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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

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 fo

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

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, 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, 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

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,

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 materia

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 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 parti

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 embryoni

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 contribu

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; producti

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, 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 deoxyribonuc

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

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. related

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 bet

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 so

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 , grow

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 ins

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 br

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 nitrog

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 l

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, wh

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

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 betw

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, medici

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 related topics: Botanical Garden Diseases of Plants Floriculture Horticulture Plant Breeding Agri

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, 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 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, 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. related articles: leaf mechanism

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 flav

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,

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 .

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 organe

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 withe

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 elec

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

. 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

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

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

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 meris

Types of Trees There are two general types of trees, angiosperms and gymnosperms . Angiosperms are flowering plants in which the ovule, or seed, is encased in a protective ovary. With about 235,000 species, the angiosperm division is the largest and most diverse plant group. It is divided into dicots , plants with two cotyledons (seed leaf structures), that include the familiar broadleaf trees such as maple and oak ; and the monocots , plants with one cotyledon that include the palms and lily trees. Unlike angiosperms, gymnosperms (Greek for “naked seed”) do not bear flowers. Their seeds lie exposed in structures such as cones or fleshy cups called arils. The group includes about 500 tree species—including three major types, needle-leaf trees (or conifers ), ginkgos , and cycads . A. Angiosperms As angiosperms, all dicots produce flowers, and their seeds are always enclosed in a protective covering. In temperate regions, most of these broadleaf trees are deciduous, meaning that they