Kids Research Express: August 2008

Almond

Almond, common name for a small tree of the rose family, and for the kernel of its fruit. The tree is characterized by the coarsely furrowed and wrinkled shell of the drupe and by the young leaves that have their sides folded along the central vein. It grows up to 9 m (30 ft) high. A native of western Asia, it now grows wild throughout southern Europe and is cultivated in the United States. The wood is hard, of reddish color, and is used by cabinetmakers. The almond is valued chiefly for its nut, which is an important article of commerce. Varieties are classified as either sweet or bitter. Sweet almonds contain a large quantity of a bland, fixed oil and emulsin, gum, and mucilage sugar; they have an agreeable taste and are nutritious. Bitter almonds contain the same substances and, in addition, a crystalline glucoside called amygdalin. The long almonds of Málaga, Spain, known as Jordan almonds, and the broad almonds of Valencia, Spain, are the most valued.

The dwarf almond tree, a low shrub, is similar to the common almond, with smaller fruit. It is common in the plains of Central Asia and is frequently planted as an ornamental shrub in England. Flowering almonds—shrubs or small trees—are cultivated extensively in the United States for their profusion of showy, white to rose blossoms.

Scientific classification: The almond belongs to the family Rosaceae. It is classified as Prunus amygdalus. The dwarf almond tree is classified as Amygdalus nana.

Plant

Plant, any member of the plant kingdom, comprising about 260,000 known species of mosses, liverworts, ferns, herbaceous and woody plants, bushes, vines, trees, and various other forms that mantle the Earth and are also found in its waters. Plants range in size and complexity from small, nonvascular mosses, which depend on direct contact with surface water, to giant sequoia trees, the largest living organisms, which can draw water and minerals through their vascular systems to elevations of more than 100 m (330 ft).

Only a tiny percentage of plant species are directly used by humans for food, shelter, fiber, and drugs. At the head of the list are rice, wheat, corn, legumes, cotton, conifers, and tobacco, on which whole economies and nations depend. Of even greater importance to humans are the indirect benefits reaped from the entire plant kingdom and its more than 1 billion years of carrying out photosynthesis. Plants have laid down the fossil fuels that provide power for industrial society, and throughout their long history plants have supplied sufficient oxygen to the atmosphere to support the evolution of higher animals. Today the world's biomass is composed overwhelmingly of plants, which not only underpin almost all food webs, but also modify climates and create and hold down soil, making what would otherwise be stony, sandy masses habitable for life.

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Poppy

Poppy, common name for a small family of herbaceous flowering plants occurring principally in the North Temperate Zone, and for its representative genus. The family contains about 23 genera and 210 species; many are important as ornamentals, and one species is the source of opium. Members of the family occupy varied habitats, but they are more common in open, well-drained areas. This preference helps explain why several members of the family, especially poppies, are bothersome weeds in cultivated fields. See also Bloodroot.

The representative genus contains about 50 species. The Oriental poppy is widely cultivated as an ornamental, and many color forms have been developed. The opium poppy produces several useful products. Its tiny seeds, produced in huge quantities in each of the plant's dry fruits, or capsules, are used in baking and produce an important drying oil. Opium is the dried sap, or latex, that is harvested from the capsules while they are still young. It contains many alkaloids, including morphine and codeine, that are useful in medicine. Heroin is synthesized from the morphine purified from the complex mixture of alkaloids in opium (see Drug Dependence).

The family shares its order with the fumitory family. This family contains about 16 genera and 400 species, also mostly found in the North Temperate Zone, with a few species located in mountainous regions of tropical Africa and South Africa. The most familiar member of the family is the bleeding heart; others are of minor ornamental importance.

The leaves are usually deeply divided and arranged in a rosette around the base of the short stem. The flowers have two to four sepals (outer floral whorls) and twice as many petals (inner floral whorls). The stamens (male floral organs) vary in number from six to many more, and the ovary (female floral organ) is superior (borne above and free from the other flower parts). The order characteristically has sap that is rich in alkaloids. The sap is clear and watery in the fumitory family, milky in the poppy family.

Scientific classification: Poppies make up the family Papaveraceae in the order Papaverales. The Oriental poppy is classified as Papaver orientale, and the opium poppy as Papaver somniferum. The family Fumariaceae is the only other family in the order Papaverales.

See picture of Opium Poppy and Golden Poppy.

Periwinkle

Periwinkle (plant), common name for herbs in a genus of the dogbane family. The leaves are opposite and evergreen. The flowers grow singly or in pairs from the axils of the leaves. The lesser periwinkle is a native of many parts of Europe, growing in woods and thickets. The greater periwinkle, which has much larger flowers and ovatocordate, or egg-shaped, leaves, is a native of southern Europe. Periwinkles are the source of alkaloids that are often used to treat cancer.

Scientific classification: Periwinkles make up the genus Vinca, of the family Apocynaceae. The lesser periwinkle is classified as Vinca minor and the greater periwinkle as Vinca major.

Cinchona

Cinchona, genus of tropical evergreen trees and shrubs of the madder family, yielding the medicinal bark variously known as Peruvian bark, Jesuits' bark, China bark, or cinchona bark, from which the drug quinine and related substances are obtained. All the cinchonas have laurel-like, entire, opposite leaves; stipules that soon fall off; and panicles of flowers that somewhat resemble those of the lilac. The flowers are white, rose, or purplish and very fragrant.

Scientific classification: Cinchonas belong to the family Rubiaceae. The species first discovered is classified as Cinchona officinalis. The important species found in Bolivia and southeastern Peru is classified as Cinchona calisaya, the species in Peru and Ecuador as Cinchona succirubra.

Ovule

Ovule, in botany, is the name applied to immature seeds, which are produced within the ovary of a flower. In flowering plants, the development of the ovule is generally as follows.

At the site of the future seed, an outgrowth, the nucellus or megasporangium, develops; this becomes covered by two integuments that grow up from its base, leaving an opening at the top called the micropyle. Within the nucellus is the megaspore mother cell. It divides into two and then into four; one of these megaspores then typically divides into eight nuclei to become the embryo sac of the female gametophyte. It is in this sac that the plant embryo will develop.

The young male plants, or male gametophytes, are popularly referred to as pollen grains; these are contained in modified leaves called stamens. When a pollen grain is placed on the stifma, it sends out a tube that grows down to the ovary and eventually enters the ovule. Two sperms are then discharged into the embryo sac; one of these fuses with the egg nucleus at the micropylar end of the embryo sac, and this fertilized egg then develops into the embryo of the seed. The other sperm fuses with two nuclei near the middle of the embryo sac; the resulting triple-fusion nucleus develops into the endosperm, which usually remains as the food storage tissue of the seed.

See Fertilization; Flower; Pollination.

Gamete

Gamete, sexual reproductive cell that fuses with another sexual cell in the process of fertilization. The cell resulting from the union of two gametes is called a zygote; the zygote usually undergoes a series of cell divisions until it develops into a complete organism.

Gametes, also called germ cells, vary widely in structure. The simplest sexual organisms are isogamous, that is, they produce a single kind of gamete. The identical gametes unite in pairs to produce zygotes. Although all isogametes are apparently alike in structure, they are thought to be different in physiological constitution, because gametes from the same individual do not successfully unite. The simplest isogametes, those of lower fungi such as molds, are small cells that grow on the ends of body filaments and become detached when mature. Other lower organisms, such as lower algae and protozoa, have gametes, which are formed by division of the protoplasm of single cells.

All higher plants are heterogamous, that is, they produce two kinds of gametes. The female gamete is called the egg; the male gamete is called the sperm. The organ of gamete production in plants is called a gametangium.

All animals and animal-like lower organisms that reproduce sexually, except a few protozoans, are also heterogamous. The male gametes are called spermatozoa; female gametes, ova or eggs. The gamete-producing organs of animals are called gonads. The formation of gametes in the gonads of animals is called gametogenesis. By this process the number of chromosomes in the sex cells is reduced in number from diploid to haploid, which is half the number of chromosomes in the normal body cells of the species. The diploid number of human chromosomes, for example, is 46. When a human sex cell divides to form two gametes, each gamete receives only half, or 23, of the normal complement of chromosomes. This type of cell division is called meiosis. The normal total of chromosomes is restored in fertilization when two gametes fuse, each contributing half of the chromosomes required by the zygote.

Fertilization

Fertilization is the process in which gametes—a male's sperm and a female's egg or ovum—fuse together, producing a single cell that develops into an adult organism. Fertilization occurs in both plants and animals that reproduce sexually—that is, when a male and a female are needed to produce an offspring (see Reproduction).

Fertilization is a precise period in the reproductive process. It begins when the sperm contacts the outer surface of the egg and it ends when the sperm's nucleus fuses with the egg's nucleus. Fertilization is not instantaneous—it may take 30 minutes in sea urchins and up to several hours in mammals. After nuclear fusion, the fertilized egg is called a zygote. When the zygote divides to a two-cell stage, it is called an embryo.

Fertilization is necessary to produce a single cell that contains a full complement of genes. When a cell undergoes meiosis, gametes are formed—a sperm cell or an egg cell. Each gamete contains only half the genetic material of the original cell. During sperm and egg fusion in fertilization, the full amount of genetic material is restored: half contributed by the male parent and half contributed by the female.

For information on plant fertilization see the articles on Seed, Pollination, and Plant Propagation.

Pollen grains

A pollen grain contains a sperm cell that fertilizes an egg. If fertilization is successful, a seed is produced. The pollen grains of each species display unique sculpting of the pollen wall, and fossilized pollen serves to identify ancient species.

Pollen grains are microscopic in size, ranging in diameter from less than 0.01mm (about 0.0000004 in) to a little over 0.5 mm (about 0.00002 in). Millions of pollen grains waft along in the clouds of pollen seen in the spring, often causing the sneezing and watery eyes associated with pollen allergies. The outer covering of pollen grains, called the pollen wall, may be intricately sculpted with designs that in some instances can be used to distinguish between plant species. A chemical in the wall called sporopollenin makes the wall resistant to decay.

Pollination

Pollination is a transfer of pollen grains from the male structure of a plant to the female structure of a plant. The pollen grains contain cells that will develop into male sex cells, or sperm. The female structure of a plant contains the female sex cells, or eggs. Pollination prepares the plant for fertilization, the union of the male and female sex cells. Virtually all grains, fruits, vegetables, wildflowers, and trees must be pollinated and fertilized to produce seed or fruit, and pollination is vital for the production of critically important agricultural crops, including corn, wheat, rice, apples, oranges, tomatoes, and squash.

Flowering plants use wind, insects, bats, mammals, and birds to transfer pollen from the stamen, or male portion of the flower, to the stigma, or female portion of the flower. Many plants have evolved closely with certain animals to ensure successful transfer of pollen. For example, many species of rain forest plants can only be pollinated by one particular species of insect, bird, or bat.

Sexual Propagation

In nature, sexual propagation begins when water, wind, insects, birds, or small mammals carry pollen randomly between plants (see Pollination). In flowering plants, this transfer of pollen enables the male sex cells, or sperm, of one flower to fertilize the female sex cell, or egg, of a second flower (see Fertilization).

The egg is located at the base of the flower in a structure called an ovule, found within the ovary. Depending on the species, an ovary contains one, several, or many ovules. The ovaries of peach and avocado flowers, for example, have one ovule, while those of watermelon and cantaloupe have many.

As the fertilized egg (or eggs) within the ovule begins to develop into an embryonic plant, it produces a variety of hormones that stimulate the outer wall of the ovule to harden into a seed coat. Other biochemical changes in the ovule produce a starchy substance that will be used as a food supply. In this way, the ovule ripens into a seed—a structure containing an embryonic plant and its food supply surrounded by a seed coat. The ovary, which houses the ovule or ovules, is also stimulated by hormones, which cause its tissues to enlarge into a fruit. The fruit contains the ripened ovules, or seeds.

Plant Propagation

Plant Propagation, growing new plants from seeds or from parts of existing plants. Plant propagation occurs in nature to ensure survival and spread of species. It is also used commercially to produce seeds and plants for agriculture, horticulture, and forestry. Plant propagation includes sexual propagation, which involves the union of sperm and egg to form seeds, and asexual propagation. Asexual propagation, also known as vegetative propagation, is the growing of new plants from a leaf, stem, or root of a single parent plant (see Vegetative Reproduction). These two forms of plant propagation transmit genetic information between plants of the same species. Genetic engineering transfers genes from one organism to another that may or may not be of the same species to introduce desirable traits into an organism. These so-called transgenic plants can then be propagated by sexual propagation or asexual propagation.

Ecology

Ecology, the study of the relationship of plants and animals to their physical and biological environment. The physical environment includes light and heat or solar radiation, moisture, wind, oxygen, carbon dioxide, nutrients in soil, water, and atmosphere. The biological environment includes organisms of the same kind as well as other plants and animals.

Because of the diverse approaches required to study organisms in their environment, ecology draws upon such fields as climatology, hydrology, oceanography, physics, chemistry, geology, and soil analysis. To study the relationships between organisms, ecology also involves such disparate sciences as animal behavior, taxonomy, physiology, and mathematics.

An increased public awareness of environmental problems has made ecology a common but often misused word. It is confused with environmental programs and environmental science (see Environment). Although the field is a distinct scientific discipline, ecology does indeed contribute to the study and understanding of environmental problems.

The term ecology was introduced by the German biologist Ernst Heinrich Haeckel in 1866; it is derived from the Greek oikos (“household”), sharing the same root word as economics. Thus, the term implies the study of the economy of nature. Modern ecology, in part, began with Charles Darwin. In developing his theory of evolution, Darwin stressed the adaptation of organisms to their environment through natural selection. Also making important contributions were plant geographers, such as Alexander von Humboldt, who were deeply interested in the “how” and “why” of vegetational distribution around the world.

Horticulture

Horticulture (Latin hortus,”garden”; cultura,”cultivation”), science and art of growing fruits, vegetables, flowers, shrubs, and trees. Horticulture originally meant the practice of gardening and, by extension, now means the cultivation of plants once grown in gardens. In contrast, the term agriculture, by derivation, referred to more open forms of culture such as the production of grains and grasses, known as agronomic crops, which are cultivated on a large scale. The original distinctions have been so blurred that many crops formerly considered either agronomic or horticultural are now categorized sometimes in one field, sometimes in the other, depending on the intended use of the crop. Thus a plant grown for home consumption may be called horticultural; the same plant cultivated for forage is regarded as an agronomic crop.

Horticulture includes the growing of fruit (especially tree fruits), known as pomology; production of vegetable crops, called olericulture; production of flowers, termed floriculture; and ornamental horticulture, known also as landscape gardening, which includes the maintenance and design of home grounds, public gardens and parks, private estates, botanical gardens, and recreational areas such as golf courses, football fields, and baseball diamonds.

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Floriculture

Floriculture, cultivation of ornamental flowering plants for aesthetic purposes, whether grown in window boxes, greenhouses, or gardens. In floriculture, plants are grown for individual effect; in landscape gardening, for total effect. Although flowers have been cultivated since the rise of civilizations, commercial cultivation in greenhouses of plants and flowers native to other countries was not established until the 19th century. See also Horticulture.

Bacteria

Bacteria, one-celled organisms visible only through a microscope. Bacteria live all around us and within us. The air is filled with bacteria, and they have even entered outer space in spacecraft. Bacteria live in the deepest parts of the ocean and deep within Earth. They are in the soil, in our food, and on plants and animals. Even our bodies are home to many different kinds of bacteria. Our lives are closely intertwined with theirs, and the health of our planet depends very much on their activities.

Bacterial cells are so small that scientists measure them in units called micrometers (µm). One micrometer equals a millionth of a meter (0.0000001 m or about 0.000039 in), and an average bacterium is about one micrometer long. Hundreds of thousands of bacteria would fit on a rounded dot made by a pencil.

Bacteria lack a true nucleus, a feature that distinguishes them from plant and animal cells. In plants and animals the saclike nucleus carries genetic material in the form of deoxyribonucleic acid (DNA). Bacteria also have DNA but it floats within the cell, usually in a loop or coil. A tough but resilient protective shell surrounds the bacterial cell.

Biologists classify all life forms as either prokaryotes or eukaryotes. Prokaryotes are simple, single-celled organisms like bacteria. They lack a defined nucleus of the sort found in plant and animal cells. More complex organisms, including all plants and animals, whose cells have a nucleus, belong to the group called eukaryotes. The word prokaryote comes from Greek words meaning “before nucleus”; eukaryote comes from Greek words for “true nucleus.”

Diseases of Plants

Diseases of Plants, deviations from the normal growth and development of plants incited by microorganisms, parasitic flowering plants, nematodes, viruses, or adverse environmental conditions. In the United States alone, known plant diseases attributable to these causes are estimated to number more than 25,000; the estimated annual losses therefrom add up to several billion dollars. Injuries to plant life due primarily to insects, mites, or animals other than nematodes are not regarded as plant diseases.

BACTERIA-INDUCED DISEASES -> Bacterial diseases are marked by various symptoms, including soft rot, leaf spot, wilt of leaves and stems, canker, leaf and twig blight, and gall formation. Fire blight, a disease of apple and pear trees, is historically interesting because it was the first plant disease in which a bacterium was shown to be the inciting agent. Infected trees exhibit a blackening of the flowers, leaves, and twigs, and the disease finally may involve the entire tree, causing serious damage and even death. Citrus canker, an Asian disease of the orange tree and its relatives, is characterized by corky growths on the fruit, leaves, and twigs. See also Bacteria.

DESTRUCTIVE FUNGI -> The majority of plant diseases are incited by fungi. Fungus diseases have been observed and commented on since ancient times. Equally large numbers of fungi in other groups produce a large array of diseases characterized by leaf spots, ulcerous lesions, blights, powdery and downy mildews, cankers, wood rots and stains, root rots, wilts, club root, and various other symptoms.

VIRAL INFECTIONS -> Typical symptoms of viral infections include mosaic patterns, yellowing of foliage, veinclearing, ring spots, stunting and premature death, malformations, and overgrowth. Under some conditions the symptoms may be masked. See also Virus.

NEMATODES -> Nematodes, or roundworms, are an important cause of disease in plants. For many years attention was focused on the root-knot nematodes, which cause fleshy root knots or galls on plants. More recent investigations were concerned with other species, including the stem or bulb nematodes, which live in the leaves, stems, bulbs, and roots of narcissus, phlox, and many other plants, and the leaf nematodes, growing in herbaceous plants including the begonia and chrysanthemum.

Botanical Garden

Botanical Garden
Botanical Garden, garden in which plants are grown and displayed primarily for scientific and educational purposes. A botanical garden consists chiefly of a collection of living plants, grown out-of-doors or under glass in greenhouses and conservatories. It usually includes, in addition, a collection of dried plants, or herbarium, and such facilities as lecture rooms, laboratories, libraries, museums, and experimental or research plantings.

The plants may be arranged according to one or more subdivisions of botanical science. The arrangements may be systematic (by plant classification), ecological (by relation to environment), or geographic (by region of origin). The larger botanical gardens often include special groupings, such as rock gardens, water gardens, wildflower gardens, and collections of horticultural groups produced by plant breeding, such as roses, tulips, or rhododendrons. A plantation restricted to exhibits of woody plants is called an arboretum.

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Plant Breeding

Plant Breeding
Plant Breeding, the practical application of genetic principles to the development of improved strains of agricultural and horticultural crops. Plant breeders can adapt old crops to new areas and uses; increase yields; improve resistance to disease; enhance the nutritional quality and flavor of fruits and vegetables; and develop traits that are useful for storage, shipping, and processing of foods. Improved wheat and rice varieties sparked the green revolution in the developing world during the 1960s and '70s. In ornamental plants, breeders have developed larger and showier flowers, greater plant vigor, and myriad types, shapes, and colors.

For more information about Plant Breeding, read the full article at wikipedia.org.

Botany Classical Studies

Gross observations and experiments on photosynthesis and the movement of water in plants can be made without knowledge of their structure, but explanations of these phenomena require knowledge of morphology—the study and interpretation of plant form, development, and life histories—and of anatomy—the study of plant tissues and their origin and relations to one another. The cellular nature of plants was first pointed out by the English scientist Robert Hooke in the 17th century, when he observed that cork bark consists of cells. In 1838 the German botanist Matthias Schleiden proposed that all plant tissues consist of cells; this implied a basic sameness of living things and laid the foundation for the development of cytology, the study of the structure and function of cells as individual units rather than as aggegrate tissue. The German pathologist Rudolf Virchow showed in 1858 that cells are derived from preexisting cells, and thus that a continuity exists between past and present living things.

Such observations were important not only in the development of plant physiology and anatomy but also in the understanding of genetics, the science of heredity, and of evolution.

Knowledge of anatomy, genetics, and evolution has greatly advanced plant classification by providing a rational basis for this subdivision of botany. The 17th-century British naturalist John Ray divided plants into nonflowering and flowering types, and flowering plants into dicots and monocots. The 18th-century Swedish botanist Carolus Linnaeus, however, provided the framework on which modern classifications are based and, just as important, a simplified system of nomenclature in which each plant is given two names: the first the name of the genus and the second the name of the species.

Botany: Historical Development

Because civilization rests in part on a knowledge of plants and their cultivation, botany can be said to have originated with the first cultivation of crops, which may date from 9000-7000 bc. Not until about 2300 years ago, however, did humans become interested in plants for their own sake. Thus, botany as a pure science began in the 4th century BC with the Greek philosopher Theophrastus, whose treatises on the classification, morphology, and reproduction of plants heavily influenced the discipline until the 17th century. Indeed, modern botany began to develop only about the 16th century, at least in part because of the invention of the microscope (1590) and of printing with movable type (1440).

The Greeks believed that plants derived their nourishment from the soil only. Not until the 17th century did the Belgian scientist Jan Baptista van Helmont show that, although only water was added to a potted willow, it gained nearly 75 kg (165 lb), whereas the soil it stood in lost only about 60 g (about 2 oz) of weight over a period of five years. This demonstrated that the soil contributes very little to the increase in the weight of plants. In the 18th century the English chemist Joseph Priestley demonstrated that growing plants “restore” air from which the oxygen has been removed (by the burning of candles or the breathing of animals), and the Dutch physiologist Jan Ingenhousz (1730-99) extended this observation by showing that light is required for plants to restore air. These and other discoveries formed the basis for modern plant physiology, that branch of botany dealing with basic plant functions.

That water moves upward through the wood and that solutes move downward through the stems of plants was discovered independently in the 17th century by Marcello Malpighi in Italy and Nehemiah Grew in England. These facts have now been known for some 300 years, but only in the last few years have acceptable theories explaining the movements of liquids in plants been developed, using a variety of refined analytical techniques.

Botany

Botany, branch of biology concerned with the study of plants (kingdom Plantae). Plants are now defined as multicellular organisms that carry out photosynthesis. Organisms that had previously been called plants, however, such as bacteria, algae, and fungi, continue to be the province of botany, because of their historical connection with the discipline and their many similarities to true plants, and because of the practicality of not fragmenting the study of organisms into too many separate fields.

Botany is concerned with all aspects of the study of plants, from the smallest and simplest forms to the largest and most complex, from the study of all aspects of an individual plant to the complex interactions of all the different members of a complicated botanical community of plants with their environment and with animals (see Ecology).

See also Historcal Development; Classical Studies.

related articles:

plant breeding
botanical garden
diseases of plants
floriculture, growing of flowering plants
horticulture, growing of cultivated plants

Controlling Garden Pests

Controlling Garden Pests
Three types of pests can plague gardens: weeds, insects, and diseases. A weed is any plant that grows where the gardener does not want it. Weeds are undesirable because they compete with garden plants for light, water, and nutrients. Common methods for controlling weeds include pulling them up by hand; digging them out; and cutting them off using a hoe or mower. One way to slow the growth of weeds is to cover the soil with a layer of mulch, which blocks out the light and air that weeds need to grow. Weeds also can be controlled by treating them with a weed killer, or herbicide. Like fertilizers, weed killers can be organic or synthetic (see See also Weed Control).

Insects damage plants by chewing leaves or other plant parts by sucking the liquid from the plant, or in some cases, by transmitting viruses. Another method for preventing insect damage is to cover young plants with a floating row cover, which is a very thin, white, gauzy blanket that keeps many insects away from the plants. Another preventive method is to grow plants bred for resistance to insect pests.

Some insects can be kept in check by introducing beneficial bacteria or insects to the garden. This method exploits the natural ecological relationships between garden pests and other organisms. Ladybugs, for instance, eat aphids, one of the more notorious garden insect pests, and certain types of bacteria kill insect larva. Another method to help control insects in vegetable and flower gardens is to rotate crops instead of growing the same type of plant in the same place every year. Many insects have a life cycle that depends on the presence of a certain type of plant. By removing the plant for at least two years, the life cycle can be interrupted, thus controlling the pest. Both organic and synthetic insect-killing materials, called insecticides, also are available to control insect pests (see Pest Control).

Diseases caused by fungi, bacteria, or viruses also can damage plants (see Diseases of Plants). In most cases, once a plant has a disease it cannot be saved, though some fungal diseases can be controlled with a fungicide. The best approach to disease prevention is to provide plants with optimum soil, nutrients, light, and water so they can fight off disease, and to grow plants that have been bred for disease resistance or have natural resistance.

Harvesting and Pruning

Gardeners harvest plants at different stages, depending on how the harvested plants or plant parts are used. Crops grown for their fruit, such as tomatoes and eggplant, are harvested when the fruit is ripe. Some plants are harvested before they flower—lettuce and spinach, for example, are grown for their tender leaves and develop a bitter flavor if allowed to flower. Plants grown for their roots, such as carrots and radishes, are harvested when the root is large enough, but before it gets tough or woody and loses its sweetness. In flower gardens, the sign of maturity is the formation of seeds. Many plants stop flowering once they set seed, so to make plants produce flowers longer, gardeners can remove the faded flowers before they produce seed, a technique called deadheading.

Gardeners may attempt to control the shape of woody plants, such as trees and shrubs, by removing, or pruning, branches growing in the wrong direction. They also prune to removed damaged, disease, or dead branches. Some shrubs, such as lilacs, bear the most flowers in young wood, so gardeners remove the oldest branches. Gardeners prune plants at different times of the year, depending on how they hope to affect the plant’s growth.

Soil Management

Soil Management, the basis of all scientific agriculture, which involves six essential practices: proper tillage; maintenance of a proper supply of organic matter in the soil; maintenance of a proper nutrient supply; control of soil pollution; maintenance of the correct soil acidity; and control of erosion.

TILLAGE

The purpose of tillage is to prepare the soil for growing crops. This preparation is traditionally accomplished by using a plow that cuts into the ground and turns over the soil. This removes or kills any weeds growing in the area, loosens and breaks up the surface layers of the soil, and provides a bed of soil that holds sufficient moisture to permit the planted seeds to germinate. Traditional tillage may harm the soil if used continuously over many years, especially if the fertile topsoil layer is thin.

MAINTENANCE OF ORGANIC MATTER

Organic matter is important in maintaining good physical conditions in the soil. It contains the entire soil reserve of nitrogen and significant amounts of other nutrients, such as phosphorus and sulfur. Soil productivity thus is affected markedly by the organic-matter balance maintained in the soil. Because most of the cultivated vegetation is harvested instead of being left to decay, organic materials that would ordinarily enter the soil upon plant decomposition are lost. To compensate for this loss, various standardized methods are employed. The two most important of these methods are crop rotation and artificial fertilization.

NUTRIENT SUPPLY

Among soil deficiencies that affect productivity, deficiency of nutrients is especially important. The nutrients most necessary for proper plant growth are nitrogen, potassium, phosphorus, iron, calcium, sulfur, and magnesium, all of which usually exist in most soils in varying quantities. In addition, most plants require minute amounts of substances known as trace elements, which are present in the soil in very small quantities and include manganese, zinc, copper, and boron. Nutrients often occur in the soil in compounds that cannot be readily utilized by plants.

SOIL POLLUTION

Soil pollution is the buildup in soils of persistent toxic compounds, chemicals, salts, radioactive materials, or disease-causing agents, which have adverse effects on plant growth and animal health. As of now, soil pollution is not widespread. Although the application of fertilizers containing the primary nutrients, nitrogen, phosphorus, and potassium, has not led to soil pollution, the application of trace elements has. The irrigation of arid lands often leads to pollution with salts. Sulfur from industrial wastes has polluted soils in the past, as has the accumulation of arsenic compounds in soils following years of spraying crops with lead arsenate. The application of pesticides has also led to short-term soil pollution. See Environment.

Watering

Water is as vital for plants as it is for other organisms. The pressure of water within the plant cells helps the plant’s leaves to remain firm. Water also is essential for most of the plant’s biochemical reactions. In addition, water stores essential dissolved nutrients. How often plants need water depends on how hot, dry, and windy the climate is, how well the plant tolerates dry conditions, and how deep the roots go into the soil. Plants can be watered at any time of day. However, to avoid plant diseases that thrive in cool, moist conditions and to reduce water lost through evaporation, gardeners water in the early morning, when the air is cool and still, but the sun will soon dry the leaves.

The best method for watering plants is to apply the water directly to the soil, rather than over the tops of the plants. The water should be applied at a rate no faster than it can percolate into the soil so that the excess will not run off and be wasted. This technique reduces water lost through evaporation and keeps leaves dry, which discourages diseases. A few tools for watering the soil efficiently include hoses with tiny holes all along their surface, called soaker hoses; plastic tubes with tiny holes punched in them at intervals for drip irrigation; and plastic jugs with small holes punched in the bottom, filled with water, and set beside a plant. Watering large, densely planted areas, such as a lawn, requires a sprinkler. Evaporation of water from the soil can be minimized by covering the soil with a protective layer known as mulch. Mulch acts as a barrier that slows evaporation by reducing the amount of air and heat that reaches the soil surface. Materials that can be used as mulch include leaves, bark chips, grass clippings, and cardboard.

Planting and Transplanting

Before planting seeds, gardeners prepare, or till, the soil using a variety of methods. Some turn over the soil with a spade, while others loosen it with a garden fork. Then, they rake it smooth before planting. Some gardeners prefer not to turn or loosen the soil because the oxygen that enters the soil when it is tilled by these methods hastens the breakdown of needed organic matter in the soil. Instead, they just dig a small hole for each seed or plant. To keep the soil loose so that roots can develop easily, they keep it covered with grass clippings, compost, or other organic matter. The presence of this organic matter encourages large populations of worms, whose tunneling breaks up the soil.

Gardeners plant seeds at different depths, depending on the seed’s size and its requirement for light. Seeds contain starch and oil, stored food that provides the energy needed for sprouting, or germination. Small seeds do not hold much food, so they are sown on or close to the soil surface, where they will not require a lot of energy to push through the soil. Larger seeds have enough food reserves to be planted deeper. This gives the root system more time to develop as the seedling, or young plant, grows up through the soil. As a general rule, a seed can be planted three times as deep as the seed is wide. Some seeds, such as lettuce, require light to germinate; these seeds must be sown on or very near the soil surface. Once the seeds are sown, the gardener gently presses down the soil to ensure that the seed touches soil, not air pockets—this soil contact helps keep the seeds moist.

Fertilizing

Fertilizing
Not all soils have enough nutrients or the right balance of nutrients. In addition, plants remove nutrients from the soil as they grow, so these nutrients must be replaced in order for the soil to remain productive. For these reasons, gardeners enhance soil by adding fertilizer, a material that contains one or more of the nutrients plants need.

Fertilizers are divided into two categories: synthetic and organic. Synthetic fertilizers are concentrated salts or minerals, some of which are produced as by-products of petroleum production. Organic fertilizers originate in plants, animals, or minerals and include compost, seaweed, and ground bone. (For advantages and disadvantages of synthetic and organic fertilizers, see Organic Farming.)

Fertilizers usually are sold in packages, on which the percentage by weight of the macronutrients nitrogen (N), phosphorus (P), and potassium (K) are listed on the label—always in the order N-P-K. For example, a fertilizer that is labeled 10-5-3 is 10 percent nitrogen, 5 percent phosphorus, and 3 percent potassium.

Understanding Soil

Healthy soil is indispensable for a healthy garden. Plants derive water, oxygen for their roots, and essential nutrients from the soil. Soil consists of two components: minerals from weathered rocks and organic matter from decayed organisms and animal wastes. The mineral content of the soil provides plants with nutrients, such as calcium, potassium, and phosphorus. Organic matter improves drainage and helps prevent waterlogged soils, reducing the occurrence of diseases such as root rot.

Soil texture, the size of the individual soil particles, affects how fast water drains and how well plants absorb nutrients. The largest soil particles are grains of sand. Sand grains fit loosely together with large gaps between them, resembling marbles in a jar. The large pores let water (and the nutrients dissolved in it) drain out too quickly for most plants to absorb it. Clay particles, on the other hand, are very tiny, and they pack closely together, resembling tiny beads in a jar. The pores between clay particles are so small that water drains very slowly. Slow drainage can lead to oxygen deprivation because the water takes the place of air in the pores. Another disadvantage of clay is that it binds water and some nutrients so tightly that most plants cannot absorb them. A third soil particle is silt, which is larger than clay but smaller than sand.

Most plants thrive in a soil type known as loam, which contains roughly 50 percent sand, 25 percent clay, and 25 percent silt. A loam soil drains water well, but not too quickly, and as a result, the plant can absorb nutrients more readily. Exceptions include desert plants, such as cacti, which do best in a sandy soil, and cottonwoods, which flourish in silty soils.

Selecting Plants

Plants differ in their tolerance for heat, cold, and moisture, so when selecting plants, gardeners must take into account the climate of their region. Gardeners also evaluate the soil type and how much sunlight falls on the proposed site, factors that affect the types of plants that can be grown in a particular area. They consider, too, the plant’s life cycle—how long it takes a plant to grow, flower, produce fruits or seeds, and die. Annuals such as petunias bloom and produce seed the same year they are planted, then die when cold temperatures set in. Biennials—hollyhocks, for example—live for two years, producing just leaves the first year. In the second year they produce flowers, and die when the weather turns cold. Perennials, which include shrubs and trees as well as flowers, are plants that live for three or more years.

A gardener can experiment with a stunning diversity of interesting garden styles and types. Herb gardens may feature culinary herbs, medicinal herbs, and fragrant herbs. Flower gardens may combine a variety of flowering plants or focus on just one type, such as roses, white-flowered plants, or flowers that bloom only at night. Specialized gardens include rock or alpine gardens, which display plants native to mountains, and water gardens, which host plants adapted to wet conditions. Botanical gardens are designed to display plants for scientific and educational purposes, and in these gardens, the plants are often labeled with their names and their optimal growing conditions.

Gardening

Gardening, growing and caring for plants as an enjoyable leisure activity, to produce food, or to create beautiful landscapes with artfully arranged flowers, shrubs, and trees. For some, gardening is a form of exercise, a way to save money on food, or a way to ensure that fruits and vegetables are free from pesticides or other chemicals. For others, gardening is a profession: landscape gardeners design, install, and maintain gardens for a living. Unlike farmers, who typically produces large quantities of crops using complex equipment, such as tractors and combines, gardeners usually produce plants in smaller quantities, relying on manual tools, such as spades, rakes, and hoes, and small power tools, such as mowers and tillers.

See also: selecting plants; understanding soil; fertilizing; planting and transplanting; watering; controlling garden pests; harvesting and pruning

Cellophane

Cellophane, originally a trade name and now a common name for a flexible, transparent film made of regenerated cellulose and used principally as a wrapping material. Cellophane is produced by dissolving wood pulp or other cellulose material in an alkali with carbon disulfide, neutralizing the alkaline solvent with an acid, extruding the precipitate into a sheet, impregnating it with glycerine, and then drying and cutting the sheets to the desired size. Cellophane was invented about 1910 by the Swiss chemist Jacques Brandenberger, who in 1912 invented the first machines for large-scale production and established a factory near Paris.

Rayon

Rayon
Rayon, artificial textile material, composed of cellulose obtained from cotton linters or from the pulp of trees such as spruce.

Rayon can be made by either the viscose process or the cuprammonium process; both produce fiber classified by the U.S. Federal Trade Commission as rayon. In the viscose process purified cellulose is treated with sodium hydroxide, then with carbon disulfide, to form a viscous yellow liquid called viscose. In the cuprammonium process purified cellulose is treated with cuprammonium liquor, then with sodium hydroxide, to form viscose. The manufacture of rayon filaments—and all manufactured fibers—is done by means of an extrusion process called spinning. In this procedure the fiber-forming liquid is forced through tiny holes in a nozzle or spinneret into a liquid bath containing chemicals that produce filaments of pure cellulose, which can be spun into yarn. The filaments are drawn together to form both fibers and yarn in a single, continuous process.

Leaf mechanism

Leaf mechanism
When the stomata are open, carbon dioxide and oxygen pass either in or out—when carbon dioxide enters, it takes part in photosynthesis, the food-making process that releases oxygen as a waste product. This oxygen passes out of the leaf. At the same time, oxygen also enters the leaf, where it takes part in respiration, a process that forms carbon dioxide as a waste product. This carbon dioxide passes out through the stomata. Water also passes out of the open stomata in the form of a vapor. This process is called transpiration. Generally, there are more stomata on the under surface of a leaf than on the upper surface. This prevents water from evaporating too quickly or in excessive amounts from the leaf's upper side, which is exposed to the sun. Stomata close at night, providing another level of water conservation.

See leaf

Transpiration

Transpiration, evaporation of water particles from plant surfaces, especially from the surface openings, or stomata, on leaves (see Leaf). Stomatal transpiration accounts for most of the water loss by a plant, but some direct evaporation also takes place through the surfaces of the epidermal cells of the leaves.

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leaf mechanism

Importance of Leaf

Importance of Leaf
Unlike leaf-bearing plants, animals cannot manufacture their own food. For this reason, animals must receive their nourishment, either directly or indirectly, from leaf-bearing plants. For example, cattle, sheep, and horses eat the leaves of grasses and other plants. These animals, in turn, are consumed by various carnivorous animals, including humans. Humans also eat many kinds of leaves directly, including artichoke, cabbage, lettuce, and spinach.

In addition to being a source of food, leaves provide many useful products. For example, the leaves of tea plants are made into a beverage, and the leaves of thyme, sage, and parsley are used for seasoning foods. Tobacco leaves may be smoked or chewed. Drugs are obtained from the leaves of foxglove, witch hazel, senna, and many other plants. Oils extracted from the leaves of geranium and citronella plants are used in manufacturing perfumes and soaps, and oils from mint and wintergreen leaves are made into flavoring extracts. Tannins, chemical substances used in preparing leather, are derived from sumac leaves, and dyes are made from indigo and henna leaves. The leaves of many plants may be used as fertilizer.

Parts of Typical Leaf

Parts of Typical Leaf
The typical green leaf is called a foliage leaf. It usually consists of two basic parts: a petiole and a blade.

The petiole is a stalklike structure that supports the leaf blade on the stem. It also serves as a passageway between the stem and the blade for water and nutrients. Another function of the petiole is to move the leaf into the best position for receiving sunlight. Most petioles are long, narrow, and cylindrical.

Many plants, such as grasses and corn, do not have petioles. In these plants the base of the blade is attached directly to the stem—the base encircles the stem as a sheath. Such leaves are called sessile leaves.

The leaf blade is usually a thin, flat structure. Its margins, or edges, may be smooth, as in the dogwood; jagged or toothed, as in the elm; or lobed, as in the oak and maple. The surface of the blade may be smooth, fuzzy, sticky, dull, or shiny. In most plants the leaves have a single blade and are referred to as simple. In other plants, such as clover, the blade is divided into separate leaflets. This kind of leaf is called a compound leaf. Most of the functions carried on by leaves take place in the blade.

A Epidermis

The blade consists of an upper and lower epidermis and a spongy layer of tissue, called the mesophyll. Running through the mesophyll is a branching system of veins.

The epidermis is the leaf blade's skin. It is a thin, usually transparent, colorless layer of cells that covers both the upper and lower surfaces of the blade. The epidermis prevents the leaf from losing excessive amounts of water and protects it against injury.

In most plants the epidermis is covered with cutin, a waxy substance secreted by the epidermal cells. The layer of cutin, called the cuticle, is responsible for the glossy appearance of some leaves. The cuticle gives the leaf additional protection by slowing down the rate at which water is lost. Generally, the cuticle is thinner on the epidermis on the underside of the leaf than on the upper epidermis, which is exposed to the sun.

In many kinds of leaves, hairs grow from the epidermis. The soft hairs of plants such as the mullein give the leaves a woolly or feltlike texture. In some plants the epidermal hairs secrete fluids. For example, in geraniums and petunias the hairs secrete a fluid that gives the leaves a clammy texture. The strong-smelling oils of the peppermint and spearmint plants come from epidermal hairs. In other plants, such as the nettle, the epidermal hairs are stiff and contain a poisonous fluid that produces a skin irritation when a person is pricked by them.

B Guard Cells

Scattered throughout the epidermis are pairs of bean-shaped cells, called guard cells. Guard cells contain chloroplasts, which are tiny granules filled with the green pigment chlorophyll. Chlorophyll gives leaves their characteristic green color. Chloroplasts enable leaves to carry on photosynthesis because they are able to absorb carbon dioxide and sunlight, which are required for the food-making process. In response to heat and light, each pair of guard cells pulls apart, and a tiny pore forms between them. The pores, called stomata, open to the outside atmosphere. See leaf mechanism.

C Water Pores

In addition to the stomata, many kinds of leaves have large specialized water pores in their epidermis. These pores, called hydathodes, permit guttation, the process by which a plant loses liquid water. Unlike the stomata, hydathodes remain open all the time.

Guttation takes place only when water is being rapidly absorbed by the roots, such as after a heavy rainfall, and when transpiration slows down, as on cool, humid nights. When these conditions occur together, droplets of water can be seen on the leaf early in the morning before they evaporate in the heat of the day. Unlike dew, which condenses on leaves from water vapor in the air and covers the entire leaf surface, guttation droplets form only on the edges and tips of leaves. Generally, the droplets are noticeable only on the leaves of strawberries and a few other kinds of plants.

D Mesophyll

The mesophyll, sandwiched between the upper and lower epidermis, consists of many thin-walled cells that are usually arranged in two layers. The palisade layer is next to the upper epidermis. It consists of cylindrical cells that are packed closely together. Next to the palisade layer and making up most of the thickness of the leaf blade is the spongy layer. The spongy layer consists of roundish cells that are packed loosely together and have numerous air spaces between them. In most plants the spongy layer extends down to the lower epidermis. However, in certain grasses, irises, and other plants whose leaves grow straight up and down, the spongy layer is wedged between two palisade layers of mesophyll. Like the guard cells, all the cells of the mesophyll contain chloroplasts.

E Veins

Running through the middle of the mesophyll and branching out to all of its cells are veins. The veins extend into the petiole and connect with other veins in the stem of the plant. A major function of the veins is to help support the leaf blade. Each type of plant has a characteristic pattern of veins forming lines and ridges in the blade.

The veins of a leaf are made up of two specialized tissues, xylem and phloem. Xylem usually forms the upper half of the vein. It consists of tubular open-ended cells that are arranged end to end. The walls of the cells are thick and rigid. Xylem conducts water and dissolved minerals to the leaf blade from the rest of the plant.

Phloem lies on the underside of the vein. It is made up of thin-walled tubular cells with tiny openings at their ends, somewhat like a sieve. These cells are also arranged end to end. Phloem carries food manufactured in the blade to the rest of the plant.

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Leaf

Leaf
Leaf, part of a plant that serves primarily as the plant's food-making organ in a process called photosynthesis. Leaves take part in other plant functions as well, including transpiration and guttation, both of which remove excess water from the plant, and respiration, the process by which a plant obtains oxygen and energy. Leaves also may store food and water and provide structural support.

A leaf is an extension of a plant's stem. Although most leaves are flat, broad, or bladelike, they also may be many other shapes, including round, oval, or feathery. In general, the leaves of trees such as hardwoods tend to be broad and relatively large, and the leaves of conifers, or cone-bearing trees, are usually small and needlelike in shape. In size, leaves range from only several millimeters (a fraction of an inch) long, as in the water plant Elodea, to 15 to 18 m (15 to 60 ft) long, as in some palm trees.

Green leaves derive their color from a green pigment called chlorophyll. The presence of additional pigments causes other leaf colors such as red in coleus and purple in cabbage. In temperate regions of the world, the leaves of some plants change color in autumn. Leaves of most garden plants turn yellow in the autumn, but those of many trees take on brilliant orange or red colors.

See Parts of Typical Leaf; Importance of Leaf.

Chlorophyll

Chlorophyll, the pigment found in plants, some algae, and some bacteria that gives them their green color and that absorbs the light necessary for photosynthesis. Chlorophyll absorbs mainly violet-blue and orange-red light. The great abundance of chlorophyll in leaves and its occasional presence in other plant tissues, such as stems, causes these plant parts to appear green. In some leaves, chlorophyll is masked by other pigments. In fall, chlorophyll wanes in the leaves of trees, and other pigments predominate.

Chlorophyll is a large molecule composed mostly of carbon and hydrogen. At the center of the molecule is a single atom of magnesium surrounded by a nitrogen-containing group of atoms called a porphyrin ring. The structure somewhat resembles that of the active constituent of hemoglobin in the blood. A long chain of carbon and hydrogen atoms proceeds from this central core and attaches the chlorophyll molecule to the inner membrane of the chloroplast, the cell organelle in which photosynthesis takes place. As a molecule of chlorophyll absorbs a photon of light, its electrons become excited and move to higher energy levels (see Photochemistry). This initiates a complex series of chemical reactions in the chloroplast that enables the energy to be stored in chemical bonds.

See also: Algae; Lichens; Photosynthesis; Leaves

Photosynthesis Variations

A majority of plants use these steps in photosynthesis. Plants such as corn and crabgrass that have evolved in hot, dry environments, however, must overcome certain obstacles to photosynthesis. On hot days, they partially close the pores in their leaves to prevent the escape of water. With the pores only slightly open, adequate amounts of carbon dioxide cannot enter the leaf, and the Calvin cycle comes to a halt. To get around this problem, certain hot-weather plants have developed a way to keep carbon dioxide flowing to the stroma without capturing it directly from the air. They open their pores slightly, take in carbon dioxide, and transport it deep within the leaves. Here they stockpile it in a chemical form that releases the carbon dioxide slowly and steadily into the Calvin cycle. With this system, these plants can continue photosynthesis on hot days, even with their pores almost completely closed. A field of corn thus remains green on blistering days when neighboring plants wither, and crabgrass thrives in lawns browned by the summer sun.

Bacteria lack chloroplasts, and instead use structures called chromatophores—membranes formed by numerous foldings of the plasma membrane, the membrane surrounding the fluid, or cytoplasm, that fills the cell. The chromatophores house thylakoids similar to plant thylakoids, which in some bacteria contain chlorophyll. For these bacteria, the process of photosynthesis is similar to that of plants, algae, and seaweed. Many of these chlorophyll-containing bacteria are abundant in oceans, lakes, and rivers, and the oxygen they release dissolves in the water and enables fish and other aquatic organisms to survive.

Certain archaebacteria, members of a group of primitive bacteria-like organisms, carry out photosynthesis in a different manner. The mud-dwelling green sulfur and purple sulfur archaebacteria use hydrogen sulfide instead of water in photosynthesis. These archaebacteria release sulfur rather than oxygen, which, along with hydrogen sulfide, imparts the rotten egg smell to mudflats. Halobacteria, archaebacteria found in the salt flats of deserts, rely on the pigment bacteriorhodopsin instead of chlorophyll for photosynthesis. These archaebacteria do not carry out the complete process of photosynthesis; although they produce ATP in a process similar to the light-dependent reaction and use it for energy, they do not produce glucose. Halobacteria are among the most ancient organisms, and may have been the starting point for the evolution of photosynthesis.

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How Photosynthesis Works

Photosynthesis
Photosynthesis is a very complex process, and for the sake of convenience and ease of understanding, plant biologists divide it into two stages. In the first stage, the light-dependent reaction, the chloroplast traps light energy and converts it into chemical energy contained in nicotinamide adenine dinucleotide phosphate (NADPH) and adenosine triphosphate (ATP), two molecules used in the second stage of photosynthesis. In the second stage, called the light-independent reaction (formerly called the dark reaction), NADPH provides the hydrogen atoms that help form glucose, and ATP provides the energy for this and other reactions used to synthesize glucose. These two stages reflect the literal meaning of the term photosynthesis, to build with light.

A - The Light-Dependent Reaction

Photosynthesis relies on flows of energy and electrons initiated by light energy. Electrons are minute particles that travel in a specific orbit around the nuclei of atoms and carry a small electrical charge. Light energy causes the electrons in chlorophyll and other light-trapping pigments to boost up and out of their orbit; the electrons instantly fall back into place, releasing resonance energy, or vibrating energy, as they go, all in millionths of a second. Chlorophyll and the other pigments are clustered next to one another in the photosystems, and the vibrating energy passes rapidly from one chlorophyll or pigment molecule to the next, like the transfer of energy in billiard balls.

Light contains many colors, each with a defined range of wavelengths measured in nanometers, or billionths of a meter. Certain red and blue wavelengths of light are the most effective in photosynthesis because they have exactly the right amount of energy to energize, or excite, chlorophyll electrons and boost them out of their orbits to a higher energy level. Other pigments, called accessory pigments, enhance the light-absorption capacity of the leaf by capturing a broader spectrum of blue and red wavelengths, along with yellow and orange wavelengths. None of the photosynthetic pigments absorb green light; as a result, green wavelengths are reflected, which is why plants appear green.

B - The Light-Independent Reaction

The chemical energy required for the light-independent reaction is supplied by the ATP and NADPH molecules produced in the light-dependent reaction. The light-independent reaction is cyclic, that is, it begins with a molecule that must be regenerated at the end of the reaction in order for the process to continue. Termed the Calvin cycle after the American chemist Melvin Calvin who discovered it, the light-independent reactions use the electrons and hydrogen ions associated with NADPH and the phosphorus associated with ATP to produce glucose. These reactions occur in the stroma, the fluid in the chloroplast surrounding the thylakoids, and each step is controlled by a different enzyme.

The light-independent reaction requires the presence of carbon dioxide molecules, which enter the plant through pores in the leaf, diffuse through the cell to the chloroplast, and disperse in the stroma. The light-independent reaction begins in the stroma when these carbon dioxide molecules link to sugar molecules called ribulose bisphosphate (RuBP) in a process known as carbon fixation.

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Where Photosynthesis Occurs

Photosynthesis

Plant photosynthesis occurs in leaves and green stems within specialized cell structures called chloroplasts. One plant leaf is composed of tens of thousands of cells, and each cell contains 40 to 50 chloroplasts. The chloroplast, an oval-shaped structure, is divided by membranes into numerous disk-shaped compartments. These disklike compartments, called thylakoids, are arranged vertically in the chloroplast like a stack of plates or pancakes. A stack of thylakoids is called a granum (plural, grana); the grana lie suspended in a fluid known as stroma.

Embedded in the membranes of the thylakoids are hundreds of molecules of chlorophyll, a light-trapping pigment required for photosynthesis. Additional light-trapping pigments, enzymes (organic substances that speed up chemical reactions), and other molecules needed for photosynthesis are also located within the thylakoid membranes. The pigments and enzymes are arranged in two types of units, Photosystem I and Photosystem II. Because a chloroplast may have dozens of thylakoids, and each thylakoid may contain thousands of photosystems, each chloroplast will contain millions of pigment molecules.

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photosynthesis
how photosynthesis works
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Photosynthesis

.
Photosynthesis, process by which green plants and certain other organisms use the energy of light to convert carbon dioxide and water into the simple sugar glucose. In so doing, photosynthesis provides the basic energy source for virtually all organisms. An extremely important byproduct of photosynthesis is oxygen, on which most organisms depend.

Photosynthesis occurs in green plants, seaweeds, algae, and certain bacteria. These organisms are veritable sugar factories, producing millions of new glucose molecules per second. Plants use much of this glucose, a carbohydrate, as an energy source to build leaves, flowers, fruits, and seeds. They also convert glucose to cellulose, the structural material used in their cell walls. Most plants produce more glucose than they use, however, and they store it in the form of starch and other carbohydrates in roots, stems, and leaves. The plants can then draw on these reserves for extra energy or building materials. Each year, photosynthesizing organisms produce about 170 billion metric tons of extra carbohydrates, about 30 metric tons for every person on earth.

Photosynthesis has far-reaching implications. Like plants, humans and other animals depend on glucose as an energy source, but they are unable to produce it on their own and must rely ultimately on the glucose produced by plants. Moreover, the oxygen humans and other animals breathe is the oxygen released during photosynthesis. Humans are also dependent on ancient products of photosynthesis, known as fossil fuels, for supplying most of our modern industrial energy. These fossil fuels, including natural gas, coal, and petroleum, are composed of a complex mix of hydrocarbons, the remains of organisms that relied on photosynthesis millions of years ago. Thus, virtually all life on earth, directly or indirectly, depends on photosynthesis as a source of food, energy, and oxygen, making it one of the most important biochemical processes known.

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Lichen

The lichen seen here growing on a tree is one of the fruticose lichens. It is made up of a layer of algal cells, a middle layer of fungal hyphae, and an outer layer of fungal tissue.

Lichen, living partnership of a fungus and an alga. The fungus component is called the mycobiont and is composed of intertwined, threadlike fibers called hyphae that are tightly packed into a tissuelike sheet. The fungus uses these hyphae to absorb food from its surroundings. The algal component, called the photobiont, makes its own food through photosynthesis and grows as a mass of green cells dispersed among the fungal hyphae. Lichens survive in a wide variety of environments, either forming small, circular crusts or leaflike structures attached to bark, rocks, or soil, or as hairlike structures hanging from tree branches.

A lichen is actually a combination of two separate organisms: an alga and a fungus. Most lichens are three-layered organisms, with an algal layer sandwiched between two layers of fungus. The alga produces the food for the lichen through photosynthesis, while the fungus absorbs water and other nutrients. Neither the fungus nor the alga can live independent of the other.

The relationship between the fungus and the alga in a lichen is an example of mutualism, in which both partners benefit from the partnership (see Symbiosis). This relationship enables each to tolerate harsh conditions where neither could survive alone. In this partnership, the fungus furnishes the alga with water, prevents overexposure to sunlight, and provides simple mineral nutrients, while the photosynthesizing alga supplies food to the fungus even if no other organic material is available. In dry, barren areas where plants have a hard time growing, such as polar tundra, deserts, rocky outcrops, or high mountains, lichens are the primary photosynthesizers. Some remarkable species even grow inside porous rocks, just below the surface where some light can still reach the algal cells.

Their ability to grow in severe conditions often makes lichens pioneers in plant succession, the process in which plants colonize bare rock or soil. Lichens release acids that break down inhospitable rock, permitting soil-trapping mosses and grasses to grab hold. In areas where soil gradually accumulates, such as a forest floor, the pioneering lichens are eventually replaced by plants and trees, although other lichens may grow on these plants and trees.

Although lichens have been used in folk medicine as purported cures for many ills, from headaches and toothaches to tuberculosis, diabetes, and asthma, their use in modern medicine is recent. The discovery in the 1940s that some fungi produce potent antibiotics stimulated an extensive screening of fungi and lichens. Since then, lichen extracts have found limited use in Europe, where lichen antibiotics have been used to treat tuberculosis and some skin diseases.

Planting and Caring for Trees

PLANTING AND CARING FOR TREES
Whether trees are being planted for reforestation, ornament, shade, or fruit, the first step is selecting the species to be grown. The choice depends on such factors as the characteristics of the soil, the location of the site, and drainage. For example, sycamore and cottonwood trees will not grow on dry exposed slopes or ridges, or in fields with a thin topsoil over a heavy compact subsoil. Walnut trees will not grow in swampy places, and jack pines grow especially well on loose sandy soils with good drainage. A good rule of thumb is to plant native trees—trees that have demonstrated their ability to thrive in the local environment without harming other local species.

The hole for a seedling should be deep enough to hold the fully expanded root system of the seedling. Larger plants should be placed in a hole 60 cm (2 ft) deep with a diameter 60 cm greater than that of the ball of the roots. In poor soil the hole should be 1.8 m (6 ft) wide and 60 cm deep for a 2.5-m (8-ft) tree, and proportionately wider for taller trees.

After the tree is placed in the hole, the soil should be firmly pressed around the roots, and the ground should be thoroughly soaked with water. Mixing bone meal or well-rotted manure into the soil will help the tree become established quickly. Most deciduous trees should be planted in the fall when they are not growing, but evergreens are usually planted in the spring, at the beginning or middle of their period of vigorous growth.

A Watering

After planting, the soil around a tree should be kept moist, but not soaked. If artificial watering is not practical, a layer of mulch 7 to 15 cm (3 to 6 in) deep should be placed around the tree to conserve moisture and to discourage the growth of weeds. Because a transplanted tree does not adequately absorb water through its damaged roots, it is important to prevent water loss from the plant by pruning top limbs to limit transpiration.

B Fertilizing

In good soils it is less important to fertilize than in poor soils. However, all trees grow better and faster and are less likely to become diseased if fertilizer is supplied in the proper amounts. This may be done most easily by placing a large handful of fertilizer in holes made by a crowbar at the edge of the spread of the tree’s limbs. The holes should be about 60 cm (2 ft) deep and about 5 cm (2 in) in diameter, and they should be spaced about 90 cm (3 ft) apart. After the fertilizer has been introduced, the holes should be filled in with soil. Organic fertilizers such as manure and mulch are preferable to chemicals that may replace or destroy natural organisms in the soil.

C Pruning

Pruning of ornamental trees maintains the form of the tree, removes weak or sickly branches, and rejuvenates old or unhealthy plants. If performed during a period of vigorous growth, pruning often also results in an increased production of flowers. In pruning, cuts are made just above the buds that point in the direction branches are desired. When large branches are removed, the cut should be made close to the trunk, and then covered for a time.

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Major Parts of a Tree

The major parts of a tree
The major parts of a tree are its roots, trunk, leaves, flowers, and seeds. These components play vital roles in a tree’s growth, development, and reproduction.

A. Roots

Trees are held in place by anchoring organs called roots. In addition to anchoring the tree, roots also absorb water and minerals through tiny structures called root hairs. From the roots the water and mineral nutrients are carried upward through the wood cells to the leaves. Although the internal structure of most kinds of roots is similar, there are often external differences. Pines, for example, have a strongly developed taproot, or main root, in addition to branching side roots. In maples, on the other hand, there is little or no central taproot, and the other roots are produced in great numbers near the surface of the soil.

Roots grow constantly, and at the growing tip of each root is a region called the meristem, which is composed of special rapidly dividing cells. Just behind the meristem the cells become elongated, and farther from the tip the cells become differentiated into various kinds of plant tissue. In rapidly growing roots the root tip is covered by a root cap, a protective coat of loose cells that are constantly being rubbed off and replaced as the root grows.

B. Trunk

Bark is the outer protective covering of tree trunks. Because bark varies so widely in color, texture, and thickness, its characteristics provide one of the most important means of identifying species of trees. Most of the total thickness of bark consists of outer bark, which is made up of dead cells. Outer bark may be very thick, as in the cork oak, or quite thin, as in young birches and maples. Openings in the outer bark allow the movement of carbon dioxide and oxygen to and from the inner tissues.

The inner bark layer, called the phloem, consists of a thin layer of living cells. These cells act together to transport food in the form of sugars, which are made in the tree’s leaves, through the trunk and stems to other parts of the tree. Phloem cells have thin walls, and their living contents are so interconnected that the sugar solutions can pass easily and rapidly from one end of the plant to the other. As old layers of outer bark are sloughed off, new ones are constantly being added from the inside, where new phloem is always being created.

Most of a tree trunk is occupied by the wood, or xylem layer, which consists almost entirely of dead cells. The living xylem cells, however, act as the tree’s plumbing system by transporting water and dissolved food through the trunk and stems. A layer of cells called the cambium separates the living xylem cells from the phloem. As the tree grows and develops, the cambium forms new phloem and xylem cells. The layers of xylem cells form rings; these rings can be counted to determine the age of the tree in areas with distinct growing seasons.

C. Leaves

In trees, as in other green plants, the principal function of the leaves is the manufacture of sugars by the process of photosynthesis. In this process, sugars are formed when carbon dioxide (from the air) and water (from the leaf cells) are combined in the presence of light and the green pigment chlorophyll. Oxygen is produced as a byproduct. Some of the newly formed sugar is used by the leaf cells for energy, but most is carried to other parts of the tree to provide energy for growth and development in those areas.

The leaves are also the chief organs involved in the loss of water from the plant, called transpiration. Many of the tree’s tissues cannot function without a constant supply of water, and water is necessary to prevent overheating or wilting of the leaves. Transpiration is responsible for the movement of water from the roots of the tree up to the top. As water is lost through the leaves, water that enters the roots is pulled upward through the xylem tissue to replace the lost moisture, ensuring a constant circulation of water through the tissues of the tree.

D. Flowers

All angiosperms bear flowers, the trees’ reproductive structures. In some trees, such as dogwoods, cherries, and some magnolias, the flowers are large and colorful. Oaks, willows, and other temperate forest trees, on the other hand, often bear small, pale, and inconspicuous flowers.

In maples and many other trees the male and female reproductive parts are carried in separate flowers on the same tree. This arrangement is known as monoecism, and such trees are called monoecious. In oaks, for example, the male pollen-producing flowers are borne in long hanging tassels, and the short-stalked or stalkless female flowers are located on the twigs. In some trees, such as the hollies and willows, the male and female flowers are borne on separate trees. This is known as dioecism, and these trees are called dioecious.

E. Seeds

Seeds, the ripened ovules of the plant that are capable of germination, are the product of fertilized flowers and are distributed in various ways. In pines, for example, each seed is surrounded by a winglike structure. As the winged seed falls from the cone, it floats down to the ground, riding air currents. Oak seeds are enclosed in acorns, which are either planted by squirrels or merely fall to the ground near the parent tree. Willow trees produce thin-walled, flask-shaped fruits that burst open, releasing the seeds. Each seed has a tuft of downy fibers, which enables it to be picked up by air currents and travel for considerable distances. Seeds of other tree species are dispersed by water, mammals, birds, and ants.

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