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.

                    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.

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.

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. 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.

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.

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.

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.

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.

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.

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.

                    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.

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.

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.

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.

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.

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.

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.

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.

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.

                    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.

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.

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.

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.

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.

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.

                    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.

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.

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.

Related Links :

How plants grow
Factors affecting plant growth
How plants reproduce
Parts of plants
L e a f
The importance of leaves
The leaf becomes fully grown
Specialized Leaves
How a leaf makes food
How to Collect Leaves
The Parts of a Flower
Variations in flower structure
The role of flowers in reproduction
Kinds of roots

Kinds of plywood

Plywood is a building material usually made of an odd number of thin layers of wood glued together. The layers, called plies or veneers, are arranged so that the grain direction (direction of the wood fibers) of each layer is at right angles to that of the layer next to it. The outside plies are called faces and backs, and the center ply or plies are called the core.

The simplest plywood is made of three plies of veneer. However, five, seven, nine, or more plies may be used. In some cases, plywood may have an even number of plies, with the grain direction of the two center plies being parallel. The term plywood is also used for door panels that have a solid lumber core up to 3 inches (7.6 centimeters) thick.

Kinds of plywood. 

Plywood is classified in two ways--by material and by use. Most softwood plywood is made of Douglas-fir or southern pine. Western hemlock, white fir, ponderosa pine, redwood, and many other trees are also used. Hardwood plywood is available in over 80 kinds of wood. They include domestic woods such as oak, red gum, poplar, birch, cherry, and walnut. Imported woods used in plywood include mahogany and other attractive tropical woods.

Interior plywood is usually made with glues that are moisture-resistant. Exterior plywood is designed to withstand severe conditions resulting from moisture and humidity. It is always made with waterproof glues.

The most commonly available types of plywood panels are 4 feet (1.2 meters) wide, 8 feet (2.4 meters) long, and from 1/4 to 3/4 inch (6 to 19 millimeters) thick. Dimensions of plywood panels usually range from 3 to 5 feet (0.9 to 1.5 meters) wide, 5 to 12 feet (1.5 to 3.7 meters) long, and 3/16 to 13/16 inches (5 to 30 millimeters) thick. Three, five, or seven plies are normally used.

Use of plywood. Plywood's main advantages over ordinary lumber are that it is lightweight and workable, yet rigid and strong. Plywood can also be cut to exact sizes and produced in large panels for ease of application, strength, and smooth surfaces. It shrinks and swells less than ordinary wood and has greater resistance to splitting at the ends. This permits carpenters to fasten plywood sheets with nails or screws close to the edges. Plywood also has little tendency to warp or twist. Decorative hardwood veneers can provide the look of expensive woods without the cost because only thin sheets are needed. Plastic or metal faces are sometimes used to provide surfaces that resist scratching. Plywood can also be made in curved shapes.

Softwood plywood is used chiefly as a structure upon which finished walls, flooring, and roofing are laid. It is also particularly suited for the forms used in shaping concrete for building, bridge, and dam foundations. Carpenters and cabinetmakers find wide use for hardwood plywood in furniture, cabinets, counters, and decorative wall paneling. Manufacturers use both hardwood and softwood in boats, recreational vehicles, office equipment, railroad cars, road signs, sporting goods, and other products.

Making plywood is done in three steps. They involve (1) the log, (2) the veneer, and (3) the lay-up.

Logs used for plywood are selected for straightness, roundness, and freedom from knots and decay. After the bark is removed and the logs cut to the desired lengths, they are often steam-heated. This softens their surfaces, and they are placed into the lathe or slicer to be converted to veneer.

Veneer is made in one of three ways. These are (1) sawing, (2) slicing, or (3) rotary cutting. Sawing is used only for fine finishing woods, such as ebony or knotty pine, which are too brittle or unsuitable for slicing. Slicing is used chiefly for fine-figured woods for furniture or wall-panel faces. Slicing is done by moving the log, called a flitch, against a heavy, stationary knife.

About nine-tenths of veneer is rotary cut with a lathe. The log is placed in a lathe and then revolved against a stationary knife extending across its length. The veneer is then unwound in a long, continuous ribbon.

The lay-up takes place after the plies are dried, trimmed, and matched. A thin layer of glue is applied to each ply. Workers then lay-up, or place, the plies with the grain in each ply opposite to that in the adjacent ply. Hydraulic presses squeeze the plies together with heat and pressure, or pressure only. Then the finished plywood is again dried, trimmed, sanded, or otherwise finished into sheets.

History. Beginning about 1500 B.C., the people of ancient Egypt made furniture that included parts made of two wood veneers glued together. In 1830, the first three-veneer plywood was created by German furniture maker Michael Thonet. The first official patent for plywood was issued to John K. Mayo of Maine in 1865.

Forest Products

Forest products have long provided people with food, shelter, clothing, and fuel. Prehistoric people ate berries and nuts that grew in forests. They built shelters from the branches of trees and wore clothing made of plant materials. About 1 1/2 million years ago, they began using wood as a fuel to make fire.

Today, wood is one of our most important raw materials. It is used in making thousands of products, from building materials, to paper, to photographic film. Despite its usefulness as a raw material, the chief use of wood throughout the world is as a fuel.

There are thousands of forest products. Most can be classified into one of five main groups: (1) wood products, (2) wood-based composite products, (3) fiber products, (4) chemical products, and (5) fuel products. Wood products are made from solid wood. Wood-based composite products contain wood and at least one other material. Manufacturers use wood fibers to produce fiber products. Chemical products are made by breaking down wood and wood fibers and chemically treating them. Such chemical products as cellophane, lacquer, and rayon are made from wood but do not feel or look like wood. Fuel products include logs, wood pellets, and charcoal. Other forest products come from the bark, fruit, gum, leaves, and sap of trees.

Wood products

Wood has many characteristics that make it an important construction material. Carpenters and woodworkers can easily shape it with tools and fasten it with nails, screws, staples, and adhesives. It is light but strong. Wood provides insulation against electric current, heat, cold, and sound. It can hold paint and other finishes, and it does not rust. Unlike metal, cement-based, or plastic construction materials, wood is a renewable resource-that is, a new supply grows after the timber has been harvested. Some of the chief wood structural materials are round timbers, lumber, and veneer products.

Round timbers include pilings, poles, and posts. Pilings driven into the ground provide foundations for buildings, wharves, and other heavy structures. Poles link overhead telephone wires and power lines. People use posts chiefly to build fences. Round timbers are simply trees that have been stripped of their branches and bark, and cut into logs. The logs are dried and treated for protection against decay and insect attack.

Lumber includes boards and larger pieces of wood that have been sawed from logs. In the United States, the construction industry uses about 50 percent of the lumber production. The rest goes to make crates, furniture, railroad ties, sporting goods, toys, and thousands of other products. 

Wood scientists classify lumber as softwood or hardwood, depending on the kind of tree. This classification does not always indicate the hardness of the wood. Various softwoods produce harder lumber than do some hardwoods. Softwood lumber comes from needleleaf trees that are also called evergreens or conifers. Builders use this type of lumber for most structural work because of its straightness and length. Softwoods include pine, larch, fir, hemlock, redwood, cypress, cedar, and Douglas-fir.

Hardwood lumber comes from trees that lose their leaves every autumn. Many hardwoods have beautiful grain patterns. For this reason, builders and furniture makers use hardwoods for cabinets, flooring, furniture, and paneling. Popular hardwoods include birch, mahogany, maple, oak, sweet gum, and walnut.

Veneer products are made of thin sheets of wood called veneers. These veneers may be cut into long strips or other shapes. Veneer products include baskets, matches, tongue depressors, and toothpicks.

Wood-based composite products

Manufacturers produce many products using wood together with at least one other material. By combining materials, they can take advantage of the best properties of each. Wood-based composite products include plywood and particleboard, which are made by combining wood with adhesive resins.

Plywood consists of a number of veneers that are glued together. The veneers are arranged so that the grain direction in each layer is at a right angle to the grain direction of the next layer. This arrangement gives plywood several advantages over lumber. For example, plywood shrinks and swells less than lumber, and it can be easily nailed near the edges without splitting. The construction and furniture industries use large amounts of plywood.

Particleboard is made from wood shavings, flakes, wafers, splinters, or sawdust. Some of these materials come from scrap left over in sawmills and paper mills. Particleboard makers mix the wood with an adhesive and press it at a high temperature and pressure to form large sheets or panels. Particleboard shrinks and swells little in length and width. It may be used as a base for flooring and furniture. One type of particleboard, called oriented strand board (OSB), has the strength of plywood and many of the same uses. To make OSB, manufacturers use waxes and resins to bond layers of wood flakes positioned with their grains running in alternating directions.

Other wood-based composite products are made by combining wood with such materials as fiberglass, metals, polyvinyl chloride, polypropylene, and portland cement. Wood-based composites commonly substitute for lumber. For example, laminated veneer lumber is made of parallel laminated sheets of veneer manufactured to standard lumber dimensions.

Fiber products

Wood is made up of many tiny fibers. Manufacturers produce paper and paperboard, hardboard, and insulation board from wood fibers. Wood fiber is also used as attic insulation, as a protective soil covering called mulch, and even as a dietary fiber in breakfast cereals.

Paper and paperboard are made from wood chips that have been reduced to a fiber pulp by chemicals, heat, or mechanical treatment. The pulp is then formed into a mat, filtered, drained, and pressed. Paper products include bags, books, cartons, packaging materials, and tissue.

Medium density fiberboard (MDF) is made from wood that has been reduced to individual fibers or fiber bundles and then been bonded with adhesive. MDF is used primarily to make tops with molded edges for tables or other furniture.

Hardboard is made by pressing wood fibers into flat sheets at a high temperature and pressure. The fibers are held together primarily by lignin, a substance that naturally occurs in and between wood fibers. Hardboard is used chiefly in furniture, siding, and paneling.

Insulation board is manufactured from wood fibers that are formed into a mat, pressed lightly, and dried. It weighs less than hardboard. Insulation board is used for acoustical tile and under siding in construction.

Chemical products

Many wood products are made from wood or bark that has been broken down into such basic chemical parts as cellulose and lignin. Cellulose is the main ingredient of wood fibers.

Cellulose products. Cellulose may be chemically treated to change its properties and to produce such compounds as cellulose acetate and cellulose nitrate. Both of these compounds are used in adhesives, lacquers, and plastics. Plastic items molded from cellulose compounds include piano keys, tool handles, and table tennis balls. Cellulose nitrate is also an ingredient in explosives. Other cellulose compounds have specialized uses in such products as paint, foods, and textiles.

Textile manufacturers process cellulose to produce rayon and acetate fibers, which are used for clothing, draperies, and upholstery. Rayon cords strengthen tires. Other materials made from cellulose include cellophane and photographic film.

Lignin products. Lignin has far fewer uses than cellulose. It is used in making printing inks, dyes, and concrete. Manufacturers use it to bind (hold together) animal food pellets and textiles. Artificial vanilla, a flavoring in many foods, is also made from lignin.

Naval stores include turpentine and rosin-materials once essential to the operation of wooden sailing ships. Almost all naval stores come from the processing of pine pulp.

Fuel products

In many developing countries, wood has long served as the primary fuel for cooking and heating. In industrialized countries, wood has been burned mainly in fireplaces and charcoal grills. After petroleum prices rose in the 1970's, wood became a popular fuel in communities near forested areas. Fuel products made from wood include split, dried logs; compressed wood pellets; charcoal; and sawmill by-products. In addition, the forest products industry burns the thick liquid that results from pulping wood.

Other forest products

Although most forest products are made from wood, some come from the bark, fruit and seeds, gum, leaves, and sap of trees. By-products from sawmills include wood chips, shavings, and sawdust. These by-products may be used in making particleboard and other products, in bedding for animals, and in floor-sweeping compounds.

The bark from the cork oak tree provides cork for such products as bottle stoppers, bulletin boards, and insulation. The bark of the hemlock and other trees furnishes tannic acid used in processing animal hides. Bark is sometimes used as fuel, ground cover, or mulch.

Fruit and seeds harvested from forest trees include many kinds of nuts. The seedpods of the kapok, or silk-cotton, tree provide kapok fibers. Kapok is widely used as a filler in jackets and sleeping bags. Latex is a milky substance produced by plants and trees of the sapodilla family. Latex is the source of natural rubber, which is used to make balloons, hoses, tires, and other items.

The leaves of some forest trees furnish ornamental greenery for Christmas wreaths and similar products. Certain evergreen and eucalyptus leaves are distilled to produce oil used in perfumes, household cleaners, soaps, and certain drugs. Sap from certain kinds of maple trees is made into maple syrup and maple sugar.

The forest products industry

The manufacture of forest products is a major industry in many industrialized countries. The United States, China, and India are the world's leading producers of forest products.

In the United States, the forest products industry employs more than 1 1/2 million people and produces more than $300 billion worth of goods annually. The industry has more than 50,000 manufacturing plants. United States forest products companies own about 70 million acres (28 million hectares) of commercially valuable forestland. They harvest timber in state and national forests under government contracts. They also buy logs from the owners of small wooded areas.

In China, economic reforms that began in 1980 have led to a greater demand for private housing. This demand has, in turn, brought a huge increase in the production of forest products for use as construction materials. In India, millions of people depend on gathering and selling forest products for cash.

Canada's forest products industry is a leading source of export income. More than 350,000 Canadians work for companies that make forest products. Each year, these firms produce goods worth more than $50 billion in U.S. dollars. Canada is the world's leading producer of newsprint, the paper on which newspapers are printed. It produces more than a fourth of the world's total supply each year. Next >>>

Contributor: Jim L. Bowyer, Ph.D., Director, Forest Products Management Development Institute, University of Minnesota.

Source : World Book 2005

Where Animals Live

Animals live in many kinds of places. The place where an animal lives is called its habitat. Each type of habitat presents a special challenge to animals. For example, animals that live in polar regions must withstand bitter cold. Those that inhabit the tropics face extreme heat. In spite of these challenges, animals can be found everywhere on Earth. They live on the highest mountains and in the deepest oceans. They roam the driest deserts and the wettest rain forests. They swim in fresh water and salt water.

Each habitat supports many kinds of animals. In most cases, the animals are the same kinds that have lived in those surroundings for thousands of years. As a result, the animals have developed bodies and ways of life that suit them to that particular habitat. No single species of animal can survive everywhere. For example, tropical fish from the Amazon River thrive in warm water but cannot withstand the cold streams of the Andes Mountains. On the other hand, many kinds of fish that live in the Arctic Ocean would die if they were exposed to the warm waters of the Caribbean Sea. However, some animals may travel between habitats from time to time. For example, African elephants eat both grass and tree parts and so move between grassland and forest. But these animals would not be able to withstand the freezing temperatures of the polar regions.

Some habitats, including many forests and grasslands, are being destroyed by human beings. The destruction of these habitats usually causes the death of many animals. When people convert grassland to farmland, for example, they destroy the homes and source of food of many species. Without these necessities, some animals will die immediately. Others may try moving to another grassland. But the new area may not have enough food and shelter to support the additional wildlife. As a result, many more animals will die.

Some of the major animals, grouped according to seven types of habitats: (1) mountains, (2) grasslands, (3) temperate forests, (4) tropical forests, (5) deserts, (6) polar regions, and (7) oceans.
Animals of the mountains

Mountains support a variety of animal life. The numbers and kinds of animals found on mountains vary with altitude. More animals and more kinds of animals live at lower altitudes than at higher ones, largely because of the differences in climate between elevations. Generally, mountain climates become colder, wetter, and windier with increasing altitude. The air also gets thinner and has less oxygen. In addition, fewer plants are found at higher elevations, and therefore less food is available for animals.


Bears, deer, elk, and mink make their homes on the forested lower slopes and in the wooded or grassy valleys of mountains. Rainbow trout and graylings swim in mountain streams. Many mountains have meadows of grasses and herbs. These meadows are home to chinchillas, ibexes, llamas, vicunas, and yaks. Butterflies, grasshoppers, and spiders also live there.

Above the timber line-that is, the line beyond which trees will not grow because of the cold-stand rocky cliffs and peaks dotted with shrubs, mosses, and other plants. Small meadows are also found there. Sure-footed bighorn sheep and mountain goats dwell among the windswept rocks, as do furry marmots and pikas. High on the snow-capped peaks, only a few insects, spiders, and ice worms can survive. Golden eagles and some other birds fly above the mountains. A large African vulture, Ruppell's griffon, has been known to soar as high as 36,600 feet (11,150 meters).

Animals of the grasslands

Grasslands include the prairies of North America, the pampas of South America, the plains of Europe, and the steppes of Asia. The savannas of east Africa have more grassland animals than any other area.

Rainfall in grasslands is seasonal, and animals sometimes travel great distances to find green grass. Gazelles, gnus, and zebras migrate by the thousands through the African savannas. Smaller groups of elephants and rhinoceroses also feed on the grasses there. Such meat-eating mammals as cheetahs, hyenas, and lions roam the savannas in search of prey. The savannas are also home to giraffes, jackals, ostriches, secretary birds, and termites. In addition, hippopotamuses live in and near bodies of water in African grasslands. Animals of other grasslands include the kangaroos and wombats of Australia, the cavies and rheas of South America, and the coyotes and prairie dogs of North America.


Many animals of the grasslands have become endangered due to loss of their habitat and to overhunting. The rich soils of grasslands are ideal for farming, and people have converted many such areas to farmland. Many of the large grassland animals are favorite big game for hunters. For example, the once-plentiful pampas deer of South America have become extremely rare. As the pampas are converted to farmland, the tall grass that grows there disappears. Without this grass, the pampas deer have no shelter and become easy prey. Bison once grazed in huge herds in the Great Plains of North America. But so many of these animals were killed by hunters or died as their grassland habitat was converted to farmland that they were nearly wiped out.

Animals of the temperate forests

Temperate forests consist largely of deciduous trees and evergreen trees. Deciduous trees shed their leaves in the fall and grow new ones in the spring. Evergreen trees have leaves that live two or more years. Some evergreens have needle-shaped leaves. Most temperate forests are in Asia, Europe, and North America. Australia also has some temperate forests. 


Many forest animals have small bodies that allow them to move easily through the underbrush. Forest mammals include chipmunks, mice, opossums, porcupines, raccoons, skunks, and squirrels. Bears, deer, and wild boars also live in temperate forests. Bobcats and wolves were once common in woodland areas. However, so many of these predators have been hunted and trapped through the years that they have become rare.

Salamanders are often plentiful in temperate forests. They hide in the leaf litter or under rocks, where they feed on insects and other small organisms. In wet forests, slugs and other snails are common. Beavers, fish, frogs, muskrats, otters, salamanders, and turtles live in or near woodland streams, ponds, and lakes. Great numbers of birds nest in the trees and shrubs.

Many temperate forests have been cleared for farms and cities, and many others have been cut down for fuel and lumber. This deforestation (destruction of forests) places woodland animals in danger. Extensive logging in the Pacific Northwest of the United States, for example, has destroyed much of the habitat of the spotted owl, threatening the existence of that species.

Animals of the tropical forests

Tropical forests stay warm all year and receive plentiful rainfall. These forests are found in Africa, Asia, Australia, Central and South America, and the Pacific Islands. More kinds of animals live in tropical forests than in any other habitat. Scientists estimate that perhaps as many as 30 million species of tropical animals have not even been discovered yet.

Insects make up the largest single group of animals that live in tropical forests. They include brightly colored butterflies, huge colonies of ants, mosquitoes, and camouflaged stick insects. Spiders are also plentiful.

Many tropical birds, such as quetzals and parrots, are spectacularly colored. The broad leaves of trees in tropical forests form a thick overhead covering called a canopy that blocks nearly all sunlight from reaching the forest floor. 

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Many kinds of animals live in the canopies of tropical forests. They include harpy eagles and toucans; tree frogs; flying dragons; spider monkeys and howlers; gibbons and orangutans; sloths; slow lorises; tree boa constrictors; bats; and wasps, beetles, and leaf-cutting ants.

Animal live throughout the world

Animals come in many shapes and sizes.

They live throughout the world. Animals walk or crawl on land and dig through the soil. They swim in the water and fly through the air. They even live inside the bodies of other animals. Bats, dogs, horses, kangaroos, and moles are all animals. So are butterflies, frogs, jellyfish, pigeons, sharks, snakes, and worms.

Most kinds of animals are less than 1 inch (2.5 centimeters) long. Many are so tiny that they can be seen only with a microscope. The largest animal is the blue whale. It is about as long as five elephants in a row. Animals are not the only kind of living things. Scientists divide living things into five main kingdoms (groups)-animals, plants, fungi, protists, and prokaryotes. Fungi include molds, mushrooms, and yeasts. Protists, such as amebas, cannot be seen without a microscope. Prokaryotes, which include bacteria and certain algae, are some of the smallest, simplest forms of life.

Animals are different from other living things in many ways. For example, the bodies of animals are made up of many cells. But the bodies of prokaryotes and most protists have only one cell. Like animals, plants and fungi also are made up of many cells. However, animals can move around. Most plants and fungi are held to one place in the soil by roots or rootlike structures. For a more complete discussion of the differences between the members of the five kingdoms.

No one knows exactly how many species (kinds) of animals there are. So far, scientists have classified (grouped) and named more than 1 1/2 million kinds of animals. Over half of these are types of insects. Many new species are discovered each year. Scientists believe there may be from 2 million to as many as 50 million kinds of animals alive today. Many other kinds of animals used to live on Earth but have died out. They include dinosaurs and dodos.

Importance of animals

Animals and the web of life. Living things depend on one another. They are connected in what is sometimes called the web of life. Plants capture the energy from sunlight and use it to make roots, stems, leaves, flowers, and fruits. Animals eat the plants, or they eat other animals that feed on the plants.


When animals die, their bodies decay and release materials that help fertilize the soil for plants. Animals and plants are also connected in other ways. When animals breathe, they take in oxygen from the air and give off carbon dioxide. Green plants take in carbon dioxide and give off oxygen in a food-making process called photosynthesis. Many plants with flowers need insects or birds to carry their pollen from plant to plant. Without this transfer of pollen, these plants are not able to reproduce (create new individuals of their own kind). Some seeds are prickly and cling to the fur or feathers of animals.

When the animals move from place to place, they take the seeds with them. In this way, the seeds get dropped in new areas where they can grow into plants. The web of life relies on balance among its parts. A change in one part may mean disaster for others. For example, if all the trees in an area are cut down, then many animals that depend on them will die. For more information on how living things are linked.

Animals and people.

Animals have provided people with food and clothing since prehistoric times. Without animals, people would not have such things as meat, honey, eggs, wool, leather, or silk. At least 10,000 years ago, people began domesticating (taming) animals. Some of these animals provide food and clothing. For example, cattle supply meat, milk, and leather. Chickens lay eggs. Sheep provide wool and meat. Some domesticated animals help people work. Water buffaloes pull plows in Asian rice fields. Horses and camels carry people from one place to another. At first, people kept cats in their houses to catch rats and mice. They raised dogs to help them hunt.


Today, cats and dogs are kept largely as pets. Certain insects are useful to people. Bees make honey, which people harvest for food. Bees also pollinate many food crops, including fruits and vegetables. Silk comes from fiber made by silkworms. Some animals harm people. On rare occasions, crocodiles, lions, and tigers attack and kill people. So do grizzly bears and polar bears. Sharks sometimes kill and eat human beings. Bites from such poisonous snakes as rattlesnakes and cobras can cause death. The black widow spider has a poison that makes people extremely sick. Some animals pass diseases along from person to person. Certain mosquitoes transmit malaria and yellow fever. Some ticks carry the bacteria that cause Lyme disease and Rocky Mountain spotted fever. Some animals cause disease themselves. Worms called flukes, which live in human organs, can cause schistosomiasis. This disease infects millions of people in many African, Asian, and Latin-American countries.

Kinds of Animals

People often divide animals into various groups based on certain similarities the animals share.

For example, some animals can be kept as pets, but others are wild. Arranging animals according to their similarities is a handy way of remembering and understanding them.

Some common ways of grouping animals. Animals can be grouped in many ways. They can be arranged according to whether they live on land or in water.

Animals that live on land are known as terrestrial animals. They include cats, dogs, lizards, mice, and worms. Animals that live in water are called aquatic animals. They include eels, fish, lobsters, octopuses, and whales.

animal dog

Animals can be arranged by the number of legs they have. Dogs, frogs, and lizards have four legs. Bats and birds have two legs. Insects have six legs, and spiders have eight. Snakes and worms have no legs.

Another way to group animals is according to how they move. Bats, most birds, and many insects fly. Whales, fish, and squid swim. Snakes and worms crawl. Antelope and cheetahs run. Frogs, kangaroos, and rabbits hop. Some animals are cold-blooded, and others are warm-blooded.

The bodies of cold-blooded animals are warm when their surroundings are warm and cool when their surroundings are cool. Warm-blooded animals, on the other hand, almost always have the same body temperature, regardless of the warmth of their surroundings.

Birds, mammals (animals whose babies drink the mother's milk), and a few species of fish and insects are warm-blooded. All other kinds of animals are cold-blooded. Animals are also commonly divided into groups according to whether they have backbones. Invertebrates do not have backbones, but vertebrates do.


The vast majority of animals are invertebrates. They include clams, insects, jellyfish, sea urchins, snails, spiders, sponges, and worms. Birds, fish, mammals, and reptiles are vertebrates. So are amphibians-frogs, salamanders, and other animals that spend part of their lives in water and part on land. The scientific classification of animals involves grouping animals according to the biological relationships among them. This orderly arrangement of animals depends in part on the features the animals share.

In general, the more features they share, the more closely they are related. However, the scientific classification of animals is based mainly on the belief that certain animals share a common ancestor. Animals with a more recent common ancestor are more closely related than those who share an ancestor further back in time. In a somewhat similar way, brothers and sisters are more closely related than are cousins. Brothers and sisters share parents.

First cousins share grandparents. In classifying animals, zoologists (scientists who study animals) divide them into ever-smaller groups that have more and more features in common. The largest group is the kingdom Animalia itself, which includes all animals. Next, each animal is placed in a group called a phylum. Each phylum is divided into groups called classes. The classes are broken down into orders, and the orders into families.

The families are split into genera, and the genera into species. The singular form of genera is genus, but the word species may be either singular or plural. Among the animals that scientists have classified are about 13,000 species of flatworms; 50,000 species of clams, oysters, and other mollusks (soft-bodied animals, most of which have a hard shell); 1,000,000 species of insects; 30,000 species of spiders; 21,000 species of fish; 4,000 species of amphibians; 6,500 species of reptiles; 9,700 species of birds; and 4,500 species of mammals.

Each species belongs to one phylum, one class, one order, one family, and one genus. For example, tigers belong to the kingdom Animalia, the phylum Chordata, the class Mammalia, the order Carnivora, the family Felidae, and the genus Panthera. They are members of the species Panthera tigris. Lions are related to tigers. They belong to the same kingdom (Animalia), phylum (Chordata), class (Mammalia), order (Carnivora), family (Felidae), and genus (Panthera) as tigers. But lions are classified in a different species-Panthera leo, also written simply as P. leo.  Next >>>
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