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Science for everybody

Science for everybody

Flower is a blossom or an entire plant that is known for its blossoms. Most plants have flowers. In some cases, however, the flowers are so small and plain that few people think of them as flowers. Most people think of flowers as being brightly colored and showy. Plants that have such blossoms include buttercups, dandelions, orchids, roses, tulips, violets, and hundreds of other garden flowers and wildflowers. Some trees, such as catalpas and horse chestnuts, have beautiful blossoms. But the trees themselves are never referred to as flowers. All the plants classified as either garden flowers or wildflowers are smaller than trees. Also, most of these flowering plants have soft stems rather than woody stems.

People prize flowers for their attractive shapes, gorgeous colors, and delightful fragrance. Because of their beauty, flowers are a favorite form of decoration. People also use flowers to express their deepest feelings. For more than 50,000 years, people have placed flowers on the graves of loved ones as a sign of remembrance and respect. Flowers are used at weddings to symbolize love, faithfulness, and long life. Certain flowers also have a religious meaning. Among Christians, for example, the white Easter lily stands for purity. Buddhists and Hindus regard the lotus, which is a type of water lily, as a sacred flower.

Originally, all flowers were wildflowers. Prehistoric people found wildflowers growing nearly everywhere, from the cold wastes of the Arctic to the steaming jungles of the tropics. In time, people learned to grow plants from seeds. They could then raise the prettiest and sweetest-smelling wildflowers in gardens. By 3000 B.C., the Egyptians and other peoples of the Middle East had begun to cultivate a variety of garden flowers, including jasmines, poppies, and water lilies.

Today, a variety of cultivated flowers are raised in every country. Thousands of species of flowering plants still grow in the wild throughout the world. But many of them are becoming rare as more wilderness areas are leveled to make room for farms and cities.

Although people admire flowers for their beauty, the function of the blossoms is to make seeds. Every blossom has male or female parts-or both male and female parts. The male and female parts together produce the seeds. The seeds develop in a female part called an ovary, a hollow structure at the base of a flower. Before the seeds can develop, however, they must be fertilized by sex cells in the pollen produced by the male parts of a flower. In many flowering plants, the pollen is carried from the male parts of one flower to the female parts of another flower. The wind pollinates some kinds of flowers, especially those that have small, plain blossoms. Insects or birds pollinate most plants that have showy or sweet-smelling flowers. In many kinds of flowering plants, the plants pollinate themselves.

Plants that have flowers are called angiosperms. The word angiosperm comes from two Greek words meaning receptacle and seed. All angiosperms bear their seeds in a protective receptacle. Before the seeds are fertilized, they are protected in the ovary. After the seeds are fertilized, the ovary grows into a structure called a fruit. The fruit encloses and protects the ripening seeds. The rest of the blossom gradually dies.

Scientists estimate that there are about 250,000 species of angiosperms throughout the world. All garden flowers and wildflowers belong to this group, as do nearly all other familiar plants, including palms, cattails, grasses, and nettles. One major exception is conifers (cone-bearing plants). Like angiosperms, conifers reproduce by means of seeds. The seeds are produced within the cones. The cones develop from structures that resemble the plain flowers of some angiosperms. But these structures lack an ovary and therefore are not considered to be flowers in the strict sense of the word.

This article discusses FLOWER (How flowers are named and classified). For detailed information about flowering plants in general, see PLANT. World Book also has hundreds of articles on individual flowers and flowering shrubs. For a list of these articles, see the Related articles with this article.







Flower/The uses of flowers

The blossoms of most flowering plants have little food value compared with other plant parts, such as the leaves and fruit. Most blossoms also lack useful chemicals or other materials that can be used in manufacturing. People use flowers mainly as decoration and in landscaping. The production and marketing of flowers for these purposes is a major industry in the United States and many other countries.

As decoration. Flowers are widely used as table decorations in homes and restaurants. In churches and other places of worship, flowers often decorate the altars. Many women wear flowers in their hair or pinned to their dress. Hawaiians often wear flower necklaces called leis. Flowers add beauty and color to many public festivals. One of the most famous of these festivals is the Parade of Roses held every New Year's Day in Pasadena, Calif. The parade features floats decorated with hundreds of thousands of roses and other flowers.

The flowers used as decoration may be either cut flowers or flowering house plants. Cut flowers are garden flowers that were harvested while in bloom. Cut flowers stay fresh several days if their stems are kept in water. Popular cut flowers include daisies, gladioli, irises, and roses. Flowering house plants have showy blossoms and can be grown indoors in containers. Such plants include African violets, azaleas, and wax begonias. Unlike cut flowers, flowering house plants may last almost indefinitely.

Many home gardeners grow their own cut flowers. Greenhouses and flower farms raise them commercially. Greenhouses also grow flowering house plants commercially, as do nurseries. Commercial producers sell their flowers to retail florists, who resell them to the public. Many florists-especially those who supply flowers for such occasions as weddings and funerals-are trained in the art of flower arranging. The section Flower hobbies discusses the art of flower arranging.

In landscaping. Flowers add greatly to the beauty of yards, gardens, parks, and other landscaped areas. The flowers may be planted in beds or borders and arranged according to size, shape, and color. Spring, summer, and fall varieties may be planted to provide a continuous display of blossoms. Some of the most popular plants used in landscaping are flowering shrubs, such as bridal wreaths, forsythias, hydrangeas, and lilacs. Flowering shrubs are especially useful in large landscaping areas because they bloom year after year and require little care.

Many public gardens and parks are noted for their beautiful displays of flowers. Bellingrath Gardens, near Mobile, Alabama, are famous for their azaleas and camellias. Golden Gate Park in San Francisco has one of the world's largest collections of rhododendrons. Irises and peonies are specialties of Longwood Gardens, near Kennett Square, Pennsylvania. The Missouri Botanical Garden in St. Louis is famous for its water lilies and roses. The Royal Botanical Gardens in Hamilton, Ontario, have one of the finest collections of flowers in Canada. The gardens are especially famous for their lilacs.

Other uses. In most cases, the flower buds or blossoms of flowering plants do not serve as food for people. There are some exceptions, however. The flower buds of broccoli, cauliflower, and globe artichoke plants are widely used as vegetables. Broccoli and cauliflower buds grow in thick clusters called heads. The heads are eaten with the stems. Artichoke buds grow singly, and only the bud is eaten. Certain seasonings also come from flower buds or flower parts. For example, cloves are the dried flower buds of the clove tree. Saffron comes from female flower parts of purple autumn crocuses. The petals of some flowers, such as roses and marigolds, have a sweet or spicy taste. They are sometimes used to flavor soups and salads, especially in Europe and Asia. Some people use dandelion and elderberry blossoms to make wine. In China, lightly fried squash flowers are a great delicacy.

Honey is made from nectar, a sugary liquid produced by flowers. Bees gather the nectar from the flowers. They eat some of it and store the rest in their hives. The nectar in the hives gradually turns into honey. Some flowers are better suited to honey production than others because they produce more and tastier nectar. Such flowers include alfalfa, buckwheat, clover, orange, and sage blossoms.

The petals of certain flowers contain sweet-smelling oils. Such flowers include jasmines, mimosas, and roses. The oils obtained from the petals of these flowers supply the fragrances for many high-quality perfumes. However, most perfumes are made synthetically from chemical substances.

Flower/Garden flowers

Garden flowers are simply cultivated wildflowers. Some kinds of garden flowers are exactly like the wild species. Other kinds have been bred scientifically so that their blooms are more attractive than those of the wild variety. Garden flowers are grown on farms and in nurseries and greenhouses as well as in home gardens. Some kinds of garden flowers also make excellent house plants.

Garden flowers can be divided into three main groups based on how long they live: (1) annuals, (2) biennials, and (3) perennials. Annuals are plants that sprout from seed, grow to full size, bloom, produce seeds, and die-all within one year or less. Biennials are plants that live two years. They do not produce flowers and seeds until their second year of growth, after which they die. Perennials live at least three years. They may or may not bloom during their first year of growth. But after perennials have begun to bloom, they may do so every year almost indefinitely, depending on their species.

Annuals and biennials live only a short time. These plants use up their food energy in producing flowers and seeds. Thus, a great number of seeds are produced in each generation. Perennials, on the other hand, store much of their food energy. They use relatively little energy to produce seeds. But since perennials live a long time, they do not need to produce many seeds each year for the species to survive.

All annuals and biennials are herbs-that is, they have soft stems. Some perennials are herbs. The stems of these herbaceous perennials wither and die at the end of each summer. But their roots survive through the winter and grow new stems in spring. Some kinds of herbaceous perennials grow from bulbs (underground stems) or from bulblike structures such as corms or rhizomes. Other perennials, called woody perennials, have woody stems. These plants include shrubs and trees. Their stems do not wither at the end of summer. However, most species of woody perennials found in temperate regions shed their leaves in fall. They then go through a period of reduced activity, called dormancy, in winter.

The great majority of garden flowers are annuals or perennials. Only a few are biennials. However, the classification of flowers as annuals, biennials, or perennials is not always precise. For example, most perennials that are native to warm climates cannot survive cold winters. These flowers therefore cannot be grown as perennials in such places as Canada and the northern parts of the United States. But some warm-weather perennials, such as gloxinias and wax begonias, bloom during their first year of growth. They can thus be grown as annuals in northern climates.

Garden annuals. Most annuals bloom about 8 to 10 weeks after the seeds are planted. In warm climates, annuals can be planted outdoors at any time of the year. In areas with cold winters, they are usually planted in spring. Certain species can survive a light frost and so may be started outdoors from seed as soon as the ground has thawed completely. These hardy annuals include bachelor's buttons, morning-glories, pansies, petunias, sunflowers, sweet alyssum, and sweet peas. However, some hardy annuals, such as pansies and petunias, grow slowly. Gardeners give these flowers a head start by planting them as seedlings. Some gardeners grow their own seedlings. Others buy them from commercial greenhouses. In either case, the seeds are planted indoors in late winter or early spring. The seedlings are then ready to be transplanted outdoors as soon as the ground has completely thawed.

Some annuals, such as garden balsams and marigolds, cannot survive even a light frost. These tender annuals should not be planted outdoors until all danger of frost has passed. In northern regions, frosts may occur for a month or more after the ground has thawed. Gardeners in these regions almost always give tender annuals a head start by sowing the seeds indoors before the growing season begins. They then plant the seedlings outdoors in spring.

Garden biennials. Gardeners who wish to start biennials outdoors usually plant the seeds in midsummer. The plants grow a few leaves by autumn. The leaves may then die, but the roots survive through the winter. The plants grow a new stem, bloom, produce seeds, and die during their second growing season. Instead of starting biennials outdoors, many gardeners buy them as seedlings in spring and raise them like annuals.

Garden perennials. Popular garden perennials in the United States and Canada include asters, bleeding hearts, chrysanthemums, columbines, day lilies, delphiniums, irises, lupines, peonies, phloxes, poppies, primroses, and violets. Most of these flowers need an annual cold or cool season for the growth of new buds. They therefore do not grow well in tropical climates. On the other hand, warm-weather perennials may be raised indoors in northern climates, and many of them are favorite house plants. Some of these perennials are pictured in the next section of the article under the heading Flowers of the tropics and subtropics. They include African violets, gloxinias, ivy geraniums, and wax begonias.

Some perennials, such as columbines and delphiniums, bloom vigorously only three or four years. Most gardeners start these plants from seeds and replace them when necessary. In many cases, perennials are started from cuttings-that is, pieces cut from the stems or roots of adult plants. When planted in water or soil, a cutting develops into a plant identical to the parent. Cuttings, like seedlings, should be started indoors. Some gardeners start cuttings taken from their own plants. Others buy cuttings that have already rooted.

Perennials should be set into the garden in spring or early autumn. In general, spring is the best time to plant perennials outdoors in Canada and the northern regions of the United States. Early autumn is usually the best time in warmer climates.

Most perennials spread by sending out shoots from their roots. The shoots develop into new stems. Most species produce new shoots soon after they have bloomed each year. Over several years, the offshoots from only one plant may cover a wide area. In most cases, however, the plants bloom better if they are dug up, divided, and replanted every few years.

Garden perennials (Bulbs). A bulb is an underground stem with a large bud, wrapped in starchy tissue. The bud develops into a new plant after the weather becomes favorable. The starchy tissue provides the developing plant with food.

Flowers that grow from bulbs or bulblike structures include crocuses, daffodils, fritillaries, gladioli, hyacinths, Madonna lilies, tuberous begonias, and tulips. The majority of these flowers can be grown throughout most of the United States. However, such plants as crocuses and tulips grow better in cool climates than in warm ones. The bulbs of daffodils, hyacinths, and some other flowers can be left in the ground through the winter in most parts of the United States. Certain other flowers, such as gladioli, cannot survive in extremely cold weather. In northern regions, the structures from which these plants grow should be dug up in fall, stored indoors, and then replanted outdoors in spring.

Garden perennials (Flowering shrubs). Shrubs, like trees, have woody stems. But shrubs do not grow as tall as trees, and most of them have two or more thin stems rather than a single thick one. As a rule, flowering shrubs grow best in areas with fairly long summers and cool winters. Most species cannot stand very cold weather. Popular kinds of flowering shrubs include azaleas, flowering quince, forsythia, honeysuckle, hydrangeas, lilacs, redbuds, roses, roses of Sharon, and spiraeas.

Most beginning gardeners buy shrubs as young plants that are ready to set into the garden. However, gardeners can easily produce their own plants from mature shrubbery. Like herbaceous perennials, many shrubs spread by sending out shoots from their roots. Such shoots will develop into new plants if they are dug up with part of the root and replanted. Shrubs that do not send out shoots can be reproduced from cuttings.

Flower/Wildflowers

Each species of flowering plant grows best in a particular type of environment. The species may be unable to grow at all in a much hotter, cooler, wetter, or drier location. Gardeners can control a plant's environment to some extent. They can therefore grow certain flowers in otherwise unfavorable locations. For example, flowers that need much moisture can be raised in dry climates if the gardener supplies the flowers with the necessary water. When flowers grow in the wild, however, they do not receive such special treatment. Each of the species can survive only in the type of environment to which it is naturally suited.

There are about 250,000 kinds of flowering plants in the world. About 165,000 species are native to the tropics. The remaining 85,000 species are native to Europe, North America, and other nontropical regions. About 20,000 of these species are native to the continental United States and Canada. Roughly half of the U.S. and Canadian species have showy blossoms and so can be classed as wildflowers.

Different species of wildflowers grow in seven major wildflower environments. These environments are (1) the Arctic tundra, (2) woodlands and forests, (3) prairies and dry plains, (4) summer-dry regions (regions with mild climates and dry summers), (5) Alpine tundras, (6) deserts, and (7) tropical and subtropical regions. In addition, a major wildflower environment may include various special environments, such as wetlands and shorelines. These special environments have their own types of wildflowers. For example, some varieties of woodland flowers grow mainly in woodland swamps. Water flowers may be found in any environment that has lakes or rivers.

Many wildflowers have spread from their native environment to similar environments in other parts of the world. In some cases, people have introduced the flowers into new environments, either accidentally or on purpose. In other cases, the seeds have been carried by the wind or by animals. Certain seeds easily stick to the bodies of animals and so may be carried long distances by animals that migrate. Wildflowers that have been introduced into North America from other parts of the world include bindweed, chicory, dandelions, furze, mullein, mustard, oxeye daisies, and Queen Anne's lace. Most of these plants are now widely distributed throughout the United States and parts of Canada. In many cases, they compete with native plants and farm crops and so are regarded as weeds.

The drawings in this section illustrate typical flowers of each major wildflower environment. Except for the tropical and subtropical flowers, most of the species pictured are native to North America. In many cases, however, flowers closely related to these species grow in similar environments in other parts of the world.

Flowers of the Arctic tundra. The Arctic tundra extends across the extreme northern parts of North America, Europe, and Asia. It is a cold, dry, treeless grassland. Most of the region has an annual frost-free period of less than two months. The ground remains frozen all year except at the surface. The surface thaws in spring and remains soggy throughout most of the summer. The tundra has few annuals. However, a variety of herbaceous perennials thrive there. These hardy plants include cinquefoils, fireweeds, louseworts, poppies, and saxifrages. They come to life suddenly in spring and brighten the brief Arctic summers with their colorful blossoms.

Flowers of woodlands and forests. Trees need considerable moisture and a yearly frost-free period of over two months to reach full size. Woodlands and forests thus grow only in regions that meet these needs. Seedlings have difficulty competing with established plants in wooded areas, and so such areas have few annuals. Nearly all the flowers are perennials.

There are two main types of forests: (1) needleleaf and (2) broadleaf. Needleleaf forests stretch south from the Arctic tundra across most of Canada, northern Europe, and northern Asia. They also extend along lower slopes of the Rocky Mountains into the Northwestern United States. Needleleaf forests have most of the same kinds of flowers as the tundra plus such species as bog orchids, columbines, and pitcher plants.

The largest broadleaf forests outside the tropics are in the eastern half of the United States, eastern Asia, and western and central Europe. The growth of flowers in these forests is regulated largely by the amount of shade or sunlight. Many woodland flowers bloom in early spring, before the trees develop leaves and the woods become heavily shaded. These early-blooming species include bloodroots, dogtooth violets, and trilliums. After the woods become shaded, flowers bloom mainly in clearings and in meadows at the edge of the woods. Such species as spiderworts and touch-me-nots blossom in late spring or early summer. Others, such as asters and goldenrods, bloom in late summer or fall.

Flowers of prairies and dry plains. Prairies and dry plains are grasslands. They receive less rainfall than woodlands and have hot summers and cold or cool winters. Prairies once covered much of the central United States and south-central Canada as well as large areas of Argentina, Russia, South Africa, and Ukraine. Today, most of these areas are used for growing crops, especially grains. However, some prairie lands have been left in their natural state. These areas are still noted for their tall grasses and traditional spring, summer, and fall flowers. Most prairie flowers are perennials. Grasses grow so thick on prairies that few seeds can penetrate the sod. Annuals have difficulty surviving as a result. Typical kinds of prairie flowers of the United States and Canada include blazing stars, pasqueflowers, coneflowers, rattlesnake masters, sunflowers, tickseeds, and wild indigo.

The prairies of the United States and Canada give way to dry plains in the west. These plains, which are called steppes, receive less moisture than prairies and so are covered with short, rather than tall, grasses. Steppes also adjoin the prairies of Argentina, Russia, South Africa, and Ukraine. The moister areas of the steppes have many of the same kinds of flowers as the prairies. The drier sections have drought-resistant species and more annuals. Typical flowers of the North American plains include prickly pear cactuses, low townsendias, scarlet globe mallows, and sunflowers.

Flowers of summer-dry regions. Summer-dry regions are found along the Mediterranean Sea, in California, and in parts of Australia, Chile, and South Africa. The mild, moderately dry climate of these regions is an ideal environment for the growth of wildflowers. A high percentage of these flowers are annuals, most of which flower in spring. Summer-dry regions include grasslands, woodlands, and chaparrals (regions of shrubs and scrubby trees). The California chaparral and its neighboring grasslands and oak woodlands have a tremendous variety of wildflowers. Some of them, such as fire poppies and fiddlenecks, are California natives. Others, such as black mustard and star thistles, have been introduced there from similar areas in the Mediterranean region.

Flowers of alpine tundras. Alpine tundras lie at high elevations in mountains throughout the world. Like the Arctic, these areas are too cold and dry for trees to grow. However, grasses, low shrubs, and a variety of wildflowers thrive. The chief alpine tundras are in the European Alps, the Himalaya of Asia, and the Rocky Mountains of North America. Most alpine flowers grow in mountain meadows, but some species are especially suited to rocky places. As in the Arctic, the yearly frost-free period is usually less than two months, and so nearly all the flowers are perennials. Most are small and grow slowly, and some do not even start to bloom until they are 10 years old or older. Many alpine and Arctic flowers are closely related, and some are identical.

Flowers of the desert. Deserts are extremely dry regions with a generally warm climate. Most deserts receive less than 10 inches (25 centimeters) of rainfall a year. In some cases, all the rain falls in one or two tremendous cloudbursts. Desert flowers must therefore be able to survive for many months without rain.

Some desert flowers are shrubs. These plants have vast networks of roots that absorb every available drop of moisture in the soil. Other desert flowers are herbaceous perennials with thick, spongy stems. The stems store water, which the plants use during the long dry spells. Cactuses are the best-known examples of this type of plant. Still other flowers are annuals. Annuals thrive in deserts because they have relatively few perennials to compete with. In addition, the seeds of many annuals can survive even the longest dry periods. The seeds lie buried until the rains return. They then sprout, and the plants complete their entire life cycle within only a few weeks.

The deserts of southwestern North America have a wide variety of flowers, including most of the familiar kinds of cactuses. Many of the cactuses have beautiful blossoms. The North American deserts also have numerous flowering shrubs and hundreds of annuals. The shrubs include such species as brittlebushes and desert mallows. Desert marigolds, devil's claws, evening primroses, ghost flowers, and sand verbenas are only a few of the many colorful annuals.

Flowers of the tropics and subtropics. Thousands of species of wildflowers grow in the humid and warm to hot climate of the tropics and subtropics. The tropical rain forests of Central and South America have the greatest variety of tropical flowers, including thousands of kinds of rare and beautiful orchids. About 950 species of flowering plants are native to Hawaii. However, many have become extinct or extremely rare as a result of land development and overpicking. Southern regions of China and South Africa have the richest assortment of subtropical flowers. Southern Florida also has many species. Some of them, such as clamshell orchids, are Florida natives. Others, such as bougainvilleas, are introduced species.

Flower/The parts of a flower

The typical flower develops at the tip of a flower stalk. The tip is somewhat enlarged, forming a cup-shaped structure called a receptacle. A bud grows from the receptacle and develops into a flower.

Most flowers have four main parts: (1) the calyx, (2) the corolla, (3) the stamens, and (4) the pistils. The calyx is the outermost part of a flower. It consists of a set of leaflike or petallike structures called sepals. The corolla consists of a flower's petals. The stamens and pistils make up the reproductive parts of flowers. The stamens are the male parts, and the pistils are the female parts. Every flower has either stamens or pistils-or both stamens and pistils. Flowers that have all four main parts are called complete flowers. Flowers that lack one or more of the parts are called incomplete flowers. In addition to the main parts, many flowers have glands that produce nectar. These glands, which are called nectaries, lie near the base of the flower.

In most flowers, each main part consists of three, four, or five elements or of multiples of three, four, or five elements. In a trillium, for example, three sepals form the calyx, and three petals form the corolla. The flower has six stamens, and the pistil is composed of three equal parts. The elements may be separate from one another, like the petals of a poppy or a rose. Or the elements may be fused (joined together). In flowers with fused petals, for example, the corolla is shaped like a tube, bell, trumpet, pouch, or saucer. Flowers that have such corollas include morning-glories, daffodils, and petunias. In such species as primroses and verbenas, the petals are fused at the base and free at the tip. The corolla thus has a tubelike or bell-like base and a fringed edge.

In buttercups, morning-glories, and most other flowers, all the main parts are arranged around the center of the flower in a circular fashion. If the flower is divided in half in any direction, the halves will be alike. Such flowers are radially symmetrical. Orchids, snapdragons, sweet peas, and certain other flowers can be divided into identical halves only if the blossoms are cut through lengthwise. Such kinds of flowers are bilaterally symmetrical.

The calyx. The sepals, which make up the calyx, are the first parts to form among the majority of flowers. They protect the developing inner parts of the flower. In most cases, the sepals remain attached to the flower after it opens.

In many flowers, such as buttercups and magnolias, the sepals are greenish, leaflike structures that are on the underpart of the flower. Other flowers have sepals that look like petals. Among many members of the iris, lily, and orchid families, the sepals and the petals look so much alike that they cannot be told apart. Botanists call these petallike structures tepals. Certain kinds of flowers have colorful sepals in place of petals. These flowers include anemones, hepaticas, larkspurs, and marsh marigolds.

The corolla, which consists of a flower's petals, is the showy, brightly colored part of most flowers. The colors of the petals-and of colored sepals-attract insects or birds that help spread a flower's pollen. The colors come from certain chemicals in a plant's tissues. These chemicals are often present in all parts of the plant, not only the petals or sepals. But they are masked in the other parts by large amounts of green or brown pigments. Many flowers also have spots, stripes, or other markings on their petals that attract insects or birds. In most cases, the odors of flowers come from oily substances in the petals. Strong odors, like bright colors, attract animals.

The stamens are the male, pollen-producing parts of a flower. They are not particularly noticeable in most flowers. In some cases, however, the stamens make up a flower's most attractive part. Male acacia flowers, for example, consist mainly of a large feathery tuft of colorful stamens.

In most flowers, each stamen has two parts-a filament and an anther. The filament is a threadlike or ribbonlike stalk with an enlarged tip. The enlarged tip forms the anther. The anther consists of four tiny baglike structures that produce pollen. After the pollen is ripe, these structures split open, which releases the pollen grains.

The stamens are separate from one another in many flowers. But in such species as hollyhocks and sweet peas, some or all of the filaments are fused and form a tube around the pistil. In some flowers, the stamens are fused with one or more other flower parts. For example, the stamens of gentians are fused to the petals, and the stamens of most orchids are fused to the pistils.

The pistils are the female, seed-bearing parts of a flower. Some flowers, including all members of a pea family, have only one pistil. But most flowers have two or more. In many species, the pistils are fused into one compound pistil. A compound pistil is often referred to simply as a pistil. The individual pistils that make up a compound pistil are called carpels.

Among most flowers, each pistil or carpel has three parts-a stigma, a style, and an ovary. The stigma is a sticky area at the top. The style consists of a slender tube that leads from the stigma to the ovary. The ovary is a hollow structure at the base. It contains one or more structures called ovules.

Variations in flower structure. Many kinds of flowers grow in clusters called inflorescences. In some species, such as bridal wreaths and snapdragons, the individual flowers in each cluster are easy to identify as flowers. In numerous other species, however, the inflorescence looks like one flower and the individual flowers that make up the inflorescence look like petals. These species include the many members of the composite family, such as asters, chrysanthemums, daisies, dandelions, and sunflowers.

Among the members of the composite family, the flowers grow from a head at the tip of the flower stalk. Each head has several or many flowers, depending on the species. A dandelion head, for example, may have 100 or more tiny yellow flowers. Each flower, or floret, looks like a petal but consists of a calyx, a corolla, stamens, and a pistil. One petal makes up the corolla. The dandelion florets grow so close together that only their corollas can be seen.

The flowers of some plants grow in plain, tassellike inflorescences called catkins. A catkin is composed of naked flowers-that is, flowers that lack both petals and sepals. Plants that have catkins include alders, poplars, and willows.

Many plants that have inflorescences also have leaflike structures called bracts just beneath each flower cluster. In most cases, bracts are small, green, and barely noticeable. But in a few species, they are so large and showy that most people mistake them for part of the flower. The showy "petals" of bougainvilleas, dogwoods, and poinsettias are bracts. The flowers themselves are small, plain-looking inflorescences at the center of the bracts.

Among most species of flowering plants, each plant bears flowers that have both stamens and pistils. Such flowers are called perfect flowers. In some species, however, each plant bears flowers that have either pistils or stamens, not both. Such flowers are called imperfect flowers. If a flower has pistils but no stamens, it is called a pistillate flower. If it has stamens but no pistils, it is called a staminate flower. In some species, the staminate and pistillate flowers are on the same plant. Such species are known as monoecious species. They include begonias, oaks, and squashes. Among dioecious species, the male and female flowers are on different plants. Dioecious species include poplars, willows, and American holly.

Flower/The role of flowers in reproduction

Flowering plants reproduce sexually. The sexual parts of their blossoms produce male and female sex cells. The male cells, called sperm, are in the pollen produced by the stamens. The female cells, called eggs, are in the ovules produced by the pistils. The sperm and egg cells unite in the ovary at the base of a pistil and develop into seeds.

Reproduction in flowers involves two main steps: (1) pollination and (2) fertilization. Pollination is the transfer of pollen from a stamen to a pistil. Fertilization is the union of a sperm with an egg cell. Fertilization occurs in much the same way in all flowering plants. However, there are two methods of pollination: (1) cross-pollination and (2) self-pollination. Cross-pollination involves the transfer of pollen from a stamen on one plant to a pistil on another plant. In self-pollination, pollen is transferred from a stamen of one flower to a pistil of the same flower or to a pistil of another flower on the same plant.

Cross-pollination is the method of pollination in most flowering plants. The method requires an agent to carry the pollen from flower to flower. Insects are the most common agents of cross-pollination.

Many insects depend on flowers for food. Bees live on nectar and pollen. Honey bees also use nectar to make honey, which they feed on in winter. Butterflies and moths also live on nectar, and certain beetles and flies feed on both nectar and pollen. As an insect travels from flower to flower in search of food, pollen grains stick to its body. Some or all of these grains may brush off onto the stigmas of some flowers that the insect visits. One or more of these flowers may thus become cross-pollinated.

When searching for food, an insect could easily fail to visit a particular kind of flower unless the insect was attracted to it. Most flowers that depend on insects for pollination are brightly colored or heavily scented. Each kind of pollinating insect is attracted by certain colors or odors and so visits certain flowers rather than others. However, more than one kind of insect pollinates most insect-pollinated flowers. For example, moths and butterflies visit many of the same flowers. A few kinds of insects and flowers have developed highly specialized relationships with each other. These flowers are pollinated only by a particular kind of insect. For example, bumble bees are the type of insect that pollinates the red clover flower.

Pollination by bees. More kinds of flowers are pollinated by bees than by any other kind of insect. Bees cannot see the color red. Otherwise, they have a keen sense of sight. They also have a well-developed sense of smell. Bees are strongly attracted by yellow and blue blossoms, especially those with a sweet odor. Unlike people, bees can see ultraviolet light. Many flowers, particularly yellow ones, have elaborate ultraviolet markings. These markings attract bees to the flowers and even pinpoint the location of the nectaries.

Many of the flowers pollinated by bees have a highly complicated structure that encourages cross-pollination and discourages self-pollination. For example, a bee can reach the nectar of a snapdragon only after brushing against the stigma. It then cannot leave the flower without touching the pollen. Furthermore, the bee cannot touch the stigma after it touched the pollen.

Pollination by butterflies and moths. Butterflies and moths are attracted to flowers that produce abundant nectar. In many such flowers, the nectaries are long and tube-shaped or are at the base of a long tube-shaped corolla. Butterflies and moths have exceptionally long, tubelike mouthparts, which enable them to reach into these structures and suck up the nectar. Butterflies, like bees, prefer flowers with sweet-smelling yellow or blue blossoms.

Unlike most bees and butterflies, many moths rest during the day and search for food at night. Many of the flowers that attract moths open only at night. Most of these flowers are pale-colored or white and so are easier to see at night than dark blossoms. Many of the flowers are also heavily scented and give off their scent only at night. Flowering tobacco and various kinds of evening primroses and honeysuckles are among the plants commonly pollinated by moths.

The yucca flowers of the American Southwest are pollinated only by the yucca moth. The female moth carries pollen from one yucca plant to another. She bores into the ovary of the second flower and lays her eggs inside it. She then deposits pollen from the first flower onto the stigma of the second. The moth eggs and the yucca seeds develop together. The eggs hatch into caterpillars, which feed on the seeds. But enough seeds remain uneaten to produce the next generation of yucca plants.

Pollination by beetles and flies. Beetles visit flowers in which both nectar and pollen are plentiful. They prefer white or dull-colored flowers with spicy odors, such as magnolias and wild roses.

Most flies do not have mouthparts that are long enough to suck nectar from tube-shaped flowers. These flies usually visit flowers that have flat corollas, such as hawthorn blossoms and buttercups. Some flowers, such as carrion flowers and skunk cabbages, give off a foul odor that attracts flies.

Pollination by other agents. Some birds feed on nectar and so help pollinate flowers. Unlike most pollinating insects, birds have a weak sense of smell. But birds have sharp vision and see red as well as they see other colors. Most odorless red flowers are pollinated by birds. In North America, hummingbirds are the chief bird pollinators. Hummingbirds are particularly attracted to red, orange, and yellow flowers, such as columbines, fuchsias, and Indian paintbrushes. Bats and the wind are also agents of pollination. Bats pollinate certain strongly scented flowers of the tropics. The wind spreads the pollen of most plants whose flowers lack petals and sepals. These plants include oaks, ragweeds, sedges, and most wild grasses.

Self-pollination. About half of all species of plants normally pollinate themselves. Such plants include barley, oats, peas, and wheat. However, self-pollination also occurs frequently in many species that depend on cross-pollination. In such cases, pollen may simply fall onto a stigma of the same plant.

Self-pollination is impossible in dioecious species because the male and female flowers are on different plants. In addition, many other plants have characteristics that discourage or prevent self-pollination. In such flowers as hibiscuses and lilies, for example, the stamens are much shorter than the pistils. Any pollen that drops from the stamens is therefore unlikely to reach a stigma of a pistil on the same plant. Many kinds of plants, such as flowering tobacco and rye, have chemicals in their cells that prevent self-pollination.

Fertilization. A pollen grain that lands on a stigma may grow a pollen tube. The tube pushes its way down the style to an ovule in the ovary. Sperm from the pollen grain travel down the tube to the ovule. Fertilization occurs when a sperm unites with an egg cell in the ovule. A seed then begins to develop. The ovary itself develops into a fruit that encloses the seed. For an illustration of this process, see PLANT (How flowering plants reproduce).

An ovary may be penetrated by many pollen tubes. But the number of seeds that develop depends on the number of ovules. An ovary that has only one ovule develops into a single-seed fruit, such as an acorn or cherry. An ovary that has many ovules develops into a fruit with many seeds, such as a milkweed pod or watermelon.

Flower/Flower hobbies

Two of the most popular flower hobbies, outdoor and indoor gardening, are discussed in GARDENING. This section deals with three other flower hobbies: (1) studying wildflowers, (2) flower arranging, and (3) flower breeding.

Studying wildflowers. To study wildflowers scientifically, you must be able to identify them. Various handbooks help provide such identification. Most of these books deal with the flowers of a particular region, such as the Northeastern or Southwestern United States. The typical handbook divides the flowers into groups according to the color of their blossoms. Each of these groups is then subdivided according to certain other characteristics of the plants, such as the number of their petals or the arrangement of their leaves. By checking a particular flower for each of the listed characteristics, you should be able to identify it.

One way to learn about wildflowers is to study them in their natural surroundings. For example, you might try to identify all the species in a particular environment, such as a meadow or woods. By taking careful notes and revisiting the location at various times of the year, you can produce a "biography" of the common flowering plants of that environment. Another way of studying wildflowers is by collecting them. However, you must follow certain rules in picking wildflowers. After you have picked the flowers, they must be properly preserved.

Rules for picking wildflowers. About 10 per cent of all the native flowering plants of the United States are so rare that they are considered to be endangered species. Unless these plants are carefully protected in their natural surroundings, they may die out completely. The U.S. government and most state governments have passed laws that prohibit people from picking wildflowers in public parks and forests. Such laws not only help conserve endangered species, but they also help preserve the blossoms so that more people can enjoy them.

In an area not protected by conservation laws, do not pick a specimen of a particular type of flower unless the species is plentiful in the area. As a general rule, wildflowers should never be uprooted. However, you may dig up one specimen by the roots, but again only if the species is abundant in the area.

Preserving wildflower specimens. The easiest way to preserve flower specimens is by pressing them. The method may be used to preserve not only the blossoms of wildflowers but also the entire plant, including the roots. While a specimen is still fresh, carefully arrange it between two sheets of newspaper. Then place the newspaper between two stacks of blotters or between the pages of an old phone book. Apply pressure by tying the blotters or phone book into a tight bundle or by using a weight. The pressure flattens the specimen and squeezes out the moisture. Change the newspaper wrapping daily and move the specimen to a dry part of the stack of blotters or phone book. After 7 to 10 days of pressing, the specimen should be dried out, unless it was especially juicy. The preserved specimen will last almost indefinitely if it is protected from moisture and insects.

Tape or glue each finished specimen to a sheet of heavy paper. Then label the mounted specimen with its common and scientific names, the place where the flower was found, the date it was picked, and any interesting facts about its growing habits. An organized collection of mounted pressed flowers is called a herbarium. See HERBARIUM.

Flower arranging. The ancient Egyptians, Greeks, and Romans all practiced the art of making decorative arrangements of cut flowers. However, the art received its fullest development in Japan. The Japanese tradition of flower arranging dates from the 500's. At that time, Japanese Buddhists began to make floral arrangements in an elaborate style for altars of their temples. Over time, the Japanese refined and simplified this style and worked out its artistic principles. These Japanese principles had a strong influence on the styles of flower arranging in many other countries.

The Japanese try above all to make each floral arrangement look natural, as if it were growing outdoors. They follow carefully worked out principles of design and color to achieve this natural effect. The Japanese use leaves and stems as major elements in many arrangements.

In Western countries, on the other hand, traditional styles of flower arranging tend to emphasize only the blossoms. Although Western principles of design and color differ from those of the Japanese, they are just as important to the overall effect.

Most flower arrangements are made of fresh flowers. However, you can use dried flowers. Flowers suited to drying include chrysanthemums, goldenrods, hydrangeas, larkspurs, and pearly everlastings. You can dry any of these flowers by hanging the blossoms head downward in a dark, dry, well-ventilated room for about three weeks. You can also dry various kinds of grasses and leaves in this way and then add the specimens to the flower arrangement. A dried floral arrangement that is prepared in late summer or fall should last through the winter.

Flower breeding has become an increasingly popular hobby among gardeners. Each year, amateur gardeners in many countries produce hundreds of new varieties of flowers. Roses are especially popular for breeding, but many gardeners also work with such flowers as chrysanthemums, irises, orchids, and water lilies. Most new varieties introduce changes in the color, shape, size, or fragrance of the blossoms. For example, breeding experiments have resulted in many dwarf varieties and numerous varieties with double flowers. Double flowers have more than the normal number of petals. Flower breeding has also produced such improvements as greater hardiness and greater resistance to diseases and insects.

Gardeners breed flowers by crossing two related species or two varieties of the same species. Each parent is selected for a desired characteristic, such as the color or size of its blossoms. The breeder takes pollen from one parent and places it on a stigma of the other parent. Some of the resulting offspring may have the desired characteristics of both parents. Such offspring are called hybrids. By repeating experiments with many parents and many varieties, gardeners can produce hybrids of greater vigor and beauty.

Flower/How flowers are named and classified

The naming of flowers. Flowers have both common names and scientific names. Many common names can be traced back hundreds or even thousands of years. The practice of giving plants scientific names of the type now in use began during the mid-1700's.

Common names. The common names of many wildflowers originated in folklore. In numerous cases, a plant's name comes from a traditional belief concerning the plant. People once used many wild plants as medicines and named the plants after the ailments they were thought to cure. For example, North American wildflowers named in this way include agueweed and colicroot. People believed that agueweed cured a fever called the ague and that colicroot cured abdominal cramps called colic. In several other cases, a plant's common name simply describes a characteristic of the plant. For example, the blossoms of lady's-slippers resemble women's shoes. The leaves of pitcher plants form a pitcherlike shape. Skunk cabbages are named after their unpleasant odor.

The English names of many wildflowers end in -wort. Such flowers include birthworts, liverworts, milkworts, ragworts, and soapworts. The ending -wort comes from the Old English word wyrt, meaning root or plant. The first part of each name refers to some special characteristic of the plant, such as its appearance or supposed healing powers. For example, ragworts were so named because their leaves have extremely ragged edges. Birthworts provided a medicine that was believed to help women during childbirth.

The English names of many poisonous or supposedly poisonous wildflowers end in -bane. These flowers include cowbane, dogbanes, fleabanes, and henbane. The word ending comes from the Old English word bana, meaning murderer. The animal mentioned in each case was supposedly the one most affected by the poison. However, many of these plants, such as cowbane and most dogbanes, are also poisonous to other animals and to people. Some of the plants, including fleabanes, are harmless.

The English names for many garden flowers can be traced back to Latin or ancient Greek. For example, the English name lily comes from the Latin name lilium. Peonies were called paeoniae in Latin, roses were rosae, and violets were violae. The English name iris comes from the Greek word iris, meaning rainbow. Hyacinths are named after Hyacinthus, a youth in Greek mythology famed for his great beauty. The names of some garden flowers come from languages other than Latin and Greek. For example, the name tulip comes from the Turkish word tulbent, meaning turban. Tulips are shaped somewhat like turbans and were introduced into Western countries from Turkey.

During modern times, a number of flowers have been named after people. For example, begonias were named in honor of Michel Begon, a governor of French Canada and an amateur botanist. Dahlias were named after Anders Dahl, a Swedish botanist who introduced the flowers into Europe from Mexico. Poinsettias were named in honor of Joel R. Poinsett, a U.S. minister to Mexico, who introduced the plants into the United States from Mexico.

Scientific names. The common names of flowers are not suitable for scientific purposes. In many cases, the same flower has more than one common name. In the United States, for example, a marsh marigold is also called a kingcup, May blob, and cowslip. In other cases, the same name is used for entirely different flowers. For example, several different species of flowers are called bluebells in various English-speaking countries. To help avoid such confusion with names, botanists refer to each species of flower by its scientific name.

The Swedish botanist Carolus Linnaeus devised the modern scientific system of plant names in the 1750's. In this system, each species is given a two-part Latin name. The first part of the name refers to the genus (group of species) to which the particular species belongs. The second part of the name refers to the species itself. Each species has only one scientific name, and each name applies to only one species. For example, the flower known as a marsh marigold, kingcup, May blob, or cowslip has the scientific name Caltha palustris. The genus name, Caltha, is the Latin word for marsh marigold. The second part of the name, palustris, is a Latin word meaning marsh loving. No other species of plant in the world is named Caltha palustris. By using scientific names, botanists can identify every species of plant precisely and without confusion.

The scientific naming and the scientific classification of flowering plants are closely related. For example, newly discovered species must be classed according to genus before they can be given scientific names. However, every species keeps the second part of its name permanently, regardless of any changes that may later be made in the classification of the species. Thus, Caltha palustris, the marsh marigold, will always keep its specific name palustris even if it is someday reclassified into a different genus.

The classifying of flowers. Flowering plants make up the class (group) of plants called Anthopsida. This class is split up into two subclasses: (1) dicotyledons, also called dicots, and (2) monocotyledons, also known as monocots. Plants are grouped based on the structure of their seeds. The seeds of dicots have two tiny leaves called cotyledons. The seeds of monocots have only one cotyledon. In addition, the petals and other flower parts of most monocots grow in threes or in multiples of three, and the veins in their leaves parallel one another. The flower parts of most dicots grow in fours or fives or in multiples of four or five, and the veins in their leaves are branched rather than parallel. Of the approximately 250,000 species of flowering plants, about 190,000 are dicots and about 60,000 are monocots.

Each of the two subclasses of flowering plants is divided into orders, each order into families, and each family into genera. The table on the following two pages lists the families that include most of the well-known garden flowers and wildflowers of the United States and Canada. These families consist mainly of herbs and shrubs, but some also include trees. The table gives (1) the approximate number of species in each family; (2) typical characteristics of most flowers in the family; and (3) the names of representative flowers.



Critically reviewed by Peter H. Raven

Questions

What are annuals? Biennials? Perennials?

What attracts insects and birds to flowers?

How do individual perennial plants spread?

What is the easiest method of preserving wildflower specimens? What steps does the method involve?

Why do annuals thrive in deserts?

What are the four main parts of a flower?

How do bees and other insects help flowering plants reproduce?

How do gardeners produce new varieties of flowers? What are such varieties called?

Why do botanists refer to flowers by their scientific names rather than by their common names?

Additional resources

Level I

Burton, Jane, and Taylor, Kim. The Nature and Science of Flowers. Gareth Stevens, 1998.

McEvoy, Paul. Flowers. Chelsea Hse., 2003.

Pascoe, Elaine. Flowers. Blackbirch Pr., 2003.

Souza, Dorothy M. Freaky Flowers. Watts, 2002.

Wildflowers. 1998. Reprint. National Geographic Soc., 2002.

Level II

Heffernan, Cecelia. Flowers A to Z: Buying, Growing, Cutting, Arranging. Abrams, 2001.

Hill, Lewis and Nancy. The Flower Gardener's Bible. Storey Bks., 2003.

Hodgson, Larry. Annuals for Every Purpose. Rodale, 2002.

Mikolajski, Andrew. The Flowering Garden: Choosing and Growing Glorious Flowers for Every Season. Southwater, 2003.

Schmid, W. George. An Encyclopedia of Shade Perennials. Timber, 2002.

Seguin-Fontes, Marthe. The Language of Flowers. Sterling Pub., 2001.

Turner, Carole B. Seed Sowing and Saving. 1998. Reprint. Storey Communications, 2003.

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Plant. Plants grow in almost every part of the world. We see such plants as flowers, grass, and trees nearly every day. Plants also grow on mountaintops, in the oceans, and in many desert and polar regions.

Without plants, there could be no life on Earth. People could not live without air or food, and thus could not live without plants. The oxygen in the air we breathe comes from plants. Our food comes from plants or from animals that eat plants. We build houses and make many useful products from lumber. Much of our clothing is made from the fibers of the cotton plant.

Scientists believe there are over 260,000 species (kinds) of plants, but no one knows for sure. Some tiny plants that grow on the forest floor can barely be seen. Others tower over people and animals. Among the largest living things on Earth are the sequoia trees of California. Some stand over 290 feet (88 meters) high and measure over 30 feet (9 meters) wide. Plants also are the oldest living things. One bristlecone pine tree in California started growing 4,000 to 5,000 years ago.

Scientists divide all living things into five main groups called kingdoms. These kingdoms are (1) plants, (2) animals, (3) fungi, (4) protists, and (5) prokaryotes. Scientists classify organisms in a particular kingdom because the organisms share certain basic characteristics. These characteristics include physical structure, means of obtaining food, and means of reproduction.

Plants have characteristics that set them apart from other living things. For example, both plants and animals are complex organisms that are made up of many types of cells. But plant cells have thick walls that consist of a material called cellulose. Animal cells do not have this material. The cells of prokaryotes and some protists, like those of plants, have cellulose walls. But prokaryotes and protists are simple organisms made up of one cell or only a few types of cells. Bacteria, including cyanobacteria (blue-green algae), are prokaryotes. Protists include other algae and diatoms and protozoans.

All plants develop from a tiny form of the plant called an embryo. Prokaryotes, protists, and fungi-such as molds and mushrooms-do not develop from embryos. Plants also obtain food in ways different from those of most other organisms. Almost all kinds of plants stay in one place for their entire lives. Most plants make their own food from air, sunlight, and water by a process called photosynthesis. Fungi cannot make their own food. They obtain the nutrients they need from the animals, plants, and decaying matter on which they live. Animals also cannot make their own food, but most animals can move about to find it.

This article discusses PLANT (Classification of plants). See the Related articles with this article for a list of articles on many kinds of plants.

The importance of plants

Plants supply people with food, clothing, and shelter. Many of our most useful medicines are also made from plants. In addition, plants add beauty and pleasure to our lives. Most people enjoy the smell of flowers, the sight of a field of waving grain, and the quiet within a forest.

Not all plants are helpful to people. Some species grow in fields and gardens as weeds that choke off useful plants. Tiny bits of pollen from certain plants cause such health problems as asthma and hay fever. Some plants are poisonous if eaten. Others, such as poison ivy and poison oak, irritate the skin.

Food. Plants are probably most important to people as food. Sometimes we eat plants themselves, as when we eat apples, peas, or potatoes. But even when we eat meat or drink milk, we are using foods that come from an animal that eats plants.

People get food from many kinds of plants-or parts of plants. The seeds of such plants as corn, rice, and wheat are the chief source of food in most parts of the world. We eat bread and many other products made from these grains, and almost all our meat comes from animals that eat them. When we eat beets, carrots, or sweet potatoes, we are eating the roots of plants. We eat the leaves of cabbage, lettuce, and spinach plants; the stems of asparagus and celery plants; and the flower buds of broccoli and cauliflower plants. The fruits of many plants also provide us with food. They include apples, bananas, berries, and oranges, as well as some nuts and vegetables. Coffee, tea, and many soft drinks get their flavor from plants.

Raw materials. Plants supply people with many important raw materials. Trees give us lumber for building homes and making furniture and other goods. Wood chips are used in manufacturing paper and paper products. Other products made from trees include cork, natural rubber, maple syrup, and turpentine. Most of the world's people wear clothing made from cotton. Threads of cotton are also woven into carpets and other goods. Rope and twine are made from hemp, jute, and sisal plants.

Plants also provide an important source of fuel. In many parts of the world, people burn wood to heat their homes or to cook their food. Other important sources of fuel-coal, oil, and natural gas-also come from plants. Coal began to form millions of years ago, when great forests and swamps covered much of Earth. As the trees in these forests died, they fell into the swamps, which were then covered by mud and sand. The increasing pressure of this mass of materials helped cause the dead plants to turn into coal. Petroleum and natural gas were formed in ancient oceans by the pressure of mud, sand, and water on decaying masses of plants and animals.

Medicines. Many useful drugs come from plants. Some of these plants have been used as medicines for hundreds of years. More than 400 years ago, for example, some Indian tribes of South America used the bark of the cinchona tree to reduce fever. The bark is still used to make quinine, a drug used to treat malaria and other diseases. Another drug, called digitalis, is used in treating heart disease. It is made from the dried leaves of the purple foxglove plant. The roots of the Mexican yam are used in producing cortisone, a drug useful in treating arthritis and a number of other diseases.

Plants and the cycle of nature. All living things-plants, animals, fungi, protists, and prokaryotes-are linked by the cycle of nature. This natural process gives people oxygen to breathe, food to eat, and heat to keep them warm. The sun supplies the energy that runs the cycle.

Plants have a complex relationship with people and animals in the cycle of nature. Plants use sunlight to make their own food, and they give off oxygen during the process. People and animals eat the plants and breathe in the oxygen. In turn, people and animals breathe out carbon dioxide. Plants combine the carbon dioxide with energy from sunlight and water and minerals from the soil to make more food. After plants and animals die, they begin to decay. The rotting process returns minerals to the soil, where plants can again use them.

Plants also play an important part in conservation, the protection of soil, water, wildlife, and other natural resources. Plants help keep the soil from being blown away by the wind or washed away by the water. They slow down the flow of water by storing it in their roots, stems, and leaves. Plants also give wild animals food to eat and a safe place to live. For more information on the importance of plants in nature, see the World Book articles on BALANCE OF NATURE, CONSERVATION, and ECOLOGY.

Kinds of plants

Each of the more than 260,000 species of plants differs from every other species in one or more ways. However, plants also have many features in common. Based on these similarities, scientists are able to classify distinct plants into groups. The study of plants is called botany, and scientists who study plants are known as botanists.

This section describes the chief kinds of plants found in the plant kingdom. It is divided into five basic groups: (1) seed plants, (2) ferns, (3) lycopsids, (4) horsetails, and (5) bryophytes. A table showing a more detailed system of plant classification that is used by many botanists appears at the end of the article. See also CLASSIFICATION, SCIENTIFIC.

Seed plants consist of a wide variety of plants that bear seeds to reproduce. Most botanists divide the seed plants into two main groups of plants-angiosperms and gymnosperms.

Angiosperms are flowering plants. They make up the vast majority of the more than 260,000 kinds of plants. They produce seeds that are enclosed in a protective seed case. The word angiosperm comes from two Greek words meaning enclosed and seed. All plants that produce flowers and fruits are angiosperms. They include most of our common plants, such as brightly colored garden plants, many kinds of wildflowers, and most trees, shrubs, and herbs. Most of the plants that produce the fruits, grains, and vegetables that people eat also are angiosperms. See ANGIOSPERM.

The sizes of angiosperms vary greatly. The smallest flowering plant, the duckweed, is only about 1/50 inch (0.5 millimeter) long. It floats on the surface of ponds. The largest angiosperms are eucalyptus trees. They grow more than 300 feet (91 meters) tall.

Some botanists divide the angiosperms into two smaller groups. Plants in one group, called monocotyledons or monocots, grow from seeds that contain one seed leaf called a cotyledon (see COTYLEDON). Plants in the other group, called dicotyledons or dicots, have two cotyledons in their seeds.

Gymnosperms include a wide variety of trees and shrubs that produce naked or uncovered seeds. Most gymnosperms bear their seeds in cones. The word gymnosperm comes from two Greek words meaning naked and seed. Gymnosperms do not produce flowers. This group is made up of such plants as conifers, cycads, ginkgoes, and gnetophytes. See GYMNOSPERM.

Conifers are the best known of the gymnosperms. They include such trees as cedars, cypresses, firs, pines, redwoods, and spruces. Most conifers have needlelike or scalelike leaves. Their seeds grow on the upper side of the scales that make up their cones. The cones of some conifers, such as junipers, look like berries. Most conifers are evergreens-that is, they shed old leaves and grow new leaves continuously and so stay green throughout the year. Wood from conifers is widely used in construction and papermaking. Conifers also provide animals with food and shelter. See CONIFER.

Cycads and ginkgoes have lived on Earth for millions of years. Large numbers of these plants once grew over wide regions of land. Most cycads look much like palm trees. They have a branchless trunk topped by a crown of long leaves. But unlike palm trees, they bear their seeds in large cones. Only one kind of ginkgo survives today. It is an ornamental tree with flat, fan-shaped leaves. It bears seeds at the ends of short stalks along its branches. See CYCAD; GINKGO.

Gnetophytes are the gymnosperms most closely related to angiosperms. They have many features that resemble those of flowering plants. For example, Gnetum has broad, oval-shaped leaves and special water-transport tubes, much like those of angiosperms. The cones of all gnetophytes are flowerlike in many details.

Ferns grow chiefly in moist, wooded regions. They vary widely in size and form. Some aquatic ferns have leaves only about 1 inch (2.5 centimeters) long. But in the tropics, tree ferns may grow more than 65 feet (20 meters) high.

Fern leaves, called fronds, usually are made up of many tiny leaflets and may be quite large. On most types of ferns, the fronds are the only parts that grow above the ground. They grow from underground stems that may run horizontally under the surface of the ground. When the fronds first appear, they are tightly coiled. The fronds unwind as they grow.

During prehistoric times, great numbers of large ferns covered Earth. These ferns, along with giant club mosses and horsetails, accounted for much of the plant life that later formed coal. See FERN.

Lycopsids include club mosses, quillworts, and selaginellas. These plants have leaves with a single, central vein. Lycopsids were among the first plants to grow on land.

Club mosses have tiny needlelike or scalelike leaves that usually grow in a spiral pattern. They are not true mosses. Club mosses are found from tropical to temperate regions. They often form a "carpet" on the forest floor. See CLUB MOSS.

Quillworts are found chiefly in moist soils around lakes and streams. They have short stems and long, grasslike leaves. The leaves usually grow to about 14 inches (36 centimeters) long. Ancient plants related to quillworts were large trees that grew up to 120 feet (37 meters) tall. These plants lived about 290 million years ago.

There are about 700 kinds of selaginellas. These plants are usually found in tropical and subtropical regions. They often grow in damp places on the forest floor. Selaginellas have small thin leaves. Their stems may either grow upright or along the ground. These plants first appeared on earth over 300 million years ago.

Horsetails are a group of small plants that have hollow, jointed stems. Horsetails grow about 2 to 3 feet (60 to 90 centimeters) tall. The plants have green stems and tiny, black leaves. The stems capture the sunlight used by the plant to make food in photosynthesis. In some horsetails, the branches grow in whorls (circles) around the main stem of the plant, and the plant resembles a horse's tail. Tiny amounts of minerals are concentrated in the stems of horsetails, including gold and silica. Silica makes the stems very coarse, like sandpaper. Some kinds of horsetails are called scouring rush because people once used these plants to scour their pots and pans. See HORSETAIL.

Bryophytes are a group made up of liverworts, mosses, and hornworts. These plants live in almost all parts of the world, from the Arctic to tropical forests. They grow in such moist, shady places as forests and ravines. Bryophytes are the only types of plants that lack vascular tissue-that is, tissue that carries water and food throughout the plant.

Most liverworts, mosses, and hornworts measure less than 8 inches (20 centimeters) tall. None of these plants have true roots. Instead, they have hairy rootlike growths called rhizoids that anchor the plants to the soil and absorb water and minerals.

Peat moss, a substance made up of thick growths of Sphagnum and other mosses, is often used in gardening. Gardeners mix peat moss into the soil to keep the soil loose and to help it hold moisture. See HORNWORT; LIVERWORT; MOSS.

Where plants live

Most species of plants live in places that have warm temperatures at least part of the year, plentiful rainfall, and rich soil. But plants can live under extreme conditions. Mosses have been found in Antarctic areas where the temperature seldom rises above 32 °F (0 °C). Many desert plants grow in areas where the temperature may rise well above 100 °F (38 °C).

Not all kinds of plants grow in all parts of the world. For example, cattails live only in such damp places as swamps and marshes. Cactuses, on the other hand, are found chiefly in deserts. Through long periods of time, many small changes have taken place in various kinds of plants. These changes have enabled the plants to survive in a particular environment. For a discussion of some of these changes, see the section of this article How plants change.

Many elements make up a plant's environment. One of the most important is the weather-sunlight, temperature, and precipitation (rain, melted snow, and other moisture). The environment of a plant also includes the soil and the other plants and the animals that live in the same area. All these elements form what scientists call a natural community.

No two natural communities are exactly alike, but many resemble one another more than they differ. Botanists divide the world into biomes-natural communities of plants, animals, and other organisms. Important land biomes include (1) the tundra, (2) forests, (3) chaparrals, (4) grasslands, (5) savannas, and (6) deserts. Forests are often subdivided into smaller biomes, such as temperate deciduous forests and tropical rain forests. In addition, many plants live in aquatic (water) regions that are not grouped as a specific biome. See BIOME.

Human beings have greatly affected the natural communities. In North America, for example, great forests once extended from the Atlantic Ocean to the Mississippi River. Most of the trees were cleared by advancing settlers, and the forests have been replaced by cities and farms. In other parts of the world, irrigation and the use of fertilizers have enabled plants to be grown on once-barren land.

This section describes the natural plant life in the important land biomes and in aquatic regions. For information on where animals live, see the ANIMAL article. For a discussion of the relationship between living things and their environment, see ECOLOGY.

The tundra is a cold, treeless area that surrounds the Arctic Ocean, near the North Pole. It extends across the uppermost parts of North America, Europe, and Asia. The land in these regions is frozen most of the year, and the annual precipitation measures only from 6 to 10 inches (15 to 25 centimeters). The upper slopes of the world's highest mountains-the Alps, the Andes, the Himalaya, and the Rockies-have conditions similar to those in the tundra.

Summers in the tundra last only about 60 days, and summer temperatures average only about 45 °F (7 °C). The top 1 foot (30 centimeters) or so of the land thaws during the summer, leaving many marshes, ponds, and swamps. Such plants as mosses, shrubs, and wildflowers grow in the tundra. These plants grow in low clumps and so are protected from the wind and cold. A thick growth of lichens (organisms made up of algae and fungi) covers much of the land. See TUNDRA.

Forests cover almost a third of Earth's land area. They consist chiefly of trees, but many other kinds of plants also grow in forests. Some botanists divide the many types of forests into three major groups: (1) coniferous forests, (2) temperate deciduous forests, and (3) tropical rain forests.

Coniferous forests are made up mainly of trees that are coniferous (cone-bearing) and evergreen. Most ecologists distinguish between boreal forests, also called taiga, and temperate coniferous forests.

Boreal forests grow in regions that have a short summer and a long, cold winter. The growing season in these regions may last less than three months. Boreal forests are found in the northernmost parts of North America, Europe, and Asia. They also grow in the high mountains of these continents. Trees found in boreal forests include such evergreen conifers as balsam firs, black spruces, jack pines, and white spruces. The pointy, triangular shape of these trees helps them shed heavy snow.

Few plants grow on the floor of boreal forests. Thick layers of old needles build up beneath the trees. These needles contain acids that are slowly released as the needles decay. Water carries the acids into the soil. The acidic water dissolves many minerals and carries them into the deeper layers of the soil. As a result, the topsoil found in boreal forests is often very sandy and unable to support many types of small plants.

Temperate coniferous forests grow in western North America in areas that have mild, wet winters and dry summers. The redwood forests of northern California and the temperate rain forests found on the Olympic Peninsula of Washington are both examples of temperate coniferous forests. Major trees of the temperate coniferous forest include redwoods and giant sequoias in the south and Douglas-firs, hemlocks, cedars, and pines in more northern areas.

Temperate deciduous forests cover large areas of North America, central Europe, east Asia, and Australia. In the United States, temperate deciduous forests grow mostly east of the Mississippi River and extend northward into the Northern States and southern Canada, where they become mixed with coniferous forests. Most of these areas have cold winters and warm, wet summers.

Most of the trees in temperate deciduous forests are called broadleaf trees because they have broad, flat leaves. They also are deciduous-that is, they lose their leaves every fall and grow new ones in the spring. Trees that grow in temperate deciduous forests include basswoods, beeches, birches, hickories, maples, oaks, poplars, tulips, and walnuts. A thick growth of wildflowers, seedlings, and shrubs covers the floor of most of these forests.

Tropical rain forests grow in regions that have warm, wet weather the year around. These regions include Central America and the northern parts of South America, central and western Africa, Southeast Asia, and the Pacific Islands.

Most trees in tropical rain forests are broadleaf trees. Because of the warm, wet weather, they never completely lose their leaves. These trees lose a few leaves at a time throughout the year. Many kinds of trees grow in tropical rain forests, including mahoganies and teaks. The trees grow so close together that little sunlight can reach the ground. As a result, only ferns and other plants that require little sunlight can grow on the forest floor. Many plants, including orchids and vines, grow high on the trees.

The heavy rainfall that occurs in tropical rain forests dissolves much of the nutrients and organic materials out of the soil. As a result, the soils found in tropical rain forests contain a very small amount of nutrients and organic matter. However, the soil is able to support the lush growth found in these forests because fresh nutrients from the decay of fallen leaves are continually being released into the soil.

Chaparrals consist of thick growths of shrubs and small trees. Cork and scrub oaks, manzanitas, and many unusual herbs are often found on chaparrals. Chaparrals occur in areas that have hot, dry summers and cool, wet winters. Such areas are found in the western part of North America, the southern regions of Europe near the Mediterranean Sea, the Middle East, northern Africa, and the southern parts of South America, Africa, and Australia.

During the dry summer season, fires are common on chaparrals. But these fires actually help to maintain the plant life. Many of the plants that grow on chaparrals are either resistant to fire or are able to grow back quickly after they burn. The fires clear the dense vegetation away and expose bare ground to allow for new growth. The heat of the fires also stimulates development in the seeds of some plants. In addition, many types of short-lived, small flowers appear only after a fire has taken place. See CHAPARRAL.

Grasslands are open areas where grasses are the most plentiful plants. In the United States and Canada, most of the natural grasslands are used to grow crops. There, farmers and ranchers grow such grains as barley, oats, and wheat where bluestem, buffalo, and grama grasses once covered the land.

Botanists divide grasslands into steppes and prairies. Only short grasses grow on steppes. These dry areas include the Great Plains of the United States and Canada, the veld of South Africa, and the plains of Kazakhstan and southern Russia. Taller grasses grow on the prairies of the American Midwest, eastern Argentina, and parts of Europe and Asia. Rolling hills, clumps of trees, and rivers and streams break up these areas. Most of the soil is rich and rainfall is plentiful. As a result, prairie land is used almost entirely to raise food crops and livestock. See GRASSLAND.

Savannas are grasslands with widely spaced trees. Some savannas are found in regions that receive little rain. Others are found in tropical regions, such as the Llamos of Venezuela, the Campos of southern Brazil, and the Sudan of Africa. Most of these areas have dry winters and wet summers. Grasses grow tall and stiff under such conditions. Acacia, baobab, and palm trees grow on many savannas. A wide variety of animals, such as antelope, giraffes, lions, and zebras, roam the savannas of Africa. See SAVANNA.

Deserts cover about a fifth of Earth's land. A huge desert region extends across northern Africa and into central Asia. This region includes three of the world's great deserts-the Arabian, the Gobi, and the Sahara. Other major desert regions of the world include the Atacama Desert along the western coast of South America, the Kalahari Desert in southern Africa, the Western Plateau of Australia, and the southwest corner of North America.

Some deserts have almost no plant life at all. Parts of the Gobi and the Sahara, for example, consist chiefly of shifting sand dunes. All deserts receive little rain and have either rocky or sandy soil. The temperature in most deserts rises above 100 °F (38 °C) for at least part of the year. Some deserts also have cold periods. But in spite of these harsh conditions, many plants live in desert regions. These plants-sometimes called xerophytes-include acacias, cactuses, creosote bushes, Joshua trees, sagebrush, and yuccas. Wildflowers are also found in the desert. See FLOWER (Flowers of the desert ).

Desert plants do not grow close together. By being spread out, each plant can get water and minerals from a large area. The roots of most desert plants extend over large areas of land, and they capture as much rain water as possible. Cactuses and other succulent (juicy) plants store water in their thick leaves and stems. See CACTUS; DESERT.

Aquatic regions are bodies of fresh or salt water. Freshwater areas include lakes, rivers, swamps, and marshes. Coastal marshes and oceans are saltwater regions. Most aquatic plants, which are also called hydrophytes, live in places that receive sunlight. These plants grow near the water surface, in shallow water, or along the shore.

Some kinds of aquatic plants, including eelgrass, live completely under the surface of the water. Other species of aquatic plants, such as duckweed, the smallest known flowering plant, float freely on the surface. Still others, such as the water marigold, grow only partly underwater. Many aquatic plants have air spaces in their stems and leaves. The air spaces help them stand erect or stay afloat.

Aquatic regions have unique conditions that make it difficult for many types of plants to grow there. For example, swamps and marshes, as well as flood plains along many streams and rivers, become flooded leaving the plants that live in these areas completely covered by water. As a result, only a few species of plants are able to survive in aquatic regions. Common freshwater plants include duckweeds, pondweeds, water lilies, sedges, and cattails. Such trees as baldcypresses, blackgums, and willows also grow in fresh water. Saltwater plants include eelgrass, cordgrass, and many types of sedges. See WATER PLANT.

Parts of plants

All plants-like all living things-are made up of cells. In plants, there are many kinds of cells that have special jobs, and together these cells form the various parts of the plant. A giant redwood tree, for example, has many billions of cells. See CELL.

A group of cells that are organized to perform a particular function is called a tissue. Plants are made up of many types of complex tissues. All plants, except bryophytes-that is, mosses, liverworts, and hornworts-have conducting tissue that carries water, minerals, and other nutrients throughout the plant body. This tissue is called vascular tissue. It is made up of two specialized tissues called xylem and phloem. The xylem tissue consists of cells that carry water and minerals from the roots to the leaves. The phloem tissue is made up of cells that carry food made by photosynthesis in the leaves to the other parts of the plant. Plants that have these special tissues are called vascular plants. Bryophytes are called nonvascular plants because they lack xylem and phloem.

A plant is made up of several important parts. Flowering plants, the most common type of plants, have four main parts: (1) roots, (2) stems, (3) leaves, and (4) flowers. The roots, stems, and leaves are called the vegetative parts of a plant. The flowers, fruits, and seeds are known as the reproductive parts.

Roots. Most roots grow underground. As the roots of a young plant spread, they absorb the water and minerals that the plant needs to grow. The roots also anchor the plant in the soil. In addition, the roots of some plants store food for the rest of the plant to use. Plants with storage-type roots include beets, carrots, radishes, and sweet potatoes.

There are two main kinds of root systems-fibrous and taproot. Grass is an example of a plant with a fibrous root system. It has many slender roots of about the same size that spread out in all directions. A plant with a taproot system has one root that is larger than the rest. Carrots and radishes have taproots. Taproots grow straight down, some as deep as 15 feet (4.6 meters).

The root is one of the first parts of a plant that starts to grow. A primary root develops from a plant's seed and quickly produces branches called secondary roots. At the tip of each root is a root cap that protects the delicate tip as it pushes through the soil. Threadlike root hairs grow farther back on the root of the plant. Few of these structures are over 1/2 inch (13 millimeters) long. But there are so many of them that they greatly increase the plant's ability to absorb water and minerals from the soil.

The roots of some aquatic plants float freely in the water. Other plants, such as orchids and some vines, have roots that attach themselves to tree branches.

The roots of almost all land plants have a special relationship with fungi. In this relationship, known as mycorrhiza, fungi cover or penetrate the growing tips of a plant's roots. Water and nutrients enter the roots through the fungi. Fungi extend the plant's root system and improve the plant's ability to absorb water and minerals. Many botanists believe the first land plants developed millions of years ago from algae that lived in water. They think mycorrhizal relationships may have helped these plants to grow on land. See ROOT.

Stems of plants differ greatly among various species. They make up the largest parts of some kinds of plants. For example, the trunk, branches, and twigs of trees are all stems. Other plants, such as cabbage and lettuce, have such short stems and large leaves that they appear to have no stems at all. The stems of still other plants, including potatoes, grow partly underground.

Most stems grow upright and support the leaves and reproductive organs of plants. The stems hold these parts up in the air where they can receive sunlight. Some stems grow along the ground or underground. Stems that grow aboveground are called aerial stems, and those underground are known as subterranean. Aerial stems are either woodyor herbaceous (nonwoody). Plants with woody stems include trees and shrubs. These plants are rigid because they contain large amounts of woody xylem tissue. Most herbaceous stems are soft and green because they contain only small amounts of xylem tissue.

In almost all plants, a stem grows in length from the end, called the apex. The cells that form this growth area are called the apical meristem. An apical meristem produces a column of new cells behind itself. These cells develop into the specialized tissues of the stem and leaves. A resting apical meristem and the cluster of developing leaves that surround it is called a bud. Buds may grow on various parts of the stem. A terminal bud is found at the end of a branch. A lateral bud develops at a point where a leaf joins the stem. This point is called a node. Buds may develop into new branches, leaves, or flowers. Some buds are covered with tiny overlapping leaves called bud scales. The bud scales protect the soft, growing tissue of the apical meristem. During the winter, the buds of many plants are dormant (inactive) and can be seen easily. In the spring, these buds resume their growth. See STEM.

Leaves make most of the food that plants need to live and grow. They produce food by a process called photosynthesis. In photosynthesis, chlorophyll in the leaves absorbs light energy from the sun. This energy is used to combine water and minerals from the soil with carbon dioxide from the air. The food formed by this process is used for growth and repair, or it is stored in special areas in the stems or roots. See PHOTOSYNTHESIS.

Leaves differ greatly in size and shape. Some plants have leaves less than 1 inch (2.5 centimeters) long and wide. The largest leaves, those of the raffia palm, grow up to 65 feet (20 meters) long and 8 feet (2.4 meters) wide. Most plants have broad, flat leaves. The edges, or margins, of these leaves may be smooth, toothed, or wavy. Grass and certain other plants have long, slender leaves. A few kinds of leaves, including the needles of pine trees and the spines of cactuses, are rounded and have sharp ends.

Most leaves are arranged in a definite pattern on a plant. The leaves of many kinds of plants grow in an alternate pattern. In this pattern, only one leaf forms at each node. On plants with the simplest kind of alternate pattern, a leaf appears first on one side of the stem and then on the other side. On plants with a more complex alternate pattern, the nodes are spaced in a spiral pattern around the stem and the leaves seem to encircle the stem from bottom to top. If two leaves grow from opposite sides of the same node, the plant has an opposite arrangement of leaves. If three or more leaves grow equally spaced around a single node on the stem, the plant has a whorled arrangement of leaves.

A leaf begins as a small bump next to the apical meristem of a stem. Most leaves develop two main parts-the blade and the petiole. The leaves of some plants also have a third part called stipules. The blade is the flat part of the leaf. Some leaves, called simple leaves, have only one blade. Leaves with two or more blades are called compound leaves. The petiole is the thin leafstalk that grows between the base of the blade and the stem. It carries water and food to and from the blade. Stipules are leaflike structures that grow where the petiole joins the stem. Most stipules look like tiny leaves.

A network of veins distributes water to the food-producing areas of a leaf. The veins also help support the leaf and hold its surface up to the sun. The upper and lower surfaces of a leaf are called the epidermis (skin). The epidermis has tiny openings called stomata. Carbon dioxide, oxygen, water vapor, and other gases pass into the leaves and out of the leaves through the stomata. See LEAF.

Flowers contain the reproductive parts of flowering plants. Flowers develop from buds along the stem of a plant. Some kinds of plants produce only one flower, but others grow many large clusters of flowers. Still others, such as dandelions and daisies, have many tiny flowers that form a single, flowerlike head.

Most flowers have four main parts: (1) the calyx, (2) the corolla, (3) the stamens, and (4) the pistils. The flower parts are attached to a place on the stem called the receptacle.

The calyx consists of small, usually green leaflike structures called sepals. The sepals protect the bud of a young flower. Inside the calyx are the petals. All the petals of a flower make up the corolla. The petals are the largest, most colorful part of most flowers. The flower's reproductive organs-the stamens and the pistils-are attached to the receptacle inside the sepals and the petals. In many flowers, the stamens and petals are fused (joined together).

A stamen is a male reproductive organ, and a pistil is a female reproductive organ. Each stamen has an enlarged part called an anther that grows on the end of a long, narrow stalk called the filament. Pollen grains, which develop sperm (male sex cells), are produced in the anther. The pistils of most flowers have three main parts: (1) a flattened structure called the stigma at the top, (2) a slender tube called the style in the middle, and (3) a round base called the ovary. The ovary contains one or more structures called ovules. Egg cells form within the ovules. The ovules become seeds when sperm cells fertilize the egg cells. The next section of this article, How plants reproduce, tells how the sperm cells unite with the egg cells to begin the formation of seeds and fruit.

Seeds vary greatly in size and shape. Some seeds, such as those of the tobacco plant, are so small that more than 2,500 may grow in a pod less than 3/4 inch (19 millimeters) long. On the other hand, the seeds of one kind of coconut tree may weigh more than 20 pounds (9 kilograms). The size of a seed has nothing to do with the size of the plant. For example, huge redwood trees grow from seeds that measure only 1/16 inch (1.6 millimeters) long.

There are two main types of seeds-naked and enclosed. Cone-bearing plants and all other nonflowering seed plants have naked, or uncovered, seeds. The seeds of these plants develop on the upper side of the scales that form their cones. All flowering plants have seeds enclosed by an ovary. The ovary develops into a fruit as the seeds mature. The ovaries of such plants as apples, berries, and grapes develop into a fleshy fruit. In other plants, including beans and peas, the ovaries form a dry fruit. Still other plants have aggregate fruits. Each tiny section of an aggregate fruit, such as a raspberry, develops from a separate ovary and has its own seed.

Seeds consist of three main parts: (1) the seed coat, (2) the embryo, and (3) the food storage tissue. The seed coat, or outer skin, protects the embryo, which contains all the parts needed to form a new plant. The embryo also contains one or more cotyledons, or embryo leaves, which absorb food from the food storage tissue. In flowering plants, the food storage tissue is called endosperm. In some plants, such as peas and beans, the embryo absorbs the endosperm, and food is stored in the cotyledons. In nonflowering seed plants, a tissue called the megagametophyte serves as a place to store food. See SEED.

How plants reproduce

Plants create more of their own kind by either sexual reproduction or asexual reproduction. In sexual reproduction, a male sperm cell joins with a female egg cell to produce a new plant. Both the egg and the sperm cells contain genes (hereditary material). Genes determine many of the characteristics of a plant. A plant that is produced by sexual reproduction inherits genes from both parent plants. It is a unique individual and has traits that may be different from either parent. Asexual reproduction can occur in many ways. It often involves the division of one plant into one or more parts that become new plants. These plants inherit genes from only one parent and have exactly the same characteristics as the parent plant. This type of asexual reproduction is called vegetative propagation. Many plants reproduce both sexually and by vegetative propagation.

Sexual reproduction. Sexual reproduction in plants occurs as a complex cycle called alternation of generations. It involves two distinct generations or phases. During one phase of the life cycle, the plant is called a gametophyte, or gamete-bearing plant. In most species of plants, the gametophyte is barely visible and is rarely noticed by people. It produces gametes-that is, the sperm and egg cells. It may produce sperm cells or egg cells, or both, depending on the species of plant. When the sperm and egg cells unite, the fertilized egg develops into the second phase of the plant's life cycle. In this phase, the plant is called a sporophyte or spore-bearing plant. When people see a plant it is most often the sporophyte phase. Sporophytes produce tiny structures called spores through a process of cell division called meiosis. The spores form in closed capsulelike structures called sporangia. Gametophytes develop from the spores, and the life cycle begins again.

In seed plants, which include flowering and cone-bearing plants, alternation of generations involves a series of complicated steps. Among these plants, only the sporophyte generation can be seen with the unaided eye. Spores are produced in the male and female reproductive organs of a plant. The spores grow into gametophytes, which remain inside the plant's reproductive organs.

In flowering plants, the reproductive parts are in the flowers. A plant's stamens are its male reproductive organs. Each stamen has an enlarged tip called an anther. The pistil is the plant's female reproductive organ. The ovary, which forms the round base of the pistil, contains the ovules. The anthers consist of structures called microsporangia, and the ovules contain structures called megasporangia. Cell divisions in the microsporangia and the megasporangia result in the production of spores.

In most species of flowering plants, one spore in each ovule grows into a microscopic female gametophyte. The female gametophyte produces one egg cell. In the anther, the spores, called pollen grains, contain microscopic male gametophytes. Each pollen grain produces two sperm cells.

For fertilization to take place, a pollen grain must be transferred from the anther to the pistil. This transfer is called pollination. If pollen from a flower reaches a pistil of the same flower, or a pistil of another flower on the same plant, the fertilization process is called self-pollination. When pollen from a flower reaches a pistil of another plant, the fertilization process is called cross-pollination.

In cross-pollinated plants, the pollen grains are carried from flower to flower by such animals as birds and insects, or by the wind. Many cross-pollinated plants have large flowers, a sweet scent, and sweet nectar. These features attract hummingbirds and such insects as ants, bees, beetles, butterflies, and moths. As these animals move from flower to flower in search of food, they carry pollen on their bodies. Most grasses and many trees and shrubs have small, inconspicuous flowers. The wind carries their pollen. It may carry pollen as far as 100 miles (160 kilometers). Some airborne pollen causes hay fever and other allergies.

If a pollen grain reaches the pistil of a plant of the same species, a pollen tube grows down through the stigma and the style to an ovule in the ovary. In the ovule, one of the two sperm cells from the pollen grain unites with the egg cell. A sporophyte embryo then begins to form. The second sperm cell unites with two structures called polar nuclei and starts to form the nutrient tissue that makes up the endosperm. Next, a seed coat forms around the embryo and the endosperm. See POLLEN; SEED.

In conifers, the reproductive parts are in the cones. A conifer has two kinds of cones. The pollen, or male, cone is the smaller and softer of the two. It also is simpler in structure. Seed, or female, cones are larger and harder than the male cones.

A pollen cone has many tiny sporangia that produce pollen grains. Each of the scales that make up a seed cone has two ovules on its surface. Every ovule produces a spore that grows into a female gametophyte. This tiny plant produces egg cells.

The wind carries pollen grains from the pollen cone to the seed cone. A pollen grain sticks to an adhesive substance near an ovule. It usually enters the pollen chamber of the ovule through an opening called the micropyle. The pollen grain then begins to form a pollen tube. Two sperm cells develop in the tube. After the pollen tube reaches the egg cell, one of the sperm cells fertilizes the egg. The second sperm cell disintegrates. The fertilized egg develops into a sporophyte embryo, and the ovule containing the embryo becomes a seed. The seed falls to the ground and, if conditions are favorable, a new sporophyte begins to grow.

In ferns and mosses, the sporophyte and gametophyte generations consist of two greatly different plants. Among ferns, the sporophytes have leaves and are much larger than the gametophytes. Clusters of sporangia called sori form on the edges or underside of each leaf. Spores develop in the sporangia. After the spores ripen, they fall to the ground and grow into barely visible, heart-shaped gametophytes. A fern gametophyte produces both male and female sex cells. If enough moisture is present, a sperm cell swims to an egg cell and unites with it. The fertilized egg then grows into an adult sporophyte.

Among mosses, a sporophyte consists of a long, erect stalk with a podlike spore-producing container at the end. The sporophyte extends from the top of a soft, leafy, green gametophyte. It depends on the gametophyte for food and water. The gametophyte is the part of the plant community recognized as moss.

Vegetative propagation. Plants can spread without sexual reproduction. Through vegetative propagation, a part of a plant may grow into a complete new plant. Vegetative propagation can take place because the pieces of the plant form the missing parts by a process called regeneration. Any part of a plant-a root, stem, leaf, or flower-may be propagated into a new plant. A plant may even grow from a single cell of another plant.

Propagation occurs most often in plants with stems that run horizontally just above or below the ground. The strawberry plant, for example, sends out long, thin stems called runners that grow along the surface of the soil. The runners, at points where they touch the ground, send out roots that produce plantlets (new leaves and stems). These plantlets are actually part of the parent plant. New plants form only when the plantlets are separated from the parent plant. Ferns, irises, many kinds of grasses, blueberries and some other shrubs, and some species of trees propagate from underground stems.

Many plants that grow as weeds are able to spread rapidly by vegetative propagation. These plants are sometimes difficult to kill because they often can regrow their lost parts by regeneration. For example, a dandelion will regrow new stems and leaves even if only part of its roots are left in the soil.

Farmers use vegetative propagation to raise many valuable food crops, such as apples, bananas, oranges, and white potatoes. For example, they cut potatoes into many parts, making sure that each part has at least one eye (bud). Each piece of potato will grow into a new potato plant. Propagation by this method produces new potato plants more quickly than do the seeds of a potato plant.

Vegetative propagation is also widely used in gardening. Many plants, including gladioli, irises, lilies, and tulips, are propagated from bulbs or corms. These plants take longer to reach the flowering stage when grown from seeds.

People propagate many plants by three chief methods. These methods are: (1) cuttage, (2) grafting, and (3) layering.

Cuttage involves the use of cuttings (parts of plants) taken from growing plants. Most cuttings are stems. When placed in water or moist soil, the majority of cuttings develop roots. The cutting then grows into a complete plant. Many species of garden plants and shrubs are propagated by stem cuttings.

Grafting also involves cuttings. But instead of putting the cutting into water or soil, it is grafted (attached) to another plant, called the stock. The stock provides the root system and lower part of the new plant. The cutting forms the upper part. Farmers use grafting to grow large numbers of some kinds of fruit, including Delicious and Winesap apples. They take cuttings from trees that have grown the type of apples they want and graft them onto apple trees with strong root systems. For a discussion of various methods of grafting, see the World Book article on GRAFTING.

Layering is a method of growing roots for a new plant. In mound layering, soil is piled up around the base of a plant. The presence of the soil causes roots to sprout from the plant's branches. A branch is then cut off and planted. In air layering, a cut about 3 inches (8 centimeters) long is made about halfway through a branch. A type of moss called sphagnum moss is placed in the cut to keep it moist, and this portion of the branch is wrapped in a waterproof covering. New roots form in the area of the cut. After they have sprouted, the branch is cut off and planted.

How plants grow

Plants can be divided into two groups, based on how they get their food. All green plants are called autotrophs. They contain chlorophyll, which enables them to capture the sunlight used in producing the food and other materials they need for growth. Other kinds of plants, called heterotrophs, lack chlorophyll and cannot make their own food. They are either parasites or saprophytes.

This section discusses the four major processes that take place in the growth of most kinds of green plants. These processes are (1) germination, (2) water movement, (3) photosynthesis, and (4) respiration. The section also discusses how a plant's heredity and environment affect its growth.

Germination is the sprouting of a seed. Most seeds have a period of inactivity called dormancy before they start to grow. In most parts of the world, this period lasts through the winter. Then, after spring arrives, the seeds start to germinate.

Seeds need three things to grow: (1) a proper temperature, (2) moisture, and (3) oxygen. Most seeds, like most kinds of plants, grow best in a temperature between 65 °F (18 °C) and 85 °F (29 °C). The seeds of plants that live in cold climates may germinate at lower temperatures, and those of tropical regions may sprout at higher temperatures. Seeds receive the moisture they need from the ground. The moisture softens the seed coat, allowing the growing parts to break through. Moisture also prepares certain materials in the seed for their part in seed growth. If a seed receives too much water, it may begin to rot. If it receives too little, germination may take place slowly or not at all. Seeds need oxygen for the changes that take place within them during germination.

The embryo of a seed has all the parts needed to produce a young plant. It may have either one or more cotyledons, which digest food from the endosperm for the growing seedling. The seed absorbs water, which makes it swell. The swelling splits the seed coat, and a tiny seedling appears. The lower part of the seedling, called the hypocotyl, develops into the primary root. This root anchors the seedling in the ground and develops a root system that supplies water and minerals. Next, the upper part of the seedling, called the epicotyl, begins to grow upward. At the tip of the epicotyl is the plumule, the bud that produces the first leaves. In some plants, such as the many kinds of beans, the growth of the epicotyl carries the cotyledons above ground. In corn and other plants, cotyledons remain underground, within the seed. After a seedling has developed its own roots and leaves, it can make its own food. It no longer needs cotyledons to supply nourishment.

Most plants grow in length only at the tips of their roots and branches. The cells in these areas are called meristematic cells. They divide and grow rapidly and develop into the various tissues that make up an adult plant. In trees and other plants that increase in thickness, new layers of cells form between the bark and wood. This area is the cambium. New layers of cells are made as the cambium grows each year. These layers form the woody rings that enable people to tell the age of a tree.

Some kinds of plants, called perennial plants, live for many years. Most perennials produce seeds yearly. Annual plants live only about one year. Biennial plants live for two years. Most annuals and biennials produce seeds only once. See ANNUAL; BIENNIAL; PERENNIAL.

Water movement. Plants must have a continuous supply of water. Each individual plant cell contains a large amount of water. Without this water, the cells could not carry on the many processes that take place within a plant. Water also carries important materials from one part of a plant to another.

Most water enters a plant through the roots. Tiny root hairs absorb moisture and certain minerals from the soil by a process called osmosis (see OSMOSIS). In many plants, fungi that grow on the roots help the plants absorb water and minerals. In vascular plants-that is, plants with special conducting tissues-these materials are transported through the xylem tissue of the roots and stems to the leaves. There, water and minerals are used in making food. Water also carries this food through the phloem tissue to other parts of the plant.

Plants give off water through a process called transpiration. Most of this water escapes through the stomata on the surfaces of the leaves. Scientists estimate that corn gives off 325,000 gallons of water per acre (3,040,000 liters per hectare) by transpiration during a growing season. Some botanists believe this water loss prevents the leaves from overheating in sunlight.

Photosynthesis is the process by which plants make food. The word photosynthesis means putting together with light. In green plants, sunlight captured by chlorophyll enables carbon dioxide from the air to unite with water and minerals from the soil and create food. This process also releases oxygen into the air. People and animals must have this oxygen to breathe.

Most photosynthesis takes place in small bodies called chloroplasts within the cells of plant leaves. These chloroplasts contain chlorophyll, which absorbs sunlight. Energy from the sun splits water molecules into hydrogen and oxygen. The hydrogen joins with carbon from the carbon dioxide to produce sugar. The sugar helps a plant make the fat, protein, starch, vitamins, and other materials that it needs to survive. See PHOTOSYNTHESIS.

Some plants, called parasites and saprophytes, have little or no chlorophyll and cannot produce their own food through photosynthesis. These plants must rely on outside sources for food. Parasites attach to living plants and take the nutrients they need from these plants. Saprophytes grow on dead and decaying organisms, or use organic substances produced by living organisms for food.

Mistletoe and dodder are common parasites found in many parts of the world. Mistletoe grows on the trunks and branches of many trees. It is called a partial parasite because it also makes some of its own food. Indian pipe is a saprophyte that grows near fungi. It uses organic materials produced by fungi for food. A plant called giant rafflesia is a parasite that grows on the roots and stems of other plants. It bears the largest flower of any known plant. Rafflesia flowers may grow over 3 feet (91 centimeters) wide.

Respiration breaks down food and releases energy for a plant. The plant uses the energy for growth, reproduction, and repair. Respiration involves the breakdown of sugar. Some of the products resulting from this breakdown combine with oxygen, releasing carbon dioxide, energy, and water. Unlike photosynthesis, which takes place only during daylight, respiration goes on day and night throughout the life of a plant. Respiration increases rapidly with the spring growth of buds and leaves, and it decreases as winter approaches.

Factors affecting plant growth. A plant's growth is shaped by both its heredity and its environment. A plant's heredity, for example, determines such characteristics as a flower's color and general size. These hereditary factors are passed on from generation to generation. Environmental factors include sunlight, climate, and soil condition.

Hereditary factors. Within the nucleus of all plant cells are tiny bodies called chromosomes that contain hereditary units called genes. These bodies contain "instructions" that direct the growth of the plant. As the cells divide and multiply, the "instructions" are passed on to each new cell. See CELL; HEREDITY.

Substances made within a plant also play a part in regulating plant growth. These substances, called hormones, control such activities as the growing of roots and the production of flowers and fruit. Botanists do not know exactly how all plant hormones work. But they have learned that certain hormones, called auxins, affect the growth of buds, leaves, roots, and stems. Other growth hormones, called gibberellins, make plants grow larger, cause blossoming, and speed seed germination. Still other hormones called cytokinins make plant cells divide.

Environmental factors. All plants need light, a suitable climate, and an ample supply of water and minerals from the soil. But some species grow best in the sun, and others thrive in the shade. Plants also differ in the amount of water they require and in the temperatures they can survive. Such environmental factors affect the rate of growth, the size, and the reproduction of all plants.

The growth of plants also is affected by the length of the periods of light and dark they receive. Some plants, including lettuce and spinach, bloom only when the photoperiod (period of daylight) is long. Such plants are called long-day plants. On the other hand, asters, chrysanthemums, and poinsettias are short-day plants. They bloom only when the dark period is long. Still other plants, among them marigolds and tomatoes, are not affected by the length of the photoperiod. They are called day-neutral plants.

Plants also are affected in other ways by their environment. For example, a plant may display a bending movement called a tropism. In a tropism, an outside stimulus (force) causes a plant to bend in one direction. A plant may have either a positive or a negative tropism, depending on whether the plant bends toward or away from the stimulus. Tropisms are named according to the stimuli that cause them. Phototropism is bending caused by light, geotropism is caused by gravity, and hydrotropism is caused by water.

A plant placed in a window exhibits positive phototropism when its stems and leaves grow toward the source of light. Roots, on the other hand, display negative phototropism and grow away from light. However, roots demonstrate positive geotropism. Even if a seed or bulb is planted upside down, its roots grow downward-toward the source of gravity. The stem of the same bulb shows negative geotropism by growing upward-away from the source of gravity. Hydrotropism occurs chiefly in roots and is almost always positive. See TROPISM.

Some plants are affected by being touched. When the sensitive plant, Mimosa pudica, is touched, its leaflets quickly fold and its branches fall against its stem. A change in pressure within certain cells of the plant causes this action. After the stimulus has been removed, the plant's branches and leaflets return to their original position. See SENSITIVE PLANT.

How plants change

Plants-like animals-compete with one another for sunlight, water, and other necessities of life. Some plants-like some animals-are better able than others to grow and reproduce. After thousands of years, those that survive may differ greatly from their ancestors. The surviving plants have adapted to their environment through a process called natural selection or survival of the fittest (see NATURAL SELECTION).

This section traces the early history of plants and discusses important forms of plant adaptation for water storage and seed dispersal (scattering). This section also describes an unusual group of plants that adapted in such a way that they capture and eat insects. It ends with a discussion of some of the ways that people have changed plants.

Early plants. The first land plants appeared on Earth over 430 million years ago during the Paleozoic Era. These plants were very simple and did not resemble any of the plants we see today. They probably had a sticklike plant body and lacked the specialized water-conducting tissue of vascular plants. Many botanists believe these early land plants are the ancestors of primitive vascular plants. The first vascular plants, called Rhyniophytes, did not have leaves or roots. They consisted of both stems that grew along the ground and stems that grew upright with Y-shaped branches. These plants probably grew as tall as 2 to 3 feet (60 to 90 centimeters).

Larger plants called Trimerophytes may have developed from the Rhyniophytes. The Trimerophytes had a more complex plant body with numerous stems and branches. But they did not have leaves, and only some of them may have had simple roots. Other small vascular plants called Zosterophyllophytes appeared shortly after the Rhyniophytes and also may have descended from them. Some botanists believe Trimerophytes and Zosterophyllophytes are the ancestors of all vascular plants that exist today. They think that ferns, horsetails, and seed-bearing plants evolved (developed through progressive change) from Trimerophytes about 408 million to 360 million years ago. Club mosses, quillworts, and selaginellas are believed to have evolved from Zosterophyllophytes about that same time.

When the first vascular plants began to grow successfully on much of the land, life on earth was very different than it is today. No leaves rustled in the breeze, few insects crawled about, and no vertebrates (animals with backbones) lived on land. However, as conditions on Earth changed, new plants and animals developed. During the Carboniferous Period, about 360 million to 286 million years ago, more complex and larger vascular plants evolved. Great forests of lycopsid trees, ferns, horsetails, and early seed plants covered Earth. The huge plants of this period died and accumulated in vast swamps. They later formed large coal deposits. Most of the coal found in the eastern and midwestern United States is made up of these plants.

Gymnosperms became the most plentiful plants during the Mesozoic Era, beginning about 248 million years ago. Conifers, cycads, and ginkgoes were among the most important plants. They served as food for the great dinosaurs that roamed the land during this period. Many now-extinct types of gymnosperms also flourished. The first angiosperms, or flowering plants, appeared near the end of the Mesozoic Era. Among them were magnolias, sycamores, willows, water lilies, and many other present-day flowering plants.

During the Cenozoic Era, beginning about 65 million years ago, forests of angiosperms covered much of the tropical and temperate regions of Earth. Grasslands and large grazing animals began to appear late in the Cenozoic Era. Some scientists believe that humanlike creatures appeared on Earth about 5 million years ago and lived in the regions between the forests and grasslands.

Water storage. Through the years, many species of plants have developed special methods for collecting and storing water that enabled them to survive in areas of little rainfall. Some cactuses, for example, have roots that spread over large areas just below the surface of the ground. These roots quickly absorb water from the light rains and sudden floods that occur on the desert. Cactuses store the water in their fleshy stems.

Through natural selection, the leaves of cactuses evolved into spines. As a result of this adaptation, cactuses have less green surface than do most plants of their size--and they lose less water through transpiration. Because cactuses have such specially shaped leaves, they carry out photosynthesis in their stems. During photosynthesis, cactuses use their stored water supply if water from their roots is not available.

Plants of the tundra also have adapted to the dry conditions created by frozen soils. The surfaces of their leaves are especially resistant to water loss. They are either hard and glossy or very hairy. In addition, tundra plants grow close to the ground, where they are covered by snow and thus protected from the strong winds of these regions.

Seed dispersal. Seeds play an important part in the distribution of plants to nearly every part of the world. If seeds simply fell to the ground, all the plants of each species would be found in the same area. People also have helped spread seeds by taking food crops and certain other plants wherever they have settled.

Seeds have many features that help them be scattered across large regions. The wind carries many seeds, including the winglike ones of the maple tree and the fluffy seeds of dandelion and milkweed plants. Some seeds, such as those of the coconut, may float on water from one land area to another.

Animals also help distribute seeds. Some plants have burs and sticky substances that cling to the fur or feathers of animals that migrate from one region to another. Many kinds of animals eat berries and fruits but do not digest the seeds. The seeds are dispersed as part of the body waste of these animals.

A few species of plants distribute their own seeds. For example, a wildflower called the touch-me-not shoots out its seeds at the slightest touch.

Insect-eating plants grow chiefly in areas where the soil lacks an adequate supply of important minerals, especially nitrogen. These plants have adapted so that they can obtain needed minerals by trapping and digesting insects in their leaves. These carnivorous plants also manufacture their own food by photosynthesis. Insect-eating plants include the pitcher plant, the sundew, and Venus's-flytrap.

Pitcher plants have tube-shaped leaves that collect rain water. Sweet substances around the rim of each tube attract insects to the plant. After an insect enters the tube, tiny, downward-pointing hairlike structures keep the struggling victim from escaping. In time, the insect becomes exhausted, slides into the water, and drowns. The plant then digests the insect by means of a fluid secreted by glands located in the leaves. See PITCHER PLANT.

The leaves of the sundew plant grow hairs that give off a sticky substance that contains digestive juices. When an insect gets stuck on this substance, the hairs wrap around it. More fluid covers and suffocates the insect, which is then gradually digested by the sundew plant. See SUNDEW.

Venus's-flytrap has hinged leaves that trap insects. The inside of each leaf has hairs, and the rim is edged with sharp bristles. When an insect lands on the hairs, the two halves of the leaf close like a trap, with the bristles interlocking. After the plant has digested the insect, the leaves open up again. See VENUS'S-FLYTRAP.

How people have changed plants. People began to play an important role in changing plants about 10,000 years ago, when they learned to raise food by farming. Early farmers noted that some plants grew better than others. They saved seeds from these plants to grow new ones. The basic food crops of the world were developed in this way. For example, the Indians of the Americas developed tiny ears of wild corn into large cobs with many kernels. By the time Christopher Columbus reached the New World in 1492, this improved corn was being raised over large areas of the Americas.

The scientific study of plants has greatly aided our attempts to make plants more useful and attractive. For example, an Austrian monk named Gregor J. Mendel conducted experiments on garden peas in the mid-1800's that laid the foundation for the field of genetics, the science of heredity. Using the laws of genetics, scientists have greatly increased the yield of such crops as corn, rice, and wheat. They also have developed plants that can resist the attacks of various diseases and insects. In 1970, Norman E. Borlaug, an American agricultural scientist, received the Nobel Peace Prize for developing high-yield, disease-resistant wheat.

Plant enemies

Various kinds of plant enemies attack and injure almost all species of plants throughout the world. Diseases and insect pests rank as the major enemies of plants. They cause serious, widespread damage to agricultural, garden, and ornamental plants, many of which have lost the natural defenses present in wild plants. In the United States, diseases, insects, and other plant enemies cause crop losses totaling about $30 billion yearly. Diseases reduce the nation's total annual crop production by 10 to 15 per cent, and insects reduce it by about another 15 per cent.

Widespread outbreaks of plant diseases can cause famine. During the 1840's, about 1 million people in Ireland died after a fungus disease destroyed the nation's potato crop. Other diseases have killed large numbers of certain species of plants. For example, a fungus disease called chestnut blight has destroyed the chestnut tree throughout North America. Insects also severely damage large numbers of plants. Swarms of grasshoppers have destroyed entire crops of alfalfa, cotton, and corn. Gypsy moths have damaged forests in the Northeastern United States and are spreading to other areas. In addition, many plants are injured or killed by animal pests, including mites, rabbits, and rodents.

Diseases in plants are caused by many kinds of microorganisms, including certain fungi, bacteria, viruses, and small worms called nematodes. Fungi cause more plant diseases than the other microorganisms. Viruses also infect plants with serious diseases.

Certain conditions in the environment can damage plant tissues and weaken plants so that they are more easily infected by disease-causing microorganisms. Such conditions include air pollution, unusually high or low temperatures, lack of proper nutrients in the soil, and low levels of light or oxygen.

Plant diseases may affect every part of the plant. Many diseases interfere with the plant's ability to carry out photosynthesis by damaging leaves or by blocking the flow of water or nutrients to stems and leaves. Fungi, bacteria, or viruses may invade plant tissues and kill cells in a small area. For example, dead spots on leaves and fruits, or yellowing and death of leaves at the edges, indicate places where microorganisms have killed plant cells. Abnormal growths, such as galls and knots, on roots, stems, and other parts of the plant also signal places of infection. Fungi or bacteria that invade the roots, stems, and leaves can prevent xylem tissue from transporting water throughout the plant. As a result, the leaves, stems, and flowers may wilt or suddenly die. In addition, fungi may secrete toxins (poisons) that cause large parts of the plant to die.

Fungal diseases are spread from plant to plant by the spores of the fungi. These spores are carried by insects, rain, wind, and even people. Some bacteria and viruses are spread in the same way. Nematodes not only cause certain diseases but also transmit viruses from diseased to healthy plants. Some bacteria and fungi live on plant refuse in the soil and infect healthy plants. Others are carried on the seeds of plants.

Some diseased plants cause serious illness when eaten by human beings and animals. For example, a fungus called ergot infects wheat, barley, and rye. It produces chemicals that can cause ergotism, an illness that afflicts people who eat bread made from the infected grain. Other fungi, if enough are present on food or animal feed, produce harmful chemicals called mycotoxins. Scientists are conducting extensive research on these chemicals, some of which may cause cancer.

Nutrient deficiencies. Plants suffer from nutrient deficiencies when they cannot obtain certain minerals and chemicals from the soil. Nutrient deficiencies are harmful to plants in a number of ways. They may cause changes in leaf color, reduction in the size of leaves, dead spots on leaves and stems, reduced growth, and wilting. Each symptom often can be linked to the lack of a specific chemical, usually nitrogen or potassium.

Plants may also be affected by chemical toxicity when the soil contains too much of certain chemicals or minerals. For example, most plants require very small amounts of zinc, iron, and copper. But people sometimes introduce excessive amounts of these substances into the soil during the mining and smelting of ore. As a result, large numbers of plants are killed. Zinc also may accumulate in the soil below fences that are coated with the mineral to prevent them from rusting. The zinc builds up in a narrow strip of soil and eventually destroys many of the plants growing there.

Some soils are naturally too rich in metals. For example, areas of serpentine, a volcanic rock that contains heavy metals, are common in western North America. These areas form barrens where few plants survive.

Pests. Insects damage or destroy plants in a number of ways. Insects with chewing mouthparts, such as beetles and grasshoppers, eat holes in leaves and stems. Other insects have piercing and sucking mouthparts with which they pierce plants and consume the plant juices. Some insects feed on flowers and fruit. The destruction of leaves by insects affects the growth and yield of crops because photosynthesis is reduced. In addition, wounds made in plants by insects provide places for disease-causing organisms to enter the plants easily.

Some insects secrete poisons or other chemical substances while feeding. These secretions may cause galls on leaves or roots or give leaves a "burned" appearance. Other insects interrupt the flow of food and water in plants by feeding on phloem and xylem tissue.

Mites, which have sucking mouthparts, injure plants by feeding on them. Rabbits and rodents gnaw on plants. Some kinds of rodents burrow into the soil and feed on the roots, seeds, and bulbs of plants.

How plants protect themselves. Insects and many other animals eat plants. To avoid being eaten, many species of plants have developed physical and chemical defenses. Many plants also protect themselves through the timing of when they produce flowers and fruits.

Physical defenses of plants include such structures as spines, thorns, and prickles. These structures, which are usually modified leaves or branches, prevent attacks by large plant-eating animals. Heavy coatings of wax or dense, stiff hairy structures on leaves and stems may repel smaller animals, especially insects. Some plants, including grasses, accumulate a hard mineral called silica in their leaves. The silica makes the leaves difficult for animals to chew and rapidly wears down their teeth.

Certain species of plants obtain protection from animal enemies through a relationship called mutualism. In this relationship, the plant provides a special type of food for a particular group of insects. The insects, in turn, protect the plant from other animals. One example of plant-insect mutualism is the relationship between ants and acacia trees in some dry regions of the world. Ants live in hollow thorns on the acacia trees, and the leaves of the trees release a sugar solution for the ants to eat. In return, the ants clear the ground around each tree and attack any other animals that enter the cleared area or that land on the trees.

Plants have a wide variety of chemical defenses against animals. The leaves and fruits of citrus plants produce sticky, strong-smelling oils that discourage many insects. Many plants contain chemicals that have an unpleasant taste or are poisonous. Such plants include nightshade, foxglove, yew, and many weeds.

Insects can quickly become immune to the chemicals plants produce. In some cases, certain kinds of insects develop a means of breaking down the toxic compounds produced by the plants. As a result, plants continually develop new chemical compounds by altering existing ones. Some scientists describe this process as a biological "arms race" between plants and their predators. In other cases, the "arms race" between an insect and plant has resulted in a unique relationship. For example, plants in the milkweed family produce a milky sap that contains poisonous chemicals. The chemicals prevent most insects from eating the plants. However, caterpillars of monarch butterflies are able to eat milkweed plants and store the poison in their bodies. The poison makes monarch butterflies distasteful and so protects them from many predators.

Many plants try to ensure the survival of their seeds through the timing of flower and fruit production. Some plants produce flowers and fruits very early in the growing season, when insect populations are small. Other plants produce so many seeds that animals cannot eat them all. For example, oak trees produce a great number of acorns every few years. When acorns are abundant, squirrels and other animals cannot eat all of them and some acorns survive to grow into new oak trees. In other years, oak trees do not produce an abundance of acorns and thus prevent animals from relying on acorns for food. If the trees produced a surplus of acorns each year, the animal population would increase and all the acorns would be eaten.

Control of diseases and pests. People fight plant diseases and pest damage by means of (1) genetic methods, (2) physical methods, (3) sanitation, (4) chemicals, (5) biological control, and (6) quarantine laws. Genetic methods include the development of resistant varieties of plants by plant breeders. Breeders cross resistant plants with other varieties of the same species to develop new varieties that combine resistance with high yield and other desirable characteristics. Such efforts by plant breeders have resulted in the development of high-yield, rust-resistant wheats, for example.

Physical methods include such barriers against plant pests as sticky bands of paper that trap insects, and wire guards to keep rodents away. Plant growers also gather and destroy insects and insect eggs found on plants. Crop rotation and plowing help prevent plant enemies from overpopulating the soil.

Sanitation includes destroying diseased plants and disinfecting planting equipment. In addition, refuse is removed from a growing area. This removal eliminates places where insects and disease-causing organisms may reproduce.

Chemicals make up the largest part of almost every program to control plant enemies. Diseases and pests may attack suddenly, and chemicals may be the only means of saving the plants. Many chemicals protect plants from diseases and pests. They include bactericides, fungicides, insecticides, nematocides, and rodenticides. In the United States, such chemicals must be approved by the Environmental Protection Agency before they can be marketed.

Biological control involves the use of natural processes to fight the insects and disease organisms that attack plants. For example, certain bacteria and viruses that cause diseases in beetles and caterpillars may be introduced into an area to control those insects. Similarly, animals that prey on insects may be introduced to control plant enemies. Another example of biological control is the capture of insects in traps baited with sex attractants, the natural chemicals that insects secrete to attract mates.

Quarantine laws regulate the shipment of plants between countries and, in the United States, between states. These laws require inspection of plants to prevent the introduction and spread of plant diseases and insect pests.

Classification of plants

Botanists classify plants by grouping them according to their shared similarities. Such an arrangement provides a logical way to organize information about plants and to show how different plants are related to each other.

Most botanists group plants by their overall appearance, their internal structure, and the form of their reproductive organs. However, not all botanists agree on how plants should be divided, and there are a number of different classifications of the plant kingdom. One frequently used classification system is described below. This system classifies plants into 10 groups or divisions. A division is the same grouping as a phylum in the animal kingdom.

One division, Bryophyta, is made up of nonvascular plants. These plants lack xylem and phloem tissues that carry water and food from one part of the plant body to another. All other divisions of the plant kingdom are made up of vascular plants that contain these specialized tissues.



Contributor: William A. DiMichele, Ph.D., Research Paleontologist and Curator of Paleobotany, National Museum of Natural History, Smithsonian Institution.

Questions

About how many kinds of plants are there?

How do animals help distribute seeds?

What are the four main parts of most flowering plants?

When did plants first appear on the land?

Describe the role of plants in the cycle of nature.

Why are flowering plants called angiosperms?

What is cross-pollination? What is self-pollination?

How do green plants make their own food?

What are the three major types of forests?

What are carnivorous plants?

Additional resources

Level I

Dow, Lesley. Incredible Plants. Time-Life Bks., 1997.

Hershey, David R. Plant Biology Science Projects. Wiley, 1995.

Kalman, Bobbie. How a Plant Grows. Crabtree Pub. Co., 1997.

Silverstein, Alvin, Robert, and Virginia. Plants. 21st Century Bks., 1996.

Taylor, Barbara. Incredible Plants. D K Pub., 1997.

Level II

Attenborough, David. The Private Life of Plants: A Natural History of Plant Behavior. Princeton, 1995.

King, John. Reaching for the Sun: How Plants Work. Cambridge, 1997.

Mabberley, D. J. The Plant-Book: A Portable Dictionary of the Vascular Plants. 2nd ed. Cambridge, 1997.

Stearn, William T. Stearn's Dictionary of Plant Names for Gardeners. Cassell, 1996.

Stern, Kingsley R. Introductory Plant Biology. 7th ed. W. C. Brown, 1997.

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Tree is the largest of all plants. The tallest trees grow higher than 30-story buildings. Many trees also live longer than other plants. Some trees live for thousands of years. They are the oldest known living things.

People do not think of trees the way they think of other plants, most of which grow only a short time and then die. People think of trees as permanent parts of the landscape. Year after year, large, old trees shade houses and streets from the sun. Their buds and flowers are a sign of spring each year, and their colorful leaves brighten in autumn in many areas.

Trees continue to grow as long as they live. A tree's leaves make food that keeps the tree alive and helps it grow. Where winters are cold, many trees lose their leaves in autumn. Other trees keep their leaves during the winter and so stay green all year long. Trees that shed their leaves in autumn rest during the winter. In spring, they grow new leaves and flowers. The flowers grow into fruits, which contain seeds for making new trees. Some tree fruits, such as apples and oranges, taste good. Fruit growers raise large amounts of these fruits for sale. Trees also make new wood each year when the weather turns warmer. Wood is one of the most valuable parts of a tree. Mills and factories use wood to manufacture lumber, paper, and many other products.

A tree differs from other plants in four main ways. (1) Most trees grow at least 15 to 20 feet (4.6 to 6.1 meters) tall. (2) They have one woody stem, which is called a trunk. (3) The stem grows at least 3 to 4 inches (8 to 10 centimeters) thick. (4) A tree's stem can stand by itself. All other plants differ from trees in at least one of these ways. For example, no plant with a soft, juicy stem is a tree. Most of these plants, called herbs, are much shorter than most trees. Shrubs, like trees, have woody stems. But most shrubs have more than one stem, and none of the stems grows so thick or so tall as a tree trunk. Some jungle vines grow more than 200 feet (61 meters) long and have a woody stem. But the stems of most vines cannot support themselves.

There are thousands of kinds of trees. But most trees belong to one of two main groups-the broadleaf trees and the needleleaf trees. These two types of trees grow in Europe, North America, and many other parts of the world. Most other types of trees, such as palms and tree ferns, grow mainly in warm regions.

This article discusses TREE (Scientific classification of trees).





Tree/The importance of trees

For thousands of years, trees have provided people with foods, fibers, and medicines. Above all, they have provided people with wood. Prehistoric people used wood to make the first spear, the first boat, and the first wheel. Throughout history, people have used wood to make tools, construct buildings, and create works of art. They have also used it for fuel. Living trees are as valuable to humankind as are tree products because they help conserve natural resources.

Wood products. Each year, loggers cut down millions of trees in the world's forests. Logs from these trees are shipped to sawmills and pulp mills. Sawmills cut the logs into lumber, which the building industry uses for many types of construction work. Manufacturers use lumber to make everything from furniture to baseball bats. Pulp mills break down the logs into wood pulp, the main raw material for making paper. The chemical industry uses wood pulp to make alcohol, plastics, and other products. See FOREST PRODUCTS; LUMBER.

Food products. People throughout the world eat fruits, nuts, and other tree products. The greatest variety of fruit trees grow in tropical and subtropical regions. These trees produce such fruits as avocados, grapefruits, mangoes, and oranges. A number of these fruits serve as basic foods in some tropical lands. Cooler, temperate regions-such as most of the United States and Europe-have fewer kinds of fruit trees. But several kinds are widely grown. For example, orchards in the United States produce vast amounts of apples, cherries, and peaches. The most important nut tree of warm regions is the coconut palm, which produces coconuts. Nut trees of temperate regions include almonds, pecans, and walnuts. Trees also supply chocolate, coffee, maple syrup, olives, and such spices as cinnamon and cloves. See FRUIT; NUT.

Other tree products are used by people in a variety of ways. The rubber tree produces latex, a milky fluid used to make natural rubber. Pine trees produce a sticky resin, used in making turpentine. The bark of oak and some other trees contains a compound called tannic acid. The tanning industry uses this compound to change animal hides into leather. The spongy bark of a type of oak that grows in Mediterranean countries provides cork. Some trees produce substances used as medicines. For example, the bark of the cinchona tree contains quinine, which doctors use to treat malaria and other diseases.

Trees in conservation. Trees help conserve soil and water. In open country, trees act as windbreaks and keep the wind from blowing away topsoil. Their roots prevent soil from being washed away by heavy rains. Tree roots also help store water in the ground. In mountain regions, forests prevent sliding snow from causing avalanches. Forests also provide shelter for wildlife and recreation areas for vacationists. See CONSERVATION.

Trees help preserve the balance of gases in the atmosphere. A tree's leaves absorb carbon dioxide from the air. They also produce oxygen and release it into the atmosphere. These two processes are necessary for people to live. People could not survive if the air had too much carbon dioxide or too little oxygen.

Tree/Kinds of trees

There are about 20,000 kinds of trees. More than 1,000 kinds grow in the United States. They range from mighty forest trees to fragile ornamentals. The greatest variety of trees grow in wet tropical regions.

Scientists who study plants divide plants with similar characteristics into various groups (see PLANT (Kinds of plants). These scientists, called botanists, do not put trees in a separate group of plants. Instead, each kind of tree is grouped with other plants that have certain features in common with it. Therefore, a group of plants may include certain trees, certain shrubs or vines, and certain herbs. For example, locust trees, broom plants, and clover all belong to the same family. These plants are grouped together because they reproduce in the same way and have similar flowers. On the other hand, some trees that look much alike, such as tree ferns and palms, belong to different groups of plants.

Trees also can be divided into six groups according to various features they have in common. These six groups are: (1) broadleaf trees; (2) needleleaf trees; (3) palm, pandanus, and lily trees; (4) cycad trees; (5) tree ferns; and (6) ginkgo trees.

Broadleaf trees are the most numerous and varied of the world's trees. They include ashes, elms, maples, oaks, walnuts, willows, and many other familiar trees of the United States and Canada. They also include most trees of the tropics, such as mahogany trees and mangrove trees.

In addition to their broad, flat leaves, broadleaf trees have other features in common. Almost all broadleaf trees of temperate regions are deciduous-that is, they lose their leaves each autumn. A few kinds of broadleaf trees in temperate regions do not lose their leaves in the fall. These broadleaf evergreens include the holly trees and live oaks of the Southeastern United States. Some tropical broadleaf trees are deciduous, but most are evergreen. See DECIDUOUS TREE; EVERGREEN.

Foresters call broadleaf trees hardwoods because many of these trees, such as beeches, maples, and oaks, have tough, hard wood. Such wood makes excellent furniture. Some broadleaf trees, including basswoods and cottonwoods, have soft, lightweight wood.

Broadleaf trees belong to a large group of plants called angiosperms. These plants have flowers which develop into fruits that completely surround the seeds. Fruits are the seed or seeds of a plant together with the parts in which they are enclosed. Botanists divide angiosperms into two classes-Monocotyledonae (monocotyledons) and Dicotyledonae (dicotyledons). Monocotyledons produce seeds that have one leafy structure called a cotyledon. These plants include palm, pandanus, and lily trees. Dicotyledons produce seeds that have two cotyledons. These plants include broadleaf trees. A few kinds of trees that do not have broad, flat leaves belong to the dicotyledon group. An example is the saguaro cactus of the Southwestern United States, which has prickly spines. See ANGIOSPERM; COTYLEDON.

Needleleaf trees include such familiar trees as firs, hemlocks, pines, redwoods, and spruces. There are about 500 species of needleleaf trees. Most of these trees have narrow, pointed, needlelike leaves. But a few types, such as cedars and junipers, have narrow, scalelike leaves.

Most needleleaf trees are evergreen, though they produce new needles each year. The oldest needles turn yellow or brown and drop, but the youngest needles remain green and do not fall. A few species of needleleaf trees are deciduous. One kind is the larch, which grows in northern forests throughout the world. Another deciduous needleleaf tree is the baldcypress that grows in swamps of the Southeastern United States.

Foresters call needleleaf trees softwoods because most of them have softer wood than broadleaf trees have. But the wood of Douglas-firs, yews, and some other needleleaf trees is hard.

Needleleaf trees belong to a group of plants called gymnosperms. Gymnosperms do not have flowers and their seeds are not enclosed to form fruits. Most gymnosperm trees bear their seeds in cones composed of hard scales. The seeds lie open on the surface of the scales. Botanists call such trees conifers. See CONIFER; GYMNOSPERM.

Most conifers grow north of the equator. The conifers belong to four families-the pine, yew, cypress, and taxodium families. The pine family is by far the largest. It includes not only pines, but also such trees as firs, hemlocks, larches, and spruces. Pine trees make up a large genus (group of species) within the pine family. Loblolly pines, ponderosa pines, and white pines are a few North American members of this genus. The yew family includes such well-known ornamental trees as English yews and Japanese yews. Although yews are classified as conifers, they do not produce cones but cup-shaped "berries." Many members of the cypress family, such as arborvitae and junipers, have scalelike leaves and give off a spicy fragrance. The taxodium family includes baldcypresses and the largest of all living trees-the redwoods and giant sequoias.

Two conifer families-the podocarpus family and the araucaria family--grow mainly south of the equator. Podocarpus trees are tall evergreens with broader leaves than those of most needleleaf trees. The araucaria family includes the Chile pine. This strange-looking tree has snakelike branches covered with sharp, scaly leaves. It is sometimes called the monkey puzzle tree because its sharp leaves make it difficult to climb.

Palm, pandanus, and lily trees belong to the large group of flowering plants called monocotyledons. These trees grow mainly in warm climates. Of the three types of trees in this group, palms are the most important.

There are about 2,500 kinds of palm trees. They range from the coconut palms of tropical islands to the date palms of desert oases. Most palm trees have no branches. The trunk has a crown of enormous leaves. The leaves are either feather-shaped or fan-shaped. See PALM.

Unlike most palms, pandanus and lily trees have branches. Each branch has a crown of sword-shaped leaves. Most pandanus trees have tall stilt roots that extend into the ground from high on the trunk or branches. Lily trees are closely related to the garden flowers called lilies, and many of the trees have attractive, fragrant flowers. The yucca trees of Mexico and the far Southern United States are lily trees. The best-known yucca is the colorful Joshua tree found in the deserts of the Southwestern United States.

Cycad trees look much like palm trees. They have a trunk without branches and a crown of long, feathery leaves. But cycads are more closely related to pine trees than to palms. They produce seeds in cones that look like large pine cones. Millions of years ago, cycads grew in nearly every part of the world. Today, they grow mainly in a few warm, moist sections of Africa, Asia, and Central America. See CYCAD.

Tree ferns. Ferns are best known as rather short plants with feathery, green fronds (leaves). But in the tropics and some areas with mild climates, many relatives of these plants are trees. Tree ferns look much like palm trees, but they belong to a different group of plants. Tree ferns do not have flowers or cones and so do not reproduce by seeds. They reproduce by means of tiny bodies called spores, which develop on the undersides of their fronds. See FERN.

Ginkgo trees are an extremely old species of tree. Millions of years ago, various kinds of ginkgoes existed. Only one species survives today. The ginkgo, like needleleaf trees, is a gymnosperm. But unlike other gymnosperm trees, the ginkgo has fan-shaped leaves. These leaves look like the fronds of a fern called the maidenhair. Ginkgoes are sometimes called maidenhair trees. They are natives of Asia, but many are grown in the United States and Europe.

Fossil trees. About 300 million years ago, there were whole forests of trees unlike most of the trees that grow today. Huge club-moss trees and horsetail trees grew along with tree ferns in steaming hot swamps. Over millions of years, the trees and other plant life in the swamps died, became buried, and turned into coal. In other places, buried forests became petrified (turned into stone). Coal deposits and petrified forests contain fossils of many trees that died out more than 100 million years ago (see FOSSIL). Two of these extinct trees are the club moss tree and horsetail tree of the coal-forming swamps. The club mosses and horsetails living today are herbs.

Tree/The parts of a tree

A tree has three main parts: (1) the trunk and branches; (2) the leaves; and (3) the roots. The branches and leaves together are called the crown. The trunk supports the crown and holds it up to the sunlight. Tree ferns, cycads, and most palms have no branches. Their crowns consist only of leaves. The roots of most trees are hidden in the ground, but they may take up as much space as the trunk and crown do above the ground. Other important parts of a tree include the seeds and the seed-forming structures.

Trunk and branches give a tree its shape. The trunks of most needleleaf trees grow straight up to the top of the tree. The branches grow out from the trunk. On most needleleaf trees, the branches near the top are shorter than those farther down, which gives the crown a spirelike shape. The trunks of most broadleaf trees do not reach to the top of the tree. Instead, the trunk divides into spreading branches near the base of the crown, giving the crown a rounded shape. The trunks of a few broadleaf trees, such as black willows and white poplars, sometimes divide so close to the ground that the trees seem to have more than one trunk.

The trunks, branches, and roots of broadleaf and needleleaf trees consist of four layers of plant tissue wrapped around one another. These layers, from innermost to outermost, are: (1) the xylem, (2) the cambium, (3) the phloem, and (4) the cork.

The xylem is the woody, central part of the trunk. It has tiny pipelines that carry water with a small amount of dissolved minerals from the roots to the leaves. This water is called sap. The cambium, which surrounds the xylem, is a thin layer of growing tissue. Its job is to make the trunk, branches, and roots grow thicker. The phloem, also called the inner bark, is a layer of soft tissue surrounding the cambium. Like the xylem, the phloem has tiny pipelines. The food made by the leaves moves through the phloem to the other parts of a tree. In palms and tree ferns, the xylem and phloem are not separate layers. Instead, bits of xylem and phloem are connected and form small double pipelines scattered throughout the trunk.

The cork layer is the outer bark of a tree. It forms a "skin" of hard, dead tissue that protects the living inner parts from injury. The bark stretches to let the trunk and branches grow thicker. The bark of some trees, such as beeches and birches, is smooth because it stretches easily. But the bark of most other trees does not stretch so well. As the trunk and branches grow thicker, they push against the bark. It finally cracks and dries and so becomes grooved and rough. Most trees replace old bark from time to time with a new layer.

Leaves of various species of trees differ greatly in size and shape. Palms have leaves over 20 feet (6 meters) long. The leaves of some needleleaf trees are less than 1/2 inch (13 millimeters) long. Some broadleaf trees have compound leaves made up of small leaflets.

The main job of the leaves is to make food for the tree. Every leaf has one or more veins, which consist of xylem and phloem tissue. The tissue that surrounds the veins contains tiny green bodies called chloroplasts. Water from the roots passes through the xylem of the trunk, branches, and leaves to the chloroplasts, which use the water to make food sugar. Only a small amount of the water carried to the leaves is used to make sugar. The leaves lose most of the water to the atmosphere through transpiration (evaporation). Like the water and dissolved minerals carried from the roots, the food made by the leaves is also called sap. It travels through the phloem of the leaves, branches, and trunk to parts of the tree where it is needed. See SAP.

Almost all leaves are green in the spring and summer. Their color comes from chlorophyll, a green substance in the chloroplasts. Most trees also have reds and yellows in their leaves. But the green conceals these colors. In late summer and early autumn, the chlorophyll in the leaves of many broadleaf trees breaks down. The leaves then die. But before the leaves fall, they reveal their hidden reds and yellows. After the chlorophyll breaks down, the leaves of many trees also develop scarlets and purples. See LEAF (The leaf changes color).

Roots are long, underground branches of the trunk. They have the same layers of tissue as the trunk. The roots anchor a tree in the ground and absorb water with dissolved minerals from the soil. The main roots branch out into small roots, which, in turn, branch out into still smaller roots. The main roots of most trees begin to branch out 1 or 2 feet (30 or 61 centimeters) under the ground. Some trees have one main root larger than the others. This root, called a taproot, extends straight down 15 feet (5 meters) or more.

A tree develops millions of small roots. Each root grows longer at its tip, which is as small as a thread. As a root tip grows, it pushes through particles of soil. Thousands of fine, white root hairs grow just back of the root tip. When the tip comes in contact with drops of water in the soil, the hairs soak up the water and dissolved minerals. The xylem layer of the roots, trunk, and branches carries this sap to the leaves.

Fungi grow on the roots of most trees in a helpful relationship called mycorrhiza. The fungi aid the roots in absorbing water and mineral nutrients. They also protect the roots from some diseases.

Seeds are the means by which all trees except tree ferns reproduce. Tree ferns reproduce by spores.

Angiosperms-broadleaf trees and palm, pandanus, and lily trees-produce seeds by means of flowers. Some broadleaf trees, such as horsechestnuts and magnolias, produce large, showy flowers. Many others have small, plain-looking flowers. Most palm, pandanus, and lily trees have small flowers that grow in bunches. Sometimes these are brightly colored and fragrant.

The seeds of angiosperms are enclosed to form a fruit. The fruits of some broadleaf trees, such as apples and cherries, have a fleshy outer covering. The fruits of other broadleaf trees, including acorns and beechnuts, are hard nuts. Ashes, elms, and maples have thin, winged fruits. Palm, pandanus, and lily trees have a variety of fruits, ranging from nuts to berries.

Gymnosperms-needleleaf trees, cycads, and ginkgoes-do not have flowers or fruits. Their seeds are produced in cones or similar structures. The seeds of needleleaf trees and cycads have no protective coverings. Ginkgo seeds have a fleshy outer covering, but the covering is not a true fruit.

Tree/How a tree grows

Most trees begin life as a seed. The young tree that develops from this seed is called a seedling. After a tree reaches a height of 6 feet (1.8 meters) or more and its trunk becomes 1 to 2 inches (2.5 to 5 centimeters) thick, it is called a sapling. Many trees reach a height of more than 100 feet (30 meters). Some old trees have trunks more than 10 feet (3 meters) in diameter.

Trees need great amounts of water. A large apple tree in full leaf may absorb 95 gallons (360 liters) from the soil daily. Most of the water goes to the leaves. On a sunny summer day, some trees move water up through their trunks at the rate of 3 feet (91 centimeters) per minute. A tree's wood is about half water.

How seeds sprout into trees. A seed contains parts that develop into the trunk and roots of a tree. It also has one or more cotyledons and a supply of plant food. After a seed has left the parent tree, it rests for a while on the ground. Water, air, and sunshine help the seed germinate (begin to grow). The part of the seed that develops into the trunk points upward toward the sunlight. As the seed absorbs water, the root part swells and bursts through the seed's shell. As the root grows, it pushes down into the soil. The food stored in the seed nourishes the tree. As the root begins to soak up water from the soil, the trunk begins to develop leaves.

How leaves make plant food. As a leaf develops, it gets sap from the roots. It also absorbs carbon dioxide from the air. The leaf uses the energy of sunlight to change the sap and carbon dioxide into sugar, a process called photosynthesis. The sugar provides food for the trunk, branches, and roots. During photosynthesis, the leaves also produce oxygen and release it into the atmosphere. See LEAF (How a leaf makes food).

How trees grow taller. Trees grow taller only at the tips of their trunk and branches. Each year, the tips of the trunk and of each branch develop a bud. The bud contains a tiny leafy green stem called a shoot. The bud is wrapped in a protective covering of bud scales. After a period of rest, the buds swell and open. The shoots that were inside the buds begin to grow and so make the trunk and branches taller. Another type of bud grows on the sides of the trunk and branches. These buds contain a shoot that develops into a leaf-bearing twig after the bud opens. As a twig grows larger, it becomes another branch of the tree. Some tree buds develop into flowers. Still others develop into twigs that bear both leaves and flowers. In warm climates, trees produce buds frequently during the year or continue to grow without forming buds. In colder climates, trees produce buds only in the summer. These buds rest in winter and open after warm weather arrives in spring.

Trees without branches-cycads, most palms, and tree ferns-grow somewhat differently. For example, a young palm tree does not grow taller for a number of years. Its short trunk thickens and produces more and larger leaves each year. After the trunk and crown reach adult size, the tree begins to grow taller. The trunk stays about the same thickness for the rest of the tree's life.

How trunks and branches grow thicker. The trunk and branches of a broadleaf or needleleaf tree grow thicker as long as the tree lives. The cambium tissue just underneath the inner bark causes this thickening. It uses the sugar produced by the leaves to make new plant tissue. On its outside, the cambium makes new phloem, or inner bark, and on its inside, new xylem, or wood.

Wood consists largely of cellulose, a tough substance made from sugar. The xylem has two kinds of wood--sapwood and heartwood. The wood nearest the cambium is the sapwood. It is living wood and contains the tiny pipelines that carry sap. In tropical climates, the sapwood thickens all year. In cooler climates, a new layer of sapwood usually forms only in early summer. As a tree ages, the wood nearest the center dies. This dead wood is the heartwood. It helps support the tree.

In regions where trees make a new layer of wood once a year, the layers form a series of annual rings. Each ring represents one year's growth. After such a tree has been cut down, a person can count the rings to determine the tree's age. Scientists have also found that slight changes in the composition of a tree's cellulose reveal the kind of weather that a tree experienced.

How trees reproduce. Most trees reproduce sexually. That is, seeds are produced only after sperm unite with eggs. Sperm are produced by pollen, which forms in the tree's male reproductive parts-either the male part of the flower or the male cone. Eggs form in the female part of the flower or in the female cone. Among many angiosperm species, the flowers have both male and female parts. The pollen from the male part can simply drop onto the female part. Other angiosperms and all gymnosperms have separate male and female flowers or cones, which may grow on the same tree or on separate trees. The pollen of these species is carried to the female flower or cone by insects, the wind, or other means. After contacting the female flower or cone, pollen produces sperm. The sperm then unite with eggs, and one or more seeds develop within a fruit or cone.

When the fruit or cone has ripened, the seeds are ready to leave the tree. The wind scatters the seeds of needleleaf trees and the winglike seeds or fruits of such broadleaf trees as ashes, maples, poplars, and willows. Birds, squirrels, and other animals scatter seeds contained in nuts or fleshy fruits. Ocean currents sometimes carry the seeds of coconut palms and mangroves.

Trees can also reproduce by a process called vegetative reproduction. After a tree has been cut or blown down, the stump may develop green sprouts. In time, one or several of these sprouts can grow into trees. A clump of birches or yellow-poplars may be produced in this way. The roots of apple trees, aspens, and some other trees sometimes develop shoots called suckers that may also grow into trees. Some spruces found in bogs grow roots from their branches. This method of reproduction is called layering. In addition, nursery workers often grow trees from cuttings-that is, twigs cut from older trees. The twigs are planted and develop roots.

Tree/Broadleaf and needleleaf trees

This section illustrates some of the chief characteristics of various broadleaf and needleleaf trees around the world. The drawings show the summer and winter appearance of each species. They also illustrate the leaf; the fruit or other seed-bearing structure; and, in most cases, the bark. For some species, the flower is shown. Each set of illustrations includes information about the tree's native geographic range-that is, the part of the world where the tree is most likely to be found. But a number of the species shown have spread or have been planted outside their native range. The average height of adult trees of each species is given in feet and in meters alongside the illustration of the tree's shape.

The drawings and other information in this section can help in identifying trees. For example, if the leaf and bark of a tree match the leaf and bark of one of the trees shown here, the tree should be fairly easy to identify. Tree guidebooks can provide additional help in identifying trees. Several guidebooks are listed in the Study aids at the end of this article.

Tree/Trees around the world

In some parts of the world, trees grow in thick forests. In other regions, they do not grow at all. To grow, trees need a period of more than two months without frost each year. The few trees that grow in the Arctic never reach full tree size. No trees can grow in the ice and bitter cold of Antarctica. Most trees also need at least 15 to 20 inches (38 to 51 centimeters) of rainfall a year. Only a few trees, such as the Joshua tree and some types of palms, can survive in deserts.

Most broadleaf trees grow best in regions that are warm and moist at least three or four months of the year. Colder, dryer climates are better suited to most needleleaf trees. But some broadleaf trees, such as birches and willows, grow well in cool climates. Some needleleaf trees, including baldcypresses and various types of pines, need fairly warm climates. Palm trees grow in warm areas throughout the world, especially the wet and the dry tropics. Pandanus trees, cycads, and tree ferns grow mainly in the wet tropics and other warm, moist regions. Lily trees also thrive in warm areas, but they do not need so much moisture as do pandanus trees, cycads, and tree ferns.

Different kinds of trees also require different soils. Many needleleaf trees grow well in poor, sandy soil. But most broadleaf trees need more fertile soil.

Some trees grow alone or in small groups. Where moisture is scarce, trees may grow only along riverbanks. Tree seeds carried by ocean currents may take root along shorelines. People plant individual trees in such places as parks and gardens. But most trees by far grow in forests. The world's forest regions consist chiefly of broadleaf and needleleaf trees.

Broadleaf forests grow in regions that have a fairly long growing season and plentiful rainfall. Every continent except Antarctica has broadleaf forests, which are also called hardwood forests. In areas with cold, snowy winters, almost all the trees in broadleaf forests lose their leaves each autumn. In tropical areas, most broadleaf trees are evergreen.

Before the 1800's, broadleaf forests covered much of the Eastern United States. They included such trees as ashes, birches, maples, and oaks. During the 1800's, most of the trees in these forests were cut down to provide lumber and fuel and to make room for farms and cities. Today, only a few parts of the Eastern United States have large broadleaf forests. Western Europe also had great forests of broadleaf trees, including ashes, beeches, and oaks. But most of these forests have been cut down.

Broadleaf forests that consist largely of quaking aspens and balsam poplars cover parts of southern Canada and large areas of southern Siberia. Forests of birches and oaks grow in eastern Europe and along the Yellow Sea coast of China and Korea. Southeastern Australia has valuable forests of eucalyptus trees. These broadleaf trees grow nearly as tall as California's needleleaf giants, the redwoods. Some eucalyptus trees stand more than 300 feet (91 meters) tall. About 600 kinds of eucalyptus trees grow in Australia. Almost all of these trees are evergreen.

In many areas, mixed forests of broadleaf and needleleaf trees grow alongside broadleaf or needleleaf forests. Central Canada, the Eastern United States, central and southern Europe, and eastern Asia all have large mixed forests.

Remarkable broadleaf forests grow in tropical regions where the weather is always hot and rain falls regularly every month of the year. In these tropical rain forests, many of the trees look alike. They are tall, and many tower more than 150 feet (46 meters). The trees have leathery, dark-green leaves. Because the trees receive plenty of moisture throughout the year, most of them are evergreen. The trees may thus look alike, but they belong to many species. Many palms grow among the broadleaf trees in the tropical rain forests. The largest rain forests are in South and Central America, central Africa, and Southeast Asia.

Needleleaf forests grow mainly in regions that have long, cold winters. These forests, which are also called softwood forests, stretch across Canada, northern Europe, and Siberia. Many firs, larches, and spruces grow in these northern forests, along with a few broadleaf trees, such as birches and willows. Some willows grow even farther north than needleleaf trees do. But they seldom reach more than shrub size. Needleleaf forests also blanket slopes in such mountain ranges as the Alps and the Rocky Mountains.

The Canadian needleleaf forests extend southward into the Western United States, where they include many of the world's largest trees. Many California redwoods tower over 300 feet (91 meters). Tall Douglas-firs also grow in the Western United States.

A few needleleaf forests grow in warmer regions. For example, the Southeastern United States has large forests of pines, such as loblolly pines and longleaf pines. These forests provide great quantities of wood for lumber and wood pulp.

How forests spread. Many forests did not always grow where they are growing now. These forests have spread from other areas. For example, broadleaf forests grow today in parts of the Northeastern United States where only needleleaf forests grew several thousand years ago. The spread of forests from one area to another is called migration. The wind helps trees migrate by carrying their seeds beyond the forests. Animals also help spread the seeds. Trees that grow from these seeds produce their own seeds, which may be spread in the same ways. Over hundreds or thousands of years, a particular kind of tree may thus spread to surrounding areas if the climate and soil are suitable.

Several hundred thousand years ago, glaciers moved down across much of North America and Europe. These glaciers caused the forests of needleleaf and broadleaf trees to migrate south. Thousands of years passed, and the ice began to melt. As the glaciers retreated northward, forests of needleleaf trees grew up again on the land that the glaciers had covered. The glaciers moved still farther north, and the climate became warm enough for broadleaf trees. Broadleaf trees usually crowd out needleleaf trees in areas where both are able to grow. As a result, broadleaf forests replaced needleleaf forests in many regions.

Forests can migrate over fairly level land but not across oceans or mountain ranges. Yet similar types of forest trees grow in areas separated by oceans or mountains. For example, the United States has oak trees much like those that grow in Europe. Most scientists believe that many millions of years ago, all the continents were connected. Needleleaf trees developed and spread across much of the earth. Broadleaf trees developed next and also spread. Over millions of years, the continents became separated-along with their trees and other forms of life--by the oceans. Mountain ranges rose up on the continents and separated the trees on each side of the mountains. In time, many of the trees on each continent and on each side of the mountain ranges developed into different species.

How people help trees spread. People have transplanted many species of trees across oceans and mountain ranges. Transplanted trees may grow well in a new region with a climate like that of their native lands. In time, these introduced species may spread and become native trees in their new surroundings. A kind of rubber tree that once grew only in Brazil was introduced into the Far East during the late 1800's. Today, whole forests of these trees grow in the Far East. About 100 years ago, Australian eucalyptus trees were planted in California. Today, many thousands of eucalyptuses shade streets and parks in several Western states. Monterey pines originally grew only in a small area of California. They now cover large areas in Australia and other countries south of the equator.

Tree/Planting and caring for trees

Homeowners plant various kinds of trees on their property. They plant shade trees for protection from the sun and ornamental trees for beauty. They may also plant trees as windbreaks. Many people enjoy having fruit trees in their yard or garden to provide shade and beauty as well as fruit.

Selecting the right tree. To grow well, a tree must be suited to the region where it is planted. Trees from faraway places should be planted only in regions with similar climates. A tree's special characteristics must also be considered. For example, trees with wide-reaching roots should not be planted near houses because the roots may damage drains and foundations, or plug sewage pipes.

Trees with full, leafy crowns make the best shade trees. These trees include ashes, basswoods, maples, and oaks, all of which are popular in the Eastern United States. Trees with showy flowers, such as the catalpa and the crab apple, are popular ornamental trees in the Eastern United States. In fairly warm areas west of the Rocky Mountains, such trees as acacias and pepper trees are planted as both shade and ornamental trees. Needleleaf trees are grown as ornamentals in many parts of the United States and Canada. They also make good windbreaks. Various broadleaf trees, including cottonwoods and Lombardy poplars, are also planted as windbreaks. Apple and cherry trees are popular fruit trees in temperate climates. In warm climates, many people grow citrus trees.

Planting the tree. A tree should be planted where it will have enough room when fully grown. The soil should be fertile and should drain well so that water does not collect and drown the roots.

It takes much time and effort to grow a tree from seed. Most people prefer to buy a tree at a nursery. If a nursery tree is taller than 15 feet (4.6 meters) or if its trunk is thicker than 3 inches (8 centimeters), special transplanting equipment may be needed.

The best time to transplant a tree is when it is resting-that is, in the fall, winter, or early spring. The roots of a deciduous tree can be dug up without a covering of soil. But they must be kept moist while out of the ground. The roots of an evergreen should be dug up with a ball of soil around them. The hole for any new tree should provide room for all the roots below ground level. A small tree may need to be supported by stakes to keep the wind from blowing it over.

Caring for the tree. A young tree should be kept moderately watered until it is well rooted. It usually takes about a year for a tree to become firmly rooted.

Pruning improves the shape of trees. Cutting off some of a young shade tree's lower buds will keep it from developing many low branches. But enough buds should be left so that the tree has a full, leafy crown. As the tree develops upper branches, more lower branches may be removed. See PRUNING.

Insects and diseases may attack a tree. With normal care, it can overcome most minor attacks. But if a tree fails to develop as many leaves as usual or if the leaves look pale, the tree may require the professional care of a tree surgeon. In some areas, air pollution threatens the health of trees.

Tree/Scientific classification of trees

Trees belong to three classes (groups) within the plant kingdom. Botanists further classify the plants in each class into subclasses, orders, and families. Plants are grouped according to the various characteristics they have in common. A few plant families consist largely of trees or shrubs, but some families have no trees at all. The table A classification of trees lists the families with the most species of trees or with one or more outstanding species. The families are arranged in the probable order of evolutionary development.



Contributor: Richard H. Waring, Ph.D., Professor Emeritus of Forest Ecology, Oregon State University.

Questions

What is sapwood? Heartwood?

What is the main job of a tree's leaves?

In what climate do most needleleaf forests grow?

What do root hairs do?

How do forests spread?

How do deciduous trees differ from evergreen trees?

When is the best time of year to transplant a tree?

How do trees help conserve soil and water?

In what four ways do trees differ from all other plants?

How do trees grow taller?

Additional resources

Level I

Burns, Diane L.Trees, Leaves, and Bark. Gareth Stevens, 1998.

Cassie, Brian.National Audubon Society First Field Guide: Trees. Scholastic, 1999.

Gardner, Robert.Science Project Ideas About Trees. Enslow, 1997.

Hickman, Pamela.Starting with Nature Tree Book. Kids Can Pr., 1999.

Staub, Frank J. America's Forests. Carolrhoda, 1999.

Level II

Benvie, Sam.The Encyclopedia of North American Trees. Firefly Bks., 2000.

Dirr, Michael A. Dirr's Hardy Trees and Shrubs. Timber, 1997.

Kourik, Robert.The Tree & Shrub Finder. Taunton Pr., 2000.

Lewington, Anna, and Parker, Edward.Ancient Trees. Collins & Brown, 1999.

Thomas, Peter.Trees: Their Natural History. Cambridge, 2000.

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Fruit is the part of a flowering plant that contains the plant's seeds. In this sense, fruits include acorns, cucumbers, tomatoes, and wheat grains. However, the word fruit commonly refers to the juicy, sweet or tart kinds that people enjoy as desserts or snacks. The word comes from the Latin word frui, meaning enjoy. Popular fruits include apples, bananas, grapes, oranges, peaches, pears, and strawberries.

Many fruits are nutritious as well as appetizing. For example, oranges and strawberries contain large amounts of vitamin C. Most fruits have a high sugar content, and so they provide quick energy. Fruits alone cannot provide a balanced diet, however, because the majority of them supply little protein.

The world's fruit growers raise millions of tons of fruit annually. Fruit growing is a branch of horticulture, a field of agriculture that also includes the raising of nuts, vegetables, flowers, and landscape crops. Most nuts are actually fruits, as are the edible portion of such vegetables as cucumbers, green peppers, and tomatoes. To prevent confusion, horticulturists define a fruit as an edible seed-bearing structure that (1) consists of fleshy tissue and (2) is produced by a perennial. A perennial is a plant that lives for more than two years without being replanted. The horticultural definition of a fruit excludes nuts and vegetables. Nuts are firm rather than fleshy. Most vegetables are annuals-that is, the plants live for only one season.

In some cases, the horticultural definition of a fruit conflicts with the definition used by botanists and with common usage. For example, watermelons and muskmelons are fruits, and most people regard them as such. But they grow on vines that must be replanted annually, and so horticulturists regard melons as vegetables. Rhubarb is sometimes considered a fruit because of its use as a dessert. But people eat the rhubarb leafstalk, not the seed-bearing structure. Therefore, horticulturists classify rhubarb as a vegetable.

This article discusses FRUIT (How botanists classify fruits).

How horticulturists classify fruits

Most of the fruits that are widely raised in North America were originally brought from other regions. For example, apples, cherries, and pears originated in Europe and western Asia. Apricots and peaches first came from China, and lemons and oranges from China and Southeast Asia. All these fruits are now grown in any part of the world that has a favorable climate.

Most fruit plants require considerable amounts of moisture. Dates and olives are among the few fruits that can be grown in dry regions without irrigation.

Horticulturists classify fruits into three groups, based on temperature requirements for growth: (1) temperate fruits, (2) subtropical fruits, and (3) tropical fruits.

Temperate fruits must have an annual cold season to grow properly. They are raised chiefly in the Temperate Zones, the regions between the tropics and the polar areas. Most temperate fruits are grown in Europe and North America, but Asia and Australia also have major producing areas.

The principal temperate fruits are apples, apricots, cherries, peaches, pears, and plums. In addition, most small fruits, which grow on plants smaller than trees, are raised mainly in the Temperate Zones. They include blueberries, cranberries, grapes, raspberries, and strawberries.

Subtropical fruits require warm or mild temperatures throughout the year but can survive an occasional light frost. They are grown chiefly in subtropical regions.

The most widely grown subtropical fruits are the citrus group, which includes grapefruit, lemons, limes, and oranges. Oranges, the leading citrus crop, are grown throughout the subtropics, from southern Japan to southern Europe. In the United States, Florida produces by far the most oranges. Other subtropical fruits include dates, figs, olives, and avocados.

Tropical fruits are raised mainly in the tropics and cannot stand even a light frost. Bananas and pineapples, the best-known tropical fruits, are grown throughout the tropics, and much of each crop is exported. Other tropical fruits include acerolas, cherimoyas, litchis, mangoes, mangosteens, and papayas.

Growing fruit

Almost all species of fruits grow on plants that have a woody stem. Such plants are trees, bushes, or woody vines. Fruits that grow on trees include apples, cherries, lemons, limes, oranges, and peaches. Most small fruits grow on bushes, but grapes come from woody vines. Bananas and strawberries grow on plants that have a nonwoody stem.

Fruit crops, unlike most other crops, are not grown from seeds. Plants grown from seeds may vary in many ways from generation to generation. But growers strive to produce plants that will bear fruits of uniform type, appearance, and quality. Such fruits bring the highest prices when marketed. Fruit plants produce fruits of uniform quality if grown vegetatively-that is, from certain parts of desirable plants, such as stems, buds, and roots. The part that is grown develops new tissues and new parts identical to those of the parent plant.

Fruit plants are produced vegetatively in three main ways: (1) by grafting, (2) from cuttings, and (3) from specialized plant structures. Most fruit trees are reproduced by grafting. In this process, a bud or piece of stem from one tree is joined to a rootstock from another. A rootstock is a root or a root plus its stem. The resulting tree will have most of the same characteristics as the tree from which the bud or stem was taken. However, the rootstock may determine such characteristics as the size and productivity of the new tree.

Some fruit plants are produced from cuttings or from specialized structures. Most cuttings are pieces of stem that grow roots when placed in water or moist soil. Specialized structures called runners are used to grow strawberry plants. Runners are long, slender shoots that mature strawberry plants send out along the ground. A runner placed in soil develops into a new plant.

Some fruit growers produce their own plants vegetatively. However, most growers buy their plants from nurseries that specialize in producing them.

The branch of horticulture that deals with fruit growing is called pomology. Pomologists have developed highly efficient methods of planting and caring for fruit crops, and most fruit farms use these techniques.

There are three main steps in growing fruit: (1) planting, (2) caring for the crop, and (3) harvesting.

Planting. Fruit crops are perennials, and so they do not have to be replanted annually as do most other crops. After the original planting, a fruit farmer need only replace plants that become unproductive. Many fruit plants remain productive for 30 to 50 years or even longer. In mild climates, farmers generally plant trees, bushes, and vines in fall. In cold climates, planting usually takes place in spring.

Most bushes are planted from 3 to 5 feet (0.9 to 1.5 meters) apart in rows that are 6 to 10 feet (1.8 to 3 meters) apart. Rows of grapevines are spaced about 10 feet (3 meters) apart. In the past, farmers almost always grew full-sized fruit trees. In most cases, the trees were planted from 20 to 40 feet (6 to 12 meters) apart to allow room for growth. Today, many farmers prefer to grow dwarf trees, which are planted close together. The branches of each tree may grow up a supporting framework called a trellis. The trellis enables all the fruit to receive maximum sunlight, and so the crop ripens better and faster than it otherwise would. Fruit is also easier to harvest from dwarf trees than from full-sized trees.

Caring for the crop. Most fruit growers use special machinery to fertilize, cultivate, and otherwise care for their crops. Fruit crops must be fertilized at least once a year. Some fertilizers are applied to the soil, and others are sprayed on the plants. Many fruit growers cultivate the soil around young fruit plants periodically. This practice helps control weeds and thus encourages crop growth. Most fruit crops grown in extremely dry regions must be irrigated. Farmers use various methods, such as ditches and sprinklers, to distribute irrigation water.

In many cases, the branches of a young fruit tree must be trained so that the tree develops a uniform shape and a sturdy structure. Training may involve propping the trunk or tying the branches, or it may consist entirely of pruning. Pruning strengthens a plant by ridding it of unproductive branches. Nearly all fruit plants have to be pruned at least once annually. In addition, most farmers remove some of the crop from the trees during the early stages of the fruit's growth. This practice, called thinning, helps increase the size of the remaining fruit.

The majority of fruit growers use chemical pesticides to protect their crops against diseases and insect pests. Most pesticides are sprayed or dusted on crops by tractor-driven machinery or specially equipped light airplanes or helicopters. Plant breeders have also developed varieties of fruit plants that resist certain diseases and harmful insects.

Sudden spring frosts can endanger fruit crops in temperate or subtropical regions. Farmers use water distributed by sprinklers to protect small-fruit crops from frosts. Water releases heat as it freezes. If it is sprinkled onto the crops continuously, it keeps the tender flowers and young fruits from freezing. Farmers use heaters to protect tree crops from spring frosts.

Harvesting. Most fruits ripen rapidly after reaching their mature size. Harvesting occurs during different stages of the growth process, depending on the type of fruit and its intended use. For example, gooseberries and cherries used in making artificial coloring are harvested when immature. Apples, bananas, peaches, and pears are usually harvested when mature, but before they ripen. Berries and most fruit picked at home orchards are harvested during the ripening stage. Most fruits taste best when they are allowed to ripen on the plant. Citrus fruits do not go through a distinct ripening process and may be harvested over a long period of time after they mature. Fruits are bruised more easily than most other crops, and so they must be harvested with greater care. Most are picked by hand. However, the increasing cost of hand labor has encouraged the use of fruit-harvesting machines. Some of these machines have arms that shake the fruit loose from the plants. The loosened fruit drops onto outstretched cloths. Other mechanical pickers have fingers that "comb" fruit from the plants.

Marketing fruit

The United States is the leading fruit-producing country in the world. It raises more than 10 percent of all the apples, pineapples, and plums; about 20 percent of the lemons, oranges, peaches, and strawberries; and about 45 percent of the grapefruit. California is the nation's chief fruit-growing state. Other leading states include Florida, Michigan, New York, Oregon, and Washington.

Most fruit scheduled to be sold fresh is taken from the orchard or field by truck and delivered to a packing house. Many large fruit farms have their own packing facilities. Commercial packing houses are centrally located in fruit-growing regions. Most large packing houses are fully mechanized. Machines wash the fruit, sort it according to size and quality, and pack each batch into containers. The fruit is then shipped to market or stored for future delivery. Railroads and trucks carry most overland shipments of fruit. Most overseas shipments travel by ocean freighter.

Fruits can be stored for varying lengths of time under controlled conditions. Temperate tree fruits must be stored at temperatures near freezing. Some kinds of apples can be kept fresh for about a year under such conditions. On the other hand, most small fruits remain fresh only a few days or weeks in cold storage. Tropical and subtropical fruits can be stored for a few weeks or months under temperature-controlled conditions. The temperatures, though cool, must be well above freezing. The amount of oxygen ordinarily present in the air promotes spoilage of fruit. The storage time for all fruits can be lengthened by reducing the oxygen supply.

Much fruit is shipped directly from farms to food processors. Processing plants preserve fruit by such methods as canning, drying, and freezing. See FOOD, FROZEN; FOOD PRESERVATION.



Developing new varieties of fruit

Over the centuries, fruits have been improved by constant selection of the most desirable plants. In selection, plants grown from seed are examined for various desirable qualities. Individual plants are singled out for high productivity or for the superior color, texture, or flavor of their fruits. If the desirable characteristics of the selected plant are reproduced when the plant is grown vegetatively, the selection may be classed as a new cultivated variety. Cultivated varieties are also known as cultivars.

Occasionally, an individual plant develops an unexpected characteristic due to a mutation, a random genetic change. For example, an apple tree may suddenly start to bear fruit of a more intense red color. Horticulturists refer to such a mutation as a sport. Growers have used sports to develop many improved varieties of fruit. The trees of Delicious apples originally produced pale-colored, striped fruits. Some branches on individual trees began to bear solid-red apples. By grafting these branches onto appropriate rootstocks, growers produced the attractively colored types of Delicious apples available today.

Horticultural plant breeders use a technique called crossing or hybridization to improve fruits. In this process, pollen is taken from a plant that has been selected for a particular desirable trait. The pollen is placed in the flower of a plant selected for another desirable quality. Some of the plants grown from the resulting seed may have the desirable characteristics of both parents. Occasionally, one of these plants may prove worthy of being named as a new variety. In many cases, hybridization and selection are repeated over many generations to create a new variety. Hybridization is a highly useful technique because it enables breeders to produce varieties with combinations of more and more desirable qualities. In the future, desirable characteristics may be transferred using techniques of genetic engineering that remove genes from one plant and insert them into another.

How botanists classify fruits

Fruit, the seed-bearing structure of a flowering plant, develops from the ovaries of the flowers. An ovary is a hollow structure near the base of a flower. It may hold one seed or more, depending on the species of the plant.

The wall of an ovary of mature fruit, in which the seed is fully developed, has three layers. The outer layer is called the exocarp, the middle layer is known as the mesocarp, and the inner layer is the endocarp. The three layers together are called the pericarp.

Botanists classify fruits into two main groups: (1) simple fruits and (2) compound fruits. A simple fruit develops from a single ovary, and a compound fruit develops from two or more ovaries.

Simple fruits are by far the largest group of fruits. They are divided into two types, depending on whether their pericarp is fleshy or dry.

Fleshy simple fruits include most of the seed-bearing structures that are commonly called fruits. The three main kinds are: (1) berries, (2) drupes, and (3) pomes.

Berries have an entirely fleshy pericarp. Botanists classify bananas, blueberries, grapes, green peppers, muskmelons, oranges, tomatoes, and watermelons as berries. Some berries, including watermelons and muskmelons, have a hard rind. Such fruits are called pepos. Other berries, including the citrus fruits, have a leathery rind. They are called hesperidiums. Raspberries, strawberries, and most of the other fruits commonly known as berries are actually compound fruits.

Drupes have an exocarp that forms a thin skin. The endocarp develops into a stone or pit, and only the mesocarp is fleshy. Such fruits include apricots, cherries, peaches, and plums.

Pomes are fleshy fruits with a paperlike core. Apples and pears are pomes.

Dry simple fruits include the pods of the bean plant, the milkweed, the pea plant, and the locust tree; the grains of the corn, rice, and wheat plants; and nuts. Botanists regard nuts as single-seed fruits with a hard pericarp called a shell. The seed is the edible part. Acorns, chestnuts, and hazelnuts are true nuts. But many so-called nuts are classed otherwise by botanists. For example, almonds are the seeds of drupes.

Compound fruits consist of a cluster of ripened ovaries. There are two main types of compound fruits, aggregate fruits and multiple fruits. Aggregate fruits develop from single flowers, each of which has many ovaries. Blackberries and raspberries are aggregate fruits. The strawberry is a special type of aggregate fruit. Each "seed" in a strawberry is actually a complete fruit. The flesh surrounding the seeds develops from the base of the flower rather than from the ovaries. Multiple fruits develop from a cluster of flowers on a single stem. Figs, mulberries, and pineapples are multiple fruits.

Contributor: Jules Janick, Ph.D., Professor of Horticulture and Landscape Architecture, Purdue University.

Additional resources

Heaton, Donald D.Nature's Harvest: A Produce Reference Guide to Fruits and Vegetables from Around the World. Food Products Pr., 1997.

Otto, Stella B.The Backyard Orchardist: A Complete Guide to Growing Fruit Trees in the Home Garden. Rev. ed. OttoGraphics, 1994.

Swenson, Allan A.Fruit Trees for the Home Gardener. Lyons Pr., 1994.

Vaughan, John G., and Geissler, Catherine.The New Oxford Book of Food Plants. Oxford, 1997.

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Biology is the scientific study of living things. There are more than 10 million species of living things on the earth. They range in size from microscopic bacteria to huge blue whales and towering redwood trees. Living things also differ greatly in where and how they live. However, all forms of life share certain characteristics that set them apart from nonliving things. These characteristics include the ability to reproduce, to grow, and to respond to changes in the environment.



Traditionally, biology has been divided into two major fields. Botany deals with plants, and zoology with animals. Botany and zoology are further divided into various branches and specialized areas of study. But most branches of biology-for example, anatomy (the study of the structure of living things) and genetics (the study of heredity)-apply to both plants and animals.

Biology may also be divided into ecology, physiology, and systematics. Ecology deals with the relationships among living things and between organisms and their environment. Physiology concerns life functions, such as digestion and respiration. Systematics, also called taxonomy, is the scientific classification of organisms.

Biologists often make use of the methods and findings of other sciences. For instance, they rely on physics and chemistry to help them understand the processes that occur in living plants and animals. They use statistics in studying changes in the size of an animal or plant population-that is, the number of organisms of a particular species in an area. Exobiologists work with astronomers in searching for life elsewhere in the universe.

Biological research has greatly affected people's lives. For example, farm production has soared as biologists have helped develop better varieties of plants and new agricultural techniques. Discoveries in biology have enabled physicians to prevent, treat, or cure many diseases. Research on the relationships between living things and their environment has helped in the management of wildlife and other natural resources.

This article discusses BIOLOGY (Careers in biology).



What biologists study

Biology is such a broad subject that most biologists specialize in some area of study. But in whatever area they work, all biologists are interested in both the parts of living things and how the parts work together.

Certain biologists study organisms that live in a specific environment. Marine biologists, for example, investigate life in the ocean. Some biologists concentrate on a particular type of organism. Ornithologists, for instance, study birds. Many biologists examine the parts of living things. Cytologists, for example, deal with the structure, composition, and functions of cells. Other biologists analyze life processes. Embryologists, for example, investigate the formation and development of animals and plants before they become independent organisms.

The techniques and tools that biologists use depend on what they are investigating. Many biologists conduct experiments to gain information and to develop and test theories. Their experiments may involve making a change in an organism's way of life or its environment and then observing the effects of that change. For example, a biologist may change the diet of an animal and study how the animal's growth and functioning are thereby affected. The microscope has long been one of the biologist's most useful tools. An entire branch of biology, called microbiology, is devoted to the study of organisms that can be seen only with a microscope. Other techniques and tools used by biologists range from aerial surveys of plant and animal populations to techniques that isolate the molecules of living cells.

History of biology



Beginnings. In prehistoric times, people gradually developed a great deal of practical biological knowledge. They learned to grow many kinds of plants and to tame and raise certain animals. In ancient times, people of China, India, and the Middle East accumulated further knowledge of plants and animals. For instance, they knew how to use numerous plants as medicines or poisons. The Egyptians learned some anatomy and physiology through embalming their dead.

The ancient Greeks made major advances in biology. Unlike most other people of the time, some Greek thinkers did not believe that gods or spirits caused natural events. Instead, they saw nature as operating according to laws that people could discover. About 400 B.C., a Greek physician named Hippocrates taught that diseases have only natural causes. He also emphasized the relationships among the parts of an organism and between an organism and its environment. Hippocrates is often called the father of modern medicine.

During the 300's B.C., the Greek philosopher Aristotle gathered a vast amount of information about plants and animals. He was one of the first thinkers to classify animals according to their own characteristics rather than according to their usefulness to people. Pliny the Elder, a Roman naturalist who lived during the first 100 years after Christ's birth, also collected many facts about plants and animals. He included the information in his 37-volume Natural History.

During the A.D. 100's, Galen, a Greek physician who practiced medicine in Rome, contributed greatly to advances in anatomy and physiology. He gained much of his knowledge from treating injured gladiators and dissecting apes and pigs.

The growth of biological knowledge slowed during the Middle Ages, a 1,000-year period in European history that began in the 400's. However, works by Hippocrates, Aristotle, Galen, and other ancient authorities were collected, preserved, and translated by Arab scholars in the Middle East. The Arabs also made major contributions of their own in biology. The works of the ancient Greek and Arab scientists eventually made their way to Europe. During the Middle Ages, the authority of the ancient writers was unquestioned, though their works contained many errors.

The Renaissance. From the early 1300's to about 1600, a new spirit of inquiry spread across western Europe. During this period, called the Renaissance, many anatomists and physiologists began to challenge the authority of the ancient writers. They believed that people should rely on experimentation and observation rather than accept without question the ideas of the ancients.

The emphasis on observation stimulated the development of a high degree of naturalism and accuracy in biological illustration. During the late 1400's and early 1500's, the great Italian artist Leonardo da Vinci made hundreds of drawings of the human body in which he paid careful attention to detail and proportion. Leonardo based his work on dissections of human corpses. The first scientific textbook on human anatomy was published in 1543. This work, titled On the Structure of the Human Body, was written by Andreas Vesalius, an anatomist born in what is now Belgium. Like Leonardo, Vesalius based his work on dissections he had made of human corpses. The book, richly illustrated with exceptionally lifelike drawings of human anatomy, corrected many of Galen's mistaken ideas.

One of the most important discoveries in physiology in the 1600's was made by William Harvey, an English physician. In 1628, Harvey published the results of his experiments showing how blood, pumped by the heart, circulates through the body.

Early discoveries with the microscope. The introduction of the microscope led to great discoveries in biology during the middle and late 1600's. In 1661, an Italian anatomist named Marcello Malpighi, with the aid of a microscope, became the first person to observe the movement of blood through the capillaries. In 1665, Robert Hooke, an English experimental scientist, published Micrographia, a book containing detailed drawings of many biological specimens as seen with a microscope. The book included the first drawings of cells. In the mid-1670's, Anton van Leeuwenhoek, a Dutch amateur scientist, discovered microscopic life forms, thus opening up a new world for investigation.

The origins of scientific classification. During the 1700's, Europeans came into increasing contact with distant parts of the world and thereby learned of many unfamiliar plants and animals. Naturalists realized that they needed a classification system that could include those plants and animals. In 1735, the Swedish naturalist Carolus Linnaeus (also called Karl von Linne) published a system of classification in which he grouped organisms according to structural similarities. His system forms the basis of scientific classification used today.

Classifying organisms according to structural similarities stimulated interest in comparative anatomy-the comparison of the anatomical structures of different organisms. The leading comparative anatomist of the late 1700's and early 1800's was Baron Cuvier of France. Cuvier noticed that most kinds of animals have one or another of a very few basic body types. He devised a system of classifying animals according to basic body types that is still used in modified form. Cuvier also applied the methods of comparative anatomy to another field he helped establish, paleontology-the study of prehistoric life.

The theory of evolution. Most biologists had long believed that each species of life had remained unchanged and no new species had appeared since the world began. However, biologists began to question those beliefs during the late 1700's. They noted that farmers had produced new varieties of plants and animals by selective breeding. In addition, voyages of exploration had revealed isolated groups of plants and animals that contained many species which varied only slightly from one another. Biologists wondered why there should be so many species with little variation. Such observations led many biologists to believe that species change over time and that some species had evolved (gradually developed) from others.

During the early 1800's, several biologists proposed explanations of how species evolve. The most convincing theory was eventually reached independently by two British naturalists-Charles Darwin and Alfred Russel Wallace. However, Darwin presented his ideas in a widely read book, and his work became better known.

Darwin detailed his theory of evolution in The Origin of Species (1859). According to Darwin, some organisms are born with traits that help them survive and reproduce. They pass the favorable traits on to their offspring. Other members of the same species that have unfavorable traits are less likely to survive and reproduce. The unfavorable traits eventually die out. Darwin proposed that species evolve as more and more favorable traits appear and are passed from generation to generation. He called the process natural selection.

Materialistic physiology and the cell theory. Many physiologists of the late 1700's had come to think of life as the total of the physical and chemical processes occurring in an organism. Unlike some other biologists, they did not believe that living things are guided in their functioning by any spiritual or supernatural forces. Instead, they felt that living things are nothing more than special combinations of materials and function like machines. Such views are called materialistic physiology or mechanistic materialist physiology.

Antoine Lavoisier, a French chemist, applied the techniques of chemistry to physiology in the late 1700's. He compared respiration to the burning of a candle because both processes use oxygen and produce heat and carbon dioxide. Beginning in the mid-1800's, the French physiologist Claude Bernard introduced a new approach to materialistic physiology. He saw living things as highly organized sets of control mechanisms that work to maintain the internal conditions necessary for life. He pointed out that in a mammal, for example, such mechanisms keep body temperature constant in spite of variations in the temperature outside the organism.

Paralleling developments in physiology was a growing understanding of the cell. In the late 1830's, two Germans-the botanist Matthias Schleiden and the physiologist Theodor Schwann-proposed that the cell was the basic structural and functional unit of all plants and animals. In 1858, Rudolf Virchow, another German scientist, published his theory that all diseases were diseases of the cell. In combination, these ideas are called the cell theory.

Building on materialistic physiology and the cell theory, Louis Pasteur, a French chemist, and Robert Koch, a German physician, firmly established a new theory of disease during the middle and late 1800's. Through their studies, Pasteur and Koch proved what was called the germ theory. According to the theory, many diseases are caused by microscopic organisms.

The growth of modern biology. During the late 1800's, Darwin's theory of evolution had stimulated much speculation among biologists about the origin, nature, and development of organisms. By the early 1900's, however, many biologists strongly rejected the emphasis on theory and speculation. Instead, they stressed the value of carefully controlled experiments and the application of mathematical techniques to biology. That method of investigation helped lead to an enormous expansion of biological knowledge, particularly in the understanding of the chemical and molecular basis of life.

Genetics was established as a branch of biology in the early 1900's. It developed chiefly from experiments conducted during the mid-1800's by Gregor Mendel, an Austrian monk. On the basis of his experiments, Mendel discovered that physical characteristics are produced by basic hereditary units that transmit traits from generation to generation. About 1910, Thomas Hunt Morgan, an American biologist, found that Mendel's hereditary units-later called genes-are located on structures called chromosomes within cells. Biologists at the time also noted that changes in hereditary traits correspond to visible changes in chromosome structure.

During the 1940's, geneticists found that genes guide the manufacture of the proteins by which cells regulate their chemical processes. In 1953, biologists James D. Watson of the United States and Francis H. C. Crick of the United Kingdom proposed a model of the molecular structure of deoxyribonucleic acid (DNA), the material in chromosomes that controls heredity. Knowing DNA structure enabled biologists to understand the molecular basis of such life processes as heredity and genetic change.

Breakthroughs in genetics helped alter biologists' approach to the study of evolution. By the 1960's, many biologists were studying evolution in terms of changes in the kinds and numbers of genes in a population.

The field of ecology began to develop dramatically in the early 1900's. Scientists had long recognized the importance of the relationships among organisms and between organisms and their environment. But the development of ecology as a separate branch of biology occurred after the introduction of such techniques as statistical analysis of complex systems of relationships. Since the 1960's, concern about environmental effects of pollution has greatly stimulated research in ecology.

During the 1900's, neurobiologists-people who study the nervous system-have learned much about how nerve cells and nerve networks function. Their work has led to a better understanding of how the brain and central nervous system process information.

Current research and issues. The study of the human immune system-that is, the body's defense system against disease and foreign substances-is one area at the frontier of biological research. Scientists are learning how our bodies produce a seemingly endless variety of disease-fighting proteins called antibodies. Each antibody is tailored to combat one of many foreign substances called antigens. Biologists have discovered that the body can produce a great number of different antibodies because certain genes rearrange themselves to produce antibodies that attack specific antigens. The study of the immune system has helped combat AIDS, a disease that immobilizes the immune system.

Since the 1950's, biologists have been collecting evidence for the theory that life began in a series of chemical reactions early in the earth's history. They have produced biological molecules in chemical experiments that reproduce conditions thought to have existed on earth billions of years ago. See LIFE (The origin of life).

Since the 1970's, a growing number of biologists have questioned the idea that evolutionary change occurs only as a result of a gradual process. Instead, they accept the idea that evolution may proceed at times by abrupt changes leading to the replacement of one species by another. Although there is questioning over details, most biologists believe in the general outlines of evolution theory. However, some other people reject the theory because of the many gaps in our understanding of how particular species evolved. Still others object to the idea of evolution because it conflicts with their religious beliefs about the creation of life. See EVOLUTION.

By the mid-1970's, scientists had learned how to remove genes from one species and insert them into another. The process is called genetic engineering. Genetic engineering offers many potential benefits in medicine, industry, and agriculture. For example, scientists have transferred to bacteria the human gene that produces insulin-a hormone that regulates the body's use of sugar. The bacteria then produce insulin, which can be used to treat people with diabetes. But some question the morality of interfering with the hereditary makeup of living things through genetic engineering. Genetic engineering has also caused concern that the release of genetically engineered organisms into the environment may have harmful effects. For this reason, scientists involved in genetic engineering guard against accidental release of such organisms. See GENETIC ENGINEERING.

In 1996, Scottish scientists led by biologist Ian Wilmut successfully cloned a mammal by taking an egg cell from an adult female sheep and replacing the cell's nucleus with one from another adult sheep. The sheep clone they produced was named Dolly. This marked the first time a mammal had been cloned in this way, and it sparked a debate about the potential cloning of human beings.

In 1990, geneticists from around the world launched the Human Genome Project. This project has helped obtain the sequence, or order, of DNA in the genome of human beings and other organisms. A genome consists of all the genes on the chromosomes of a cell. In the early 2000's, the Human Genome Project and Celera Genomics Corporation, a private company, completed the sequencing of essentially the entire human genome. American and British scientists used these findings to determine that a human genome has about 30,000 to 40,000 genes, far fewer than previously believed. Researchers also found that human beings share many genes with such primitive organisms as bacteria.

Careers in biology

To prepare for a career in biology, students should take such high school and college courses as chemistry, mathematics, and physics as well as biology. A bachelor's degree is sufficient for some biology careers, but many positions require a graduate degree. Some people with a bachelor's degree teach in junior high and high schools. Others work as technicians in research laboratories. Many biologists with advanced degrees teach and conduct research at universities.

Job opportunities for biologists in agricultural research and in industry are increasing, especially in the areas of genetic engineering and ecology. Such biologists may work to develop new varieties of food crops or to create organisms capable of producing drugs.

Many government agencies responsible for public health, sanitation, and water quality employ biologists. Careers in biology also include work in zoos and botanical gardens. Some specialists in ecology and wildlife management work in state and national parks. In addition, some companies and government agencies hire biologists to study the environmental effects of pollution and of proposed construction projects.

Contributor: Garland E. Allen, Ph.D., Professor of Biology, Washington University.

Questions

What is genetic engineering?

How did Antoine Lavoisier describe respiration?

What is the cell theory?

How did Galen gain much of his knowledge of anatomy?

Why did many biologists of the late 1700's come to believe that species change over time?

How do living things and nonliving things differ?

Whose book contained the first drawings of cells?

What is natural selection?

Who is often called the father of modern medicine?

What are some careers in biology?

Additional resources

Diagram Group.The Facts on File Biology Handbook. Facts on File, 2000.

Mayr, Ernst.This Is Biology. Belknap, 1997.

Mertz, Leslie A.Recent Advances and Issues in Biology. Oryx, 2000.

Miller, Louise.Careers for Nature Lovers and Other Outdoor Types. 2nd ed. VGM Career, 2001.

Serafini, Anthony.The Epic History of Biology. 1993. Reprint. Perseus Pub., 2001.

Smith, Miranda.Eyewitness Living Earth. DK Pub., 1996.

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Classification, Scientific. Scientific classification is a method scientists have developed to arrange all of the world's organisms in related groups. It is the orderly arrangement of all living things. Scientific classification indicates certain relationships among all organisms. Detailed scientific classifications also show how ancient and extinct biological groups fit into this arrangement. The classification of organisms is a science called taxonomy or systematics. Scientific classification is an interpretation of facts. It is based on the opinion and judgment a biologist forms after studying many living and preserved dead organisms. Most biologists use the same basic framework for classification. But not all biologists agree on how different groups of organisms fit into this scheme, and so classifications often differ in details.

The language of classification. Latin and Greek words are used in scientific classification, because early scholars used these languages. Every known organism belongs to a particular species. Each species has a two-part scientific name. Most of these names come from Greek or Latin words. We call this system of names the binomial system of nomenclature, or binomial nomenclature. These are Latin terms that mean two-name naming. The two names identify an organism by indicating which species it belongs to.

Organisms are known by different common names in different regions of the world. However, each organism has only one correct scientific name, and scientists anywhere in the world can recognize the organism by its scientific name. For example, the same large member of the cat family may be known in various parts of North America and South America as a puma, cougar, mountain lion, panther, or leon. The cat's scientific name is Felis concolor. Scientists can identify the animal by that name no matter what language they speak.

International commissions of scientists establish the rules for adopting scientific names. Some scientific names are descriptive. The scientific name of the spotted skunk, for example, is Spilogale putorius, which means smelly, spotted weasel. But many scientific names have no descriptive meaning.

Groups in classification. Seven chief groups make up a system in scientific classification. The groups are: (1) kingdom, (2) phylum or division, (3) class, (4) order, (5) family, (6) genus, and (7) species. The kingdom is the largest group. The species is the smallest. Every known organism has a particular place in each group.

Kingdom is the largest unit of biological classification. Until the 1960's, most biologists formally recognized only two major kingdoms-Animalia, the animal kingdom, and Plantae, the plant kingdom. But as more information about the microscopic structure and biochemistry of organisms became known, scientists realized that a two-kingdom classification system was not exact enough. Today, most biologists use a system that recognizes five kingdoms of organisms. These are Animalia, Plantae, Fungi, Protista, and Prokaryotae.

The kingdom Animalia is the largest kingdom. It has more than 1 million named species. These species include the organisms that most people easily recognize as animals, such as human beings, deer, fish, insects, and snails. The kingdom Plantae consists of more than 260,000 known species. It includes those organisms that most people easily recognize as plants, such as magnolias, sunflowers, grasses, pine trees, ferns, and mosses. The kingdom Fungi has more than 100,000 known species. These species include fungi, such as mushrooms and bread molds, as well as the lichens. The kingdom Protista has more than 100,000 known species. This kingdom includes green, golden, brown, and red algae; ciliates; sporozoans; sarcodines; and flagellates. The kingdom Prokaryotae consists of bacteria, including blue-green algae or cyanobacteria. There are more than 10,000 known species in this kingdom.

Division, or phylum, is the second largest group. The kingdoms Protista, Fungi, and Plantae are classified into divisions. In the animal kingdom, the term phylum is used instead of division. Scientists disagree on which term should be used for the kingdom Prokaryotae.

The animal kingdom may be divided into 20 or more phyla. All animals with backbones belong to the phylum Chordata. The plant kingdom has 2 divisions. All plants that have flowers belong to the division Tracheophyta.

Class members have more characteristics in common than do members of a division or phylum. For example, mammals, reptiles, and birds all belong to the phylum Chordata. But each belongs to a different class. Apes, bears, and mice are in the class Mammalia. Mammals have hair on their bodies and feed milk to their young. Reptiles, including lizards, snakes, and turtles, make up the class Reptilia. Scales cover the bodies of all reptiles, and none of them feed milk to their young. Birds make up the class Aves. Feathers grow on their bodies, and they do not feed milk to their young.

Order consists of groups that are more alike than those in a class. In the class Mammalia, all the animals produce milk for their young. Dogs, moles, raccoons, and shrews are all mammals. But dogs and raccoons eat flesh, and are grouped together in the order Carnivora, with other flesh-eating animals. Moles and shrews eat insects, and are classified in the order Insectivora, with other insect-eating animals.

Family is made up of groups that are even more alike than those in the order. Wolves and cats are both in the order Carnivora. But wolves are in the family Canidae. All members of this family have long snouts and bushy tails. Cats belong to the family Felidae. Members of this family have short snouts and short-haired tails.

Genus consists of very similar groups, but members of different groups usually cannot breed with one another. Both the coyote and the timber wolf are in the genus Canis. But coyotes and timber wolves generally do not breed with one another.

Species is the basic unit of scientific classification. Members of a species have many common characteristics, but they differ from all other forms of life in one or more ways. Members of a species can breed with one another, and the young grow up to look very much like the parents. No two species in a genus have the same scientific name. The coyote is Canis latrans, and the gray wolf is Canis lupus. Sometimes groups within a species differ enough from other groups in the species that they are called subspecies or varieties.

Development of classification. For thousands of years, people have tried to classify living things. Early human beings divided all organisms into two groups: (1) useful, and (2) harmful. As people began to recognize more kinds of living things, they developed new ways to classify them. One of the most useful was suggested by the Greek philosopher and naturalist Aristotle, who lived during the 300's B.C. Only about a thousand organisms were known in his time. He classified animals as those with red blood-animals with backbones-and those with no red blood-animals without backbones. He divided plants by size and appearance as herbs, shrubs, or trees. Aristotle's scheme served as the basis for classification for almost 2,000 years.

During the 1600's, the English biologist John Ray first suggested the idea of species in classification. But the basic design for modern classification began with the work of the Swedish naturalist Carolus Linnaeus in the 1700's. Linnaeus classified organisms according to their structure and gave distinctive two-word names to each species. Many of Linnaeus's groupings from species through orders still are accepted today. But his higher groupings often were based on superficial physical resemblances. Modern classifications are based on more microscopic structural and biochemical characteristics, as well as on presumed evolutionary relationships among the organisms. Classifications continue to change as more information becomes available.

Contributor: Theodore J. Crovello, Ph.D., Professor of Biology and Dean, Graduate Studies and Research, California State University, Los Angeles.

Additional resources

Margulis, Lynn, and Schwartz, K. V.Five Kingdoms. 3rd ed. W. H. Freeman, 1998.

Ritvo, Harriet.The Platypus and the Mermaid and Other Figments of the Classifying Imagination. Harvard Univ. Pr., 1997.

Wallace, Holly.Classification. Heinemann Lib., 2001. Younger readers.

Winston, Judith E.Describing Species: Practical Taxonomic Procedure for Biologists. Columbia Univ. Pr., 1999..

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