel (1822-1884)

Mendelian genetics is the classical mechanistic explanation of heredity in sexually reproducing organisms. It explains how genetic information is passed from one generation to another.

In 1866 the Augustinian monk Gregor Mendel (1822-1884) published a paper titled “Versuch über Pflanzenhybriden” (Experiments in Plant Hybridisation, 1910), describing the heredity of mutant characteristics of garden peas.

Mendel founded the modern science of genetics with these experiments, because they led him to propose the existence of hereditary factors, now called genes, and rules describing their inheritance, now referred to as Mendel’s laws. The importance of Mendel’s work was not recognized until 1900, sixteen years after his death, when the movements of chromosomes during cell division were carefully studied.

Genetic mutation

A mutation is a heritable, sudden change in the structure of a gene, which has no relation to the individual’s ancestry. The change can occur spontaneously or can be the result of exposure to ultraviolet radiation or chemicals.

In 1901 Hugo de Vries coined the term mutation to describe changes in the hereditary material of evening primrose (Oenothera). “Mutation” is a derivative of the Latin verb mutare, meaning “to move or change.” The word was first used to describe spontaneous, heritable changes in the phenotype of an organism.

In the modern era of genomics, mutations can be defined as changes in DNA (deoxyribonucleic acid) sequences, that is, changes in the structure of a gene. The changes can occur spontaneously or can be induced via ionizing radiation (ultraviolet radiation) or chemicals, such as aflatoxin B1 and ethylmethane sulfonate.

Thirty years after the work of Gregor Mendel in the nineteenth century, several rediscoveries of his work in genetics brought his theories to the fore. At about the same time, the discovery of chromosomes, coupled with the earlier knowledge, took genetics in a new direction.

Gregor Mendel (1822-1884) is often considered the founder of the science of genetics. Though his experiments with pea plants became the basis for understanding genetics in all plants and animals, he died unknown. In 1900 three simultaneous “rediscoveries” of Mendel’s studies put his name at the forefront of biology.

With the reintroduction to the world of Mendel’s genetic laws, biologists began to look more closely at genetic phenomena. These researchers used Mendel’s laws as the basis for more in-depth studies of genetics, which led to the modern understanding of genes, chromosomes, and their inheritance.

Germination

With germination, the growth of a seedling, spore, or bud begins. Seedling development begins with the close of germination. To germinate, seeds must be nondormant and in a suitable environment. Seeds germinate within a restricted range of temperatures, moisture, oxygen, light, and freedom from chemical inhibitors.

Wild seeds display many adaptations that predispose germination within specific habitats and seasons. By contrast, seeds of crops and other cultured plants usually lack controls that prevent germination.

The control system was lost because some seeds in the population lacked controls and were chosen when they germinated in the care of a culturist. For that reason, most cultivated plants that start from seeds show little or no germination control. Most of the information on germination control, therefore, covers wild species of plants.

Ginkgo

The ginkgos, phylum Ginkgophyta, constitute one of four phyla of the gymnosperms in the kingdom Plantae. Ginkgo biloba, the maiden hair tree, is the only living representative of the ginkgo family, Ginkgoaceae, a group of plants that have lived for millions of years and are identified by an abundant fossil record.

The ginkgo is a hairless, deciduous tree with a straight trunk and pyramid-shaped foliage usually sparsely branched when young, becoming denser with age. Leaves are fan-shaped, 2 to 3 inches (5 to 7.5 centimeters) across, sometimes divided into two lobes.

The ginkgo normally reaches heights of 80 to 100 feet (24 to 30meters) and under favorable conditions grows to 125 feet (38 meters) or more. The bark is reddish-gray and corky, with irregular, wide fissures dividing rough plates. On old trees, the bark becomes gray, rough, and deeply furrowed.

Glycolysis

Glycolysis is the beginning of the process of extracting usable energy from food. The disposal of the products of glycolysis when there is no oxygen available is the process of fermentation.

The simple sugar glucose is generally considered the starting point for looking at glycolysis and fermentation. Glucose is a simple carbohydrate, consisting of carbon, hydrogen, and oxygen.

Most glucose is produced by plants; organisms that cannot photosynthesize must obtain glucose (or more complex carbohydrates) from their surroundings. Animals obtain food molecules by eating. Simpler forms of life, such as bacteria and yeast, simply absorb their food from their environment.

Gnetophytes

The gnetophytes are a small group of vascular seed plants composing the phylum Gnetophyta, which is one of four phyla of gymnosperms that have living representatives.

The Gnetophyta include only three genera Ephedra, Gnetum, and Welwitschia—each of which belongs to a separate family, in a single order, the Gnetales. The gnetophytes have a number of features in common with the flowering plants (phylum Anthophyta, the angiosperms), which has sparked scientific interest in the evolutionary relationships between the two groups; they are the only gymnosperms, for example, in which vessels occur.

There are about ninety species of gnetophytes. They are diverse in form and size, and their distribution varies widely, from moist, tropical environments to extremely dry deserts. Most gnetophytes are shrubs or woody vines. The leaves occur oppositely or in whorls of three.

Grains

Grains are the fruits or seedlike fruits of plants, particularly members of the grass family, Poaceae. Important cereal grain crops are all produced by annual grasses and are dry (desiccant) fruits with the ovary wall fused to the seed coat. Inside the fruit wall-seed coat covering (the bran) is a small embryo (germ) and a large amount of stored food (endosperm).

Grains were the first domesticated crops and allowed the development of all of the great early civilizations. Several factors contribute to the importance of grains in agriculture: ease of growth, storage, and preparation; high yields; and high-energy, easily digestible content (starch).

The wild relatives of cereal grains all disperse their seeds by the shattering, or breaking apart, of mature fruiting stalks. Harvesting these wild species is a problem because the seeds are flung everywhere when the fruiting stalk is disturbed. A first step in the domestication of all grains was the elimination of shattering so that inflorescences could be harvested.

Grasses

Grasses are monocotyledonous flowering plants (phylum Anthophyta, the angiosperms) belonging to the family Poaceae, formerly Gramineae. The family is widespread and economically very valuable. All the grasses are herbaceous except for the bamboos, some of which are treelike.

Grasses arose seventy million to eighty million years ago, in the Late Cretaceous period of the Mesozoic era. They succeeded partly because they concentrate their growth lower down on the leaf and stem than other plants do and thus can regenerate quickly when fire or herbivores remove the top part of the plant.

This makes them ideal for human uses such as lawns and pasture for domestic livestock. Grasses are a very important food source for humans. Grasses provide all cereal grains—barley, corn, millet, oats, rice, rye, and wheat. Other grasses used for food include sorghum, sugarcane, and bamboo.

Mongolian grassland

Grasslands are areas of intermittent rainfall which favor grass growth. The grass helps the soil become rich by facilitating the accumulation of nutrients and decaying plant material.

Grasslands once covered about a quarter of the world’s land surface. Grasses’ growth patterns help enrich the soil immensely. Because their soils become among the world’s richest, grasslands are so intensely farmed and grazed that only small patches of natural grassland remain.

Climate and Geographic Location

Annual precipitation between 10 and 32 inches (25-80 centimeters), often with a dry period late in the growing season, supports grassland. Grassland temperature patterns vary. Fire and grazing favor grasses and often combine with climate to maintain grasslands.

An algae covered rock

The green algae are a diverse group of eukaryotic organisms classified in the phylum Chlorophyta. They are considered eukaryotic because individual cells possess a prominent structural feature known as a nucleus, which houses the chemicals responsible for heredity and metabolic regulation.

The phylum is one of several algal phyla in the kingdom Protista, where algae are grouped based upon pigmentation, carbohydrate storage reserves, and cell wall composition.

Green algae are found in moist soils and fresh-water and saltwater habitats; most are believed to be freshwater-dwelling. The phylum consists of at least eight thousand species. Some estimates place this number at seventeen thousand species. Several shared characteristics support the hypothesis that these organisms and terrestrial plants derived from a common ancestor.

Green revolution

The Green Revolution implemented advances in agricultural science to raise food production levels, particularly in developing countries. These advances are associated with the spreading use of high-yield varieties (HYV) of wheat, rice, and corn developed through advanced methods of genetics and plant breeding.

Yield Increases

The Green Revolution can be traced back to a 1940 request from Mexico for technical assistance from the United States to increase wheat production. By 1944, with the financial support of the Rockefeller Foundation, a group of U.S. scientists were researching methods of adapting the new high-yield variety (HYV) wheat that had been successfully used on American farms in the 1930’s to Mexico’s varied environments.

A major breakthrough is attributed to Norman Borlaug, who by the late 1940’s was director of the research in Mexico. For his research and work in the global dissemination of the Mexican HYV wheat, Borlaugwon the 1970 Nobel Peace Prize.

Growth and Growth Control

The processes of primary and secondary growth take a plant from early cell division to its adult form.Growth control factors regulate these processes.

Plant growth is of two distinct types: primary growth and secondary growth. Primary growth results in increased length of stems or roots. Secondary growth increases the width of the plant and allows differentiation of cells into various distinct tissue types. Both types of growth occur in plant tissues called meristems.

A meristem consists of tissue where extensive cell division takes place, and thus plant growth. There are two general types of meristems. Primary growth occurs at the apical meristems, and secondary growth occurs at the lateral meristems, which are known as the vascular cambium and the cork cambium.

Growth Habits

Plant habit, also known as plant life form, is the characteristic shape, appearance, or growth form of a plant species. It develops from specific genetic patterns of growth in combination with environmental factors and is part of the organization of every plant.

Growth of Plants

Development of a plant body is accomplished through growth, defined as increase in number of cells and size of a species. Rates of growth in plants are achieved in two ways: first, by geometric increase, in which all cells of the organism divide simultaneously, especially in a young embryonic plant; second, by arithmetic increase, in which only one cell undergoes division, especially in mature plants with localized growth in a region at the root and shoot apices.

Generally, plants grow by a combination of both kinds of cell division to produce variations of form that finally develop a specific habit that is unique to a particular plant species.

Gymnosperms: pine trees

Pine trees are a familiar example of gymnosperms, a series of evolutionary lines of woody vascular seed plants that produce seeds not encased in an ovary.

Two kinds of higher plants—the gymnosperms and angiosperms—have developed to become the dominant type of land plant. With the exception of a few aquatic angiosperms, they do not require water for pollen transfer and are thus free to live in a wide variety of habitats.

Gymnosperms and angiosperms differ primarily in the amount of protection they provide their ovules (the part that, after fertilization, becomes a weed), with gymnosperms usually providing less than angiosperms.

Halophytes: Sea Arrowgrass (Triglochin maritimum)

Halophytes are salt-resistant or salt-tolerant plants that thrive and complete their life cycles in soils or waters containing high salt concentrations. Despite high salt content in the tissues of halophytes, they can be grown and harvested as food or animal fodder.

The Salinity-Plant Relationship

Salts are ionic molecules that typically dissolve in water and split into cations (positively charged ions) and anions (negatively charged ions). Rain dissolves salts from minerals in rocks and soil.

These dissolved salts then either enter surface waters or percolate down to an aquifer. In arid regions where there is low rainfall, water evaporates quickly, leaving its salt load behind, leading to the development of saline soils, salt lakes, and brackish groundwater.

Haptophytes

The algal phylum Prymnesiophyta, or Haptophyta, is a monophyletic taxon that contains two hundred to three hundred extant species that are 4-40 microns in size.

The phylum Haptophyta is divided into two subclasses, the Prymnesiophycidae and Pavlovophycidae, that differ significantly from one another. The phylum is usually classified within the kingdom Chromista with other algae containing chlorophyll a and c (excluding dinoflagellates), but their exact relationship to the heterokont algae remains unclear.

Although a few freshwater species are known, most are marine, and species diversity within the phylum is greatest in nutrient-poor waters of the tropical and subtropical open oceans. Most are motile biflagellate single cells or nonmotile coccoid (walled unicells) cells. Colonial, filamentous, and palmelloid forms are also known.

Hardy-Weinberg Theorem

The Hardy-Weinberg theorem is the principal that, in the absence of external pressures for change, the genetic makeup of an ideal population of randomly mating, sexually reproducing diploid organisms will remain the same, at what is called Hardy-Weinberg equilibrium.

Population genetics is the branch of genetics that studies the behavior of genes in populations. The two main subfields of population genetics are theoretical (or mathematical) population genetics, which uses formal analysis of the properties of ideal populations, and experimental population genetics,which examines the behavior of real genes in natural or laboratory populations.

Population genetics began as an attempt to extend Gregor Mendel’s laws of inheritance to populations. In 1908 Godfrey H. Hardy, an English mathematician, and Wilhelm Weinberg, a German physician, each independently derived a description of the behavior of allele and genotype frequencies in an ideal population of randomly mating, sexually reproducing diploid organisms.

Heliotropism

Heliotropism is a growth movement in plants that is induced by sunlight. It is sometimes called solar tracking, a directional response to the sun. Because plants react in a similar way toward artificial sources of light, heliotropism is sometimes termed phototropism, a growth movement induced by any light stimulus.

Plants that orient their leaves to receive maximum sunlight are called diaheliotropic. Diaheliotropism is the tendency of leaves or other organs of a plant to track the sun by turning their surfaces toward it. Tracking the sun maximizes the amount of direct solar radiation received. Diaheliotropic movements can increase radiation interception, enhance photosynthesis, and increase growth rates of plants.

Plants that move their leaves to avoid sunlight are called paraheliotropic. Paraheliotropism is a plant response to minimize surface exposure to the sun. By orienting leaves and other plant organs parallel to the sun’s rays, light absorption is minimized. It is a process that some plants use to reduce dehydration by reducing leaf temperatures and water loss during times of drought.

Spraying herbicides

Herbicides are a class of pesticide used to kill or otherwise control weeds and other unwanted vegetation.

Herbicides are used to control grasses, weeds, and other plant pests. These chemical compounds kill plants or inhibit their normal growth. In general, herbicides work by interfering with photosynthesis, so that a plant dies from lack of energy or by a combination of defoliation (leaf removal) and systemic herbicidal action.

Herbicides are used to clear rights-of-way beneath power lines and along railways and roads. In agriculture and forest management, they are used to control weeds or to remove the leaves from some crop plants to facilitate harvesting.

Herbs

The term “herb” has a variety of meanings but is most frequently used in one of three ways. To a botanist, an herb is a plant with a soft, flexible stem and a life cycle that is completed in one growing season. A person interested in medicinal plants would use the word “herb” to describe any plant with medicinal properties. In a culinary situation, an herb is a plant used to impart flavor to food.

Herb or Spice?

Herbs and spices are both used in cooking to modify the taste and smell of food. A clear distinction between herbs and spices is difficult to draw, but there are some broad differences that are useful to know.

Plants referred to as herbs, such as basil (Ocimum basilicum) or rosemary (Rosmarinus officinalis), typically have been used in temperate regions throughout much of recorded history.

Heterokonts

Heterokonts are a group of closely related phyla with flagella in pairs, one long and one short. They include oomycetes, chrysophytes, diatoms, and brown algae.

The term “heterokont” refers either to the flagellar arrangement of biflagellate cells in which the two flagella differ in length (as in anisokonts), type of motion, or ornamentation, or to those organisms (and organisms evolutionarily derived from such lineages) in which biflagellate cells with heterokont flagella are produced at some point during their life cycle.

The most common heterokont flagellar arrangement consists of a posteriorly directed whiplash flagellum and an anteriorly directed tinsel flagellum. The tubular tinsel flagellum characteristically bears two rows of stiff, glycoproteinaceous, tripartite hairs previously referred to as mastigonemes but now increasingly referred to as stramenopili. Such organisms are often referred to as stramenopiles.

High-yield Crops

High-yield agricultural crops are those that have been bred, genetically modified, or fertilized to increase their production yields.

The health and well-being of the world’s growing population are largely dependent on the ability of the agricultural industry to raise high yielding food and fiber crops. No one knows for certain when the first crops were cultivated.

At some time in the past, people discovered that seeds from certain wild grasses could be collected and later planted where they could be controlled during the growing process and eventually harvested for food. Agriculture was firmly established in Asia, India, Mesopotamia, Egypt, Mexico, Central America, and South America at least six thousand years ago.

Plant hormones

Plant hormones are the major group of chemical messengers by which most plant activities are controlled. The five different groups of hormones regulate virtually every aspect of plant growth and development.

The majority of higher plants begin life as seeds. When seeds germinate, the embryonic tissues begin to grow and undergo differentiation until, ultimately, the various parts of the mature plant are formed.

Every aspect of this growth and development is regulated by a group of chemical messengers called hormones. These plant hormones, or phytohormones, function as plant regulators. A plant regulator is an organic compound, other than a nutrient, which in small amounts promotes, inhibits, or otherwise modifies a basic plant process.

Hornworts

Hornworts are small, short, nonflowering, nonvascular plants which live both on land and in water. They represent an early land plant group and belong to three hundred species in the order Bryophyta. Related to mosses and liverworts, hornworts are sometimes called horned liverworts.

Evolutionary biologists have found a few fossils that could be the first known hornwort specimens. Fossilized spores dating from the late Cretaceous period, about 100 million years ago, are considered to be from hornworts.

Other fossils dated earlier in that period resemble hornworts but might actually have been liverworts. Spores from the Miocene epoch, approximately twenty-six million years ago, are the most common hornwort fossils.

Horsetails

The plants known as horsetails or scouring rushes belong to the genus Equisetum, the only remaining genus in the phylum Sphenophyta, a group of seedless vascular plants.

Members of the phylum Sphenophyta, the horsetails, reached their maximum diversity during the Late Devonian and Carboniferous periods. One fossil group of the Sphenophyta, the calamites, grew from 12 to 18 meters (24 to 60 feet) in height, with trunks as much as 45 centimeters (more than 3 feet) in diameter.

Today these ancient plants survive in the single genus Equisetum. Equisetumis found throughout the world and, depending on the classification scheme, comprises between fifteen and twenty-five species.

Horticulture

Horticulture is the branch of agriculture that is connected with the production of plants that are directly used by people for food, medicine, and aesthetic purposes.

The ability to produce crops, particularly those crops associated with food and fiber, is the multidisciplinary science of intensively cultivating plants to be used by humans for food, medicinal purposes, or aesthetic satisfaction.

Crop production is largely determined by a variety of environmental conditions, including soil,water, light, temperature, and atmosphere. Therefore, horticulture science is primarily concerned with the study of how to manipulate the plants or these environmental factors to achieve maximum yield.

Human Population Growth

Since the Industrial Revolution of the nineteenth century, human populations have experienced a period of explosive growth. Overpopulation now poses a real threat to plant lives, ecosystems, and the long-term sustainability of the earth’s current ecological balance.

Just eleven thousand years ago, there were only about five million humans who lived on the planet Earth. The initial population growth was slow, largely because of the way humans lived—by hunting. Such a mobile lifestyle limited the size of families for practical reasons.

When simple means of birth control, often abstention from sex, failed, a woman would elect abortion or, more commonly, infanticide to limit her family size. Furthermore, a high mortality rate among the very young, the old, the ill, and the disabled acted as a natural barrier to rapid population growth.

Hybrid zones

Hybrids are offspring of parents from different species. Hybrid zones are areas where such different species overlap and crossbreed. Hybridization and hybrid zones have played a major role in the formation of new species in a number of plant groups and represent major factors in the evolutionary process.

A hybrid individual is produced from successful matings (cross-pollination) between individuals from different species or between individuals from different populations that differ markedly in one or more heritable traits. The mating process by which hybrid offspring are produced is hybridization. A distinction needs to be made between natural and artificial hybrids.

An artificial hybrid typically involves direct human intervention in an effort to obtain plants with agricultural or horticultural properties superior to those of either parent. Natural hybrids do not involve human intervention; they occur naturally.

Hybridization

Hybridization is the process of crossing two genetically different individuals to result in a third individual with a different, often preferred, set of traits. Plants of the same species cross easily and produce fertile progeny. Wide crosses are difficult to make and generally produce sterile progeny because of chromosome-pairing difficulties during meiosis.

Hybridization is the process of crossing two genetically different individuals to create new genotypes. For example, a cross between a parent 1, with the genetic makeup (genotype) BB, and parent 2, with bb, produces progeny with the genetic makeup Bb, which is a hybrid (the first filial generation or F1). Hybridization was the basis of Gregor Mendel’s historic experiments with garden peas. Inheritance studies require crossing plants with contrasting or complementary traits.

Hybridization of plants occurs in nature through various mechanisms. Some plants (such as the oil palm) are insect-pollinated, and others (such as maize, or corn) are wind-pollinated.

hydrologic cycle

The hydrologic cycle is a continuous system through which water circulates through vegetation, in the atmosphere, in the ground, on land, and in surface water such as rivers and oceans.

The sun and the force of gravity provide the energy to drive the cycle that provides clean, pure water at the earth’s surface. The total amount of water on earth is an estimated 1.36 billion cubic kilometers. Of this water, 97.2 percent is found in the earth’s oceans. The ice caps and glaciers contain 2.15 percent of the earth’s water.

The remainder, 0.65 percent, is divided among rivers (0.0001 percent), freshwater and saline lakes (0.017 percent), groundwater (0.61 percent), soil moisture (0.005 percent), the atmosphere (0.001 percent), and the biosphere and groundwater below 4,000 meters (0.0169 percent). While the percentages of water appear to be small for these water reservoirs, the total volume of water contained in each is immense.

Hydroponics

Literally “water culture,” hydroponics originally referred to the growth of plants in a liquid medium. It now applies to all systems used to grow plants in nutrient solutions with or without the addition of synthetic soil for mechanical support.

Hydroponics has become an important method of crop production with the increase in the number of commercial greenhouse operations. Greenhouses are utilized in the production of a wide array of bedding plants, flowers, trees, and shrubs for commercial as well as home and garden use.

Cash receipts from greenhouse and nursery crops total more than $4 billion annually. In some arid regions, the majority of vegetable crops are produced in greenhouses.

Inflorescence

The term "inflorescence" refers to the arrangement of flowers on a floral axis. Most schemes that define inflorescence types separate solitary flowers from flower clusters and stipulate that an inflorescence is a cluster of two or more flowers.

It is not always easy to distinguish between solitary flowers and an inflorescence. An examination of the evolutionary development of the flower and the inflorescence provides some insight into the problem. It generally is accepted that the flower arose as a modified stem tip that bore male and female reproductive structures at its apex.

These reproductive structures became the pistils and stamen of the flower. Leaves that immediately subtended the reproductive structures became the sterile parts of the flower (petals and sepals) and are typically more leaflike as distance from the apex increases.

Irrigation

Irrigation techniques supply additional water to arid and semiarid horticulture or farming regions where few, if any, crops could otherwise be grown.

Approximately 350 million acres (142 million hectares) of land worldwide are irrigated. In the United States more than 10 percent of the crops, encompassing approximately 50 million acres (20 million hectares), receive water through irrigation techniques; 80 percent of these are west of the Mississippi River.

In countries such as India, Israel, North Korea, and South Korea, more than one-half of food production requires irrigation. From 1950 to 1980, the acreage of irrigated cropland doubled worldwide. Increases since then have been more modest.

Leaf Abscission

In the process of leaf abscission, plants periodically shed their leaves. Leaf abscission involves a number of biochemical and physical changes that are largely controlled by plant hormones.

Plants are primarily categorized as annuals, biennials, or perennials, based on their growth patterns. Annuals are those plants that undergo a complete life cycle from seed to seed in one growing season.

Biennials require two growing seasons to complete a life cycle; during the first year, only vegetative growth takes place. The above ground portion dies through the winter, and in the next growing season the roots send up a reproductive shoot that produces the seeds.

Leaf Anatomy

The leaf has evolved as the chief part of the plant for gathering light energy from the sun and conducting photosynthesis to transform that light energy into biochemical energy. Hence, its structure is adapted to that function.

Leaves are formed by a plant to manufacture food. Photosynthesis—a complicated chemical reaction in which carbon dioxide from the air and water from the soil, in the presence of light, produce sugar—is carried out in the chloroplasts found packed within the leaf cells.

Because energy is derived from light by chlorophyll, either the leaf must be thin enough for light to penetrate all the cell layers or, in the case of plants with succulent leaves, chloroplasts must be most concentrated near the surface of the leaves.

Leaf Arrangements

The study of leaf arrangements, or phyllotaxy, considers not only the descriptive classification of leaf arrangements but also theories regarding the cause of such arrangements.

The function of the arrangement of leaves (phyllotaxy) is to increase a plant’s ability to carry on photosynthesis by positioning the leaves in such away as tomaximize the surface area available to intercept sunlight. Leaves may be either caulescent (on obvious stems) or acaulescent (with no obvious stems).

Flowering plants have three basic types of arrangements: alternate spiral; opposite; and whorled or verticillate. The alternate spiral arrangement is generally considered to be the most primitive condition, with the opposite and whorled conditions being derived by suppression of internode development.

types of leaves

The pattern of leaf lobes (projections) or divisions, leaf arrangement, the number, and the shape of leaflets composing compound leaves are often useful characteristics for identification of plants.

Leaves, the main photosynthetic organs of plants, are usually green, flattened structures that are formed as lateral outgrowths at stem nodes. Simple leaves are composed of a single lamina, or blade, which may be attached to the stem via a cylindrical structure called a petiole.

Leaves lacking a petiole are called sessile. Laminae of simple leaves may exhibit various patterns and degrees of lobing, which are often characteristic of individual species of plants and, together with reproductive features, are used in plant identification.

Other species have compound leaves, in which the leaf laminae are subdivided into smaller leaflets. The pattern of arrangement, the number, and the shape of leaflets comprising compound leaves are often useful characteristics for identification of plants.

Some species of plants exhibit either gradual or abrupt changes in leaf lobing and division during development and are called heterophyllous. For example, some species exhibit a mixture of pinnatifid (pinnately lobed) and pinnatisect (pinnately compound) leaves on the same stem.

Heterophylly is often observed in water plants, with one form of leaf being produced where the plant stem is submerged and another being produced where the stem is above water.

Leaf margins

The flattened part of the leaf is the leaf blade or lamina,which can be subdivided into three discrete regions: The tip or apex is the part of the lamina farthest removed from the point of attachment of the leaf to the stem. The base of the leaf is the part of the lamina that is closest to the point of attachment of the leaf to the stem. The margin is the perimeter of the leaf between the apex and base.

The form, ormorphology, of leaves is often characteristic of individual species of plants and, like the reproductive features, is an important base of plant identification.

Some plants have a more or less cylindrical petiole that joins the base of the leaf to the stem, while others lack a petiole and are called sessile. The midrib is the prominent vein that subdivides the leaf into two halves from base to apex.

Leaf shapes

The overall shapes of leaves and leaflets are often characteristic of individual species of plants and, together with reproductive features, are used in plant identification.

To assess the shape of a leaf, one examines the outline formed by the apex, margin, and base of the leaf or leaflet. If the leaf has teeth or lobes along its margin, one imagines a smooth curve interconnecting the tips of the teeth or lobes to assess the overall shape.

The shapes of compound leaves can likewise be assessed by imagining a smooth curve connecting the tips of the leaflets that form the compound leaf blade. The terminology below is used to describe the shape of laminae of simple and compound leaves as well as the laminae of the leaflets of compound leaves.

Elongated Leaves

Linear leaves have a long and very narrow leaf shape, with sides that are almost parallel with one another and are usually more than four times longer than broad. Oblong leaves have a rectangular leaf blade two to four times longer than it is wide, with sides that are almost parallel to each other.

Legumes

Legumes, including peas and beans, are among the most important staple food crops worldwide. Legumes are also planted among other crops because of their ability to enrich soil nitrogen content.

Legumes are plants of the pea or bean family classed in the family Leguminosae, which is referred to as the family Fabaceae in North America. With 18,000 species and 650 genera, the legumes are one of the largest families of plants in the world.

The vast majority of legumes are herbaceous plants, but family members range in size from dwarf willow like herbs of Arctic and alpine habitats to massive tropical trees. Most are flowering plants, but the family also includes a number of shrubs and trees, some quite tall and many bearing thorns or spines.

Lichens

Lichens are composed of two distinct species, a fungus and a photosynthetic alga (or bacterium) that have coevolved to live in a symbiotic relationship with each other as a single life-form that grows on rocks, trees, and other substrates.

Lichens are classified as members of the kingdom Fungi, with most being placed under the phyla Ascomycota and Basidiomycota. It is estimated there are seventeen thousand species of lichen, representatives of which have been found nearly everywhere in the world.

Symbiosis

Symbiosis is an extreme form of an ecological relationship known as mutualism between members of different species, in which each partner in the union derives benefits from the other. In symbiotic unions, the partners are so dependent on each other they can no longer independently survive.

Lipids

Lipids are a diverse group of compounds sharing the common property of being hydrophobic (insoluble in water). Lipids include fatty acids, fats, oils, steroids (sterols), waxes, cutin, suberin, glycerophospholipids (phospholipids), glyceroglycolipids (glycosylglycerides), terpenes, and tochopherols.

Lipids are ubiquitous in plants, serving many important functions, including storage of metabolic energy, protection against dehydration and pathogens, the carrying of electrons, and the absorption of light. Lipids also contribute to the structure of membranes. In addition, plant lipids are agricultural commodities important to the food, medical, and manufacturing industries.

Fatty Acids

Fatty acids, the simplest of the lipids, are highly reduced compounds with a hydrophilic (watersoluble) carboxylic acid group and a hydrophobic hydrocarbon chain. Hundreds of different fatty acids have been isolated from plants.

Liquid Transport Systems

Liquid transport systems are structures that facilitate the movement of water, via the xylem, from a plant’s roots to its leaves. Water then evaporates from the leaves through the stomata in the process of transpiration.

Water is the most abundant compound in plant cells. It accounts for 85-95 percent of the weight of most plants. It even makes up 5-10 percent of the weight of “dry” seeds. More than 95 percent of the water gathered by a plant, however, evaporates back into the atmosphere, often within hours after being absorbed. This evaporation of water from a living plant is called transpiration. Most transpiration is from leaves.

Plants transpire huge amounts of water. On a warm, dry day, an average-size maple tree transpires more than 200 liters per hour, while herbaceous plants transpire their own weight in water several times per day.

Liverworts

Liverworts (phylum Hepatophyta) are one of three ancient lines of bryophytes (liverworts, hornworts, and mosses): low-growing land plants that depend on free water (rain) for fertilization.

Liverworts, with about six thousand species, generally prefer somewhat cooler, moister, shadier, and more acidic habitats than mosses. Like any bryophyte, a liverwort has a dominant (conspicuous) green gametophyte and a small, attached sporophyte, which is a single-stalked sporangium that developed from a fertilized egg.

As in hornworts, liverwort gametophytes are typically dorsiventrally symmetrical (flattened). A unique feature of liverworts is the presence, in the gametophyte, of oil bodies, cellular organelles that produce aromatic terpenoids.

Lycophytes

The lycophytes, which compose the phylum Lycophyta, are one of four phyla of seedless plants having vascular, or conducting, tissue. The living lycophytes are all small and herbaceous, whereas the extinct lycophytes included large trees, which were important in the formation of coal.

There are at least twelve genera and twelve hundred species of living lycophytes. These include plants known as club mosses and spike mosses (though none are true mosses) and quillworts. The lycophytes consist of three families, each belonging to a separate order. The family Selaginellaceae has a single genus, Selaginella.

Similarly, the family Isoetaceae has a single genus, Isoetes. The remaining genera belong to the family Lycopodiaceae. The living lycophytes are widely distributed but reach their greatest species diversity in the tropics. The lycophytes are similar to the higher vascular plants—the gymnosperms and angiosperms—in having vascular tissue and true leaves, stems, and roots.

Marine plants

Marine plants grow near the surface of salt water and ice, within reach of sunlight necessary for photosynthesis. Algae, the most plentiful type of marine plant, form the foundation of the food chain and crucial to a balanced ecosystem.

Water is essential to life. The earliest plants, primarily algae, formed in bodies of saline water covering prehistoric Earth. During the Silurian period, approximately 441 million to 410 million years ago, some aquatic plants began to grow on land, but many plants remained solely water based.

These marine plants have provided fundamental nourishment in the food chain. No marine animals would have evolved or been able to survive if marine plants had not existed.

Medicinal plant

Because plants are so biochemically diverse, they produce thousands of substances commonly referred to as secondary metabolites. Many of these secondary metabolites have medicinal properties that have proven to be beneficial to humankind.

The use of plants for medicinal purposes predates recorded history. Primitive people’s use of trial and error in their constant search for edible plants led them to discover plants containing substances that cause appetite suppression, stimulation, hallucinations, or other effects. Written records show that drugs such as opium have been in use for more than five thousand years.

From antiquity until fairly recent times, most physicianswere also botanists or at least herbalists. Because modern commercial medicines are marketed in neat packages, most people do not realize that many of these drugs were first extracted from plants.