Vacuoles

Vacuoles
Vacuoles
Vacuoles are receptacles within plant cells that hold water, enzymes, eacids, waste products, pigments, or other substances that serve the plant.

Vacuoles are the largest organelles in most mature plant cells. Frequently constituting more than 90 percent of the volume of a cell, the vacuole presses the rest of the protoplasm against the cell wall. Vacuoles are surrounded by a single fragile membrane called the vacuolar membrane, or tonoplast.

The contents of the vacuole, referred to as vacuolar sap, is 90 to 98 percent water. The vacuole of a typical plant cell occupies approximately 500,000 cubic micrometers. It would take approximately two million of these vacuoles to equal the volume of a sugar cube.

Vegetable Crops

Vegetable Crops
Vegetable Crops
Vegetables are plant products, either fruits or other fleshy parts of the plant, consumed in human diets. Although the word itself has no precise taxonomical significance, agriculturally vegetables constitute a significant sector of world food crops.

Although most of the world’s people consume “vegetables” daily, the word itself has no precise botanical or scientific meaning. Various vegetative (non reproductive) parts of plants are eaten as vegetables, but reproductive parts of plants such as cucumbers and tomatoes, which are technically fruits, are also consumed as “vegetables.”

Vegetative “vegetables” include the stems of celery, the tubers Irish (white) potatoes, and asparagus shoots. Other vegetables commonly consumed include roots: sweet potatoes, carrots, parsnips, and radishes, among others. Leaves eaten as vegetables include lettuce, spinach, collards, and cabbage.

Vesicle-Mediated Transport

Vesicle-Mediated Transport
Vesicle-Mediated Transport
Large substances such as proteins, some amino acids, and poly saccharides are transported into and out of plant cells by vesicle-mediated transport, which involves interaction with and fragmentation of the cell membrane to create a membrane-bound vesicle for internal distribution or external export. Once formed, the vesicle can be transported to its destination within the cell.

Plant cells use several methods to transport ions, polar molecules, and macro molecules through the cell membrane. Some of these can permeate the member via membrane via osmosis.

Small substances, mostly ions, can diffuse through pores composed of trans membrane proteins. Other substances, however—such as glucose, glycogen, and some amino acids—must be transported by membrane-bound carrier molecules in a process called vesicle-mediated transport.

Viruses and Viroids

Viruses are extremely small parasites that have none of the structures characteristic of living cells. Many viruses are little more than a protein-coated and particle-containing DNA or RNA.

The protein coat is specifically adapted for breaching the plasma membranes of host organisms. Viroids are even simpler parasites comprising small, single-stranded molecules of RNA with no protein coat. They are the smallest known agents of infectious disease.

The concept of a virus dates back to the late nineteenth century investigations of the mosaic disease of tobacco by Dmitri Iosifovich Ivanovsky in Russia and Martinus Willem Beijerinck in Holland.

Water and Solute Movement in Plants

Water and Solute Movement in Plants
Water and Solute Movement in Plants
Plants have two separate transport systems for conducting essential nutrients and water into and through the plant. These take the form of two types of vascular tissue.

One, for water and minerals, the xylem, originates in the root and moves water and minerals upward. The second, the phloem, moves dissolved carbohydrates out of the leaves to other plant parts in which they are used for growth or stored.

Vascular plant tissue is designed to meet the nutritional transport needs of land plants. Xylem tissue has two types of transport cells; both are non living when functional.The smaller in diameter is the tracheid. These have a narrow bore and tapering, overlapping ends.

Wetlands

Wetlands
Wetlands
Wetlands, transitional areas between aquatic and terrestrial habitats, are home to a variety of flood-tolerant and salt-tolerant plant species.

Wetlands represent one of the most biologically unique and productive of all natural habitats. In their unaltered state, these water-influenced areas are used by a variety of wildlife species.

These habitats also have the ability to take up and store water during floods, and their soils and plants have the ability to remove nutrients and heavy metals from water.

Wheat

Wheat
Wheat
Wheat (Triticum sativum) is the world’s most important grain crop, serving as a natural food source for much of the world’s population. The wheat grain is easily refined to raw foods such as flour, which can be used in countless recipes.

Throughout the world, large portions of agricultural land are devoted to the production of wheat. Wheat is the national food staple for more than forty nations and provides 20 percent of the total food calories for the world’s population; it is the major staple for about 35 percent of the people of the world.

In the United States, wheat constitutes a large part of the domestic economy, makes up a large part of the nation’s exports, and serves as the national bread crop.

Wood

Wood is a fibrous plant tissue that functions in support and water conduction. It composes the bulk of stems and roots in the magnoliids and eudicots of the angiosperms (phylum Anthophyta), as well as in the conifers (phylum Coniferophyta).

It is formed by thickening growth,which persistently adds newlayers that accumulate as a cylinder of wood between the pith and the bark.

Secondary Xylem

Technically, wood is secondary xylem. The growth in girth that produces it is called secondary growth. This growth occurs after the stem or root segment has completed its increase in length, or primary growth. Secondary growth also yields, in much smaller amounts, secondary phloem, which becomes a part of the bark.

Wood and Charcoal as Fuel Resources

Wood
Wood
Globally, the amount of wood and charcoal used for fuel exceeds the combined amount of wood used for all other purposes. Between 60 percent and 95 percent of the total energy needs of some developing countries are met by wood.

Wood is one of the oldest energy sources. Rough wood and bark may be burned directly for fuel, or wood may be converted into charcoal by charring in a kiln from which air has been excluded.

According to the Food and Agriculture Organization of the United Nations, more than half of all the wood utilized in the world at the end of the twentieth century was used for energy production.

Wood and Timber

Wood and Timber
Wood and Timber
The use of products derived from woody plants, notably timber, takes advantages of wood’s insulating ability, strength, workability, and abundance as a construction and engineering material.

No other material has all the advantages of wood. One material may equal wood in insulating quality but lack its abundance and low cost. Another may rival it in strength but fail on the point of workability. A third may rank with it in work-ability but fail to measure in durability. If wood were a newly discovered material, its properties would startle the world.

Since the human race first started to build crude shelters at the dawn of civilization, wood has been available as a construction material. Wood has long been used in the construction of buildings, bridges, and boats.

Yeast

Yeast
Yeast
The term “yeast” does not refer to any recognized taxonomic name but instead describes fungi that are unicellular and usually reproduce asexually by budding. The term yeast is also used, more specifically, for those species in the genus Saccharomyces that are used to leaven bread and ferment alcoholic beverages.

Among mycologists, there is some disagreement over what should be called a yeast. Many mycologists use the term to describe any fungus that has a unicellular budding format any time in its life. They often use the term “monomorphic” to describe those that are always unicellular and the term “dimorphic” to describe those that can have both unicellular and filamentous growth.

Others, however, reserve the name yeast for those species that are permanently unicellular and use the term “yeastlike” to describe those fungi that can alternate between mycelial and unicellular forms. Because some species that have traditionally been called yeasts have later been shown to have a mycelial form, the former broader definition will be used here.

Zosterophyllophyta

The Zosterophyllophyta are a phylum of extinct seedless vascular plants that have been recovered from fossils in the stratum ranging from the Early to the Late Devonian, approximately 408 million to 370 million years ago.

Today, scientists believe that zosterophyllophytes and lycopods arose at about the same time from a common but unknown ancestor. The zosterophyllophytes went extinct, but the lycopods have survived until today.

Characteristics

The Zosterophyllophyta evolved independently from the Rhyniophyta, but both groups shared a number of characteristics. Their aerial stems arose from a horizontal axis, the rhizome, when one of two branches formed by the forking rhizome turned and grew upward. Some of the zosterophyllophytes had no leaves, the common condition in the rhyniophytes.

Zygomycetes

Zygomycetes
Zygomycetes
Zygomycetes are a group of fungi that constitute the phylum Zygomycota. Also called zygote fungi, zygomycetes include about 750 species.

Most are saprobes, living on decaying plant and animal matter in the soil; some are parasites of plants, of insects, or of small soil animals; some cause the familiar soft fruit rot and black bread mold; and a few occasionally cause severe infections in humans and farm animals.

Zygomycetes share many common features with members of other phyla in kingdom Fungi. They are rapidly growing, nonphotosynthetic organisms that characteristically form filaments called hyphae.