Plant is any member of a diverse kingdom of living things that generally have more than one cell, live in one place without moving around, and make their own food using sunlight. Plants grow in almost every part of the world. They grow in parks and yards, in forests, on plains, on mountains, in wetlands, in the oceans, and in deserts. Most of us see flowers, grasses, and trees nearly every day.
Without plants, people could not survive. The oxygen we breathe comes mostly from plants. We get nearly all our food from plants or from animals that eat plants. We build houses and make many useful products from lumber. The fibers of bamboo, cotton, and flax plants supply clothing. Plants also serve as sources of medicine.
Scientists have identified hundreds of thousands of species (kinds) of plants. But no one knows the total number of plant species in the world. Some tiny plants that grow on the forest floor can barely be seen. Others tower over people and animals. The sequoia trees of California rank among the largest living things on Earth. Some stand over 290 feet (88 meters) tall and have trunks over 30 feet (9 meters) in diameter. Some plants have extremely long lifespans. A bristlecone pine tree in California, for example, has been growing since 4,000 to 5,000 years ago.
Plants have characteristics that set them apart from other living things. For example, nearly all kinds of plants stay in one place, from the moment they put down roots until they die. Most plants make their own food from air, sunlight, and water by a process called photosynthesis. Like animals, plants are complex living things made up of many types of cells. But plant cells have thick walls made of a material called cellulose. Animal cells do not have walls.
The importance of plants
Plants supply people with food, clothing, and shelter. Some of our most useful medicines come from plants. Plants also 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 unwanted in fields and gardens as weeds, competing with more desirable plants for resources. Tiny grains 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.
As food.
Plants are probably most important to people as a source of food. Sometimes we eat plant parts directly, as when we eat apples, peas, or potatoes. But even when we eat meat or drink milk, we are consuming foods from animals that eat plants.
People get food from many kinds of plants. But 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, pasta, and many other products made from these grains. In addition, almost all our meat except seafood comes from animals that eat grain.
Different parts of plants serve as food. 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; the leaf stalks of celery plants; and the flower buds of broccoli and cauliflower plants. The fruits of many plants also provide 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.
As raw materials.
Plants supply people with 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, maple syrup, natural rubber, 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. Fossil fuels, such as coal and natural gas, can 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. Mud and sand covered them. The increasing pressure of this mass of materials helped turn the dead plants into coal. People can also use plants to make biofuel. Biofuel is any fuel made from biological materials. For example, corn and sugar cane can be processed into ethanol. Soybeans and oil palms are used to make biodiesel. These fuels can replace fossil fuels in transportation and industry.
As medicines.
A variety of useful drugs come from plants. Some of these plants have been used as medicines for hundreds of years. For example, more than 400 years ago, 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. The roots of the Mexican yam are used in producing cortisone, a drug useful in treating arthritis and a number of other diseases.
In ecosystems.
Plants are a vital part of ecosystems. An ecosystem consists of all the living and nonliving things in an area and the relationships among them. Plants use the energy in 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 carbon dioxide with water to make food, using the energy in sunlight. After plants and animals die, they begin to decay. The rotting process returns nutrients (nourishing substances) to the soil, where living plants can use them. For more information on the importance of plants in nature, see the World Book articles Balance of nature and Ecology .
Kinds of plants
The study of plants is called botany, and scientists who study plants are known as botanists. Botanists classify plants according to common descent—that is, by dividing them into groups that share a common ancestor. The result somewhat resembles a family tree. Closely related plants share many similarities. Distant relatives share fewer similarities. Some definitions of plants include green algae or even other organisms, but this article considers only land plants and their aquatic (water-dwelling) descendants.
One major group consists of nonvascular plants. These plants lack vascular tissue—that is, tissue that carries water and food throughout the plant. The other major group consists of vascular plants, which contain these tissues. Within these groups are many smaller groupings. A table showing a more detailed system of plant classification appears with this article.
Nonvascular plants
consist of hornworts, liverworts, and mosses. Nonvascular plants are traditionally called bryophytes << BRY uh fyts >> , though many scientists consider mosses to be the only true bryophytes. Nonvascular plants make up only a small fraction of the total number of plants. They live in almost all parts of the world, from the Arctic to tropical forests. They typically grow in such moist, shady places as forests and ravines.
Most hornworts, liverworts, and mosses measure less than 8 inches (20 centimeters) tall. None of these plants has true roots. Instead, they have hairy rootlike growths called rhizoids that anchor them to the soil and absorb water and minerals. See Hornwort ; Liverwort ; Moss.
Vascular plants
have tissue that carries water and food throughout the plant. The vast majority of plants are vascular plants. There are four main groups of vascular plants: (1) lycophytes << LY kuh fyts >> , (2) ferns and related plants, (3) gymnosperms << JIHM nuh spurmz >> , and (4) angiosperms << AN jee uh spermz >> .
Lycophytes
include club mosses, quillworts, and selaginellas << suh LAHJ ih `nehl` uhz >> . These plants have leaves with a single, central vein. Lycophytes were among the first vascular plants. Although modern lycophytes are small, some prehistoric lycophytes grew to the size of trees. Plant matter from lycophytes and ferns later formed much of the world’s coal.
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 (mild) 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 grasslike leaves that 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.
Selaginellas 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.
Ferns and related plants
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.
Most fern leaves, called fronds, consist of many tiny leaflets and may grow large. On most types of ferns, the fronds are the only parts that grow above the ground. They sprout 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. See Fern .
Whisk ferns and horsetails have mostly hollow, jointed stems. Whisk ferns have many slender, highly branched stems. They grow mostly in tropical and subtropical regions. Horsetails grow about 2 to 3 feet (60 to 90 centimeters) tall. The plants have green stems and tiny, dark 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. See Horsetail.
Gymnosperms
include a wide variety of trees and shrubs that produce naked (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 true flowers. This group is made up of such plants as conifers, cycads << SY kadz >> , gingkoes, and gnetophytes << NEE tuh fyts >> . 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. See Conifer.
Cycads and ginkgoes flourished for much of the time that dinosaurs lived. 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. Many cycads are in danger of becoming extinct. 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 have many features that resemble those of flowering plants. For example, a group of tropical trees and vines called 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.
Angiosperms
are flowering plants. They produce seeds that are enclosed in a protective seedcase. The word angiosperm comes from two Greek words meaning vessel and seed. All plants that produce flowers and fruits are angiosperms. They are by far the largest group of plants, greatly outnumbering all other plants put together. They include most of our common plants, such as brightly colored garden plants, the many kinds of wildflowers, and most herbs, shrubs, and trees. Most of the plants that produce the fruits, grains, and vegetables that people eat are angiosperms. See Angiosperm.
Angiosperms vary greatly in size. The smallest flowering plants, the duckweeds, are only about 1/50 inch (0.5 millimeter) long. The largest angiosperms are eucalyptus trees. They grow over 300 feet (90 meters) tall.
Botanists divide angiosperms into several groups. These include the monocotyledons << MAHN uh `kaht` uh `lee` duhnz >> , also called monocots, and the eudicotyledons << YOO duh `kaht` uh `lee` duhnz >> , also called eudicots. Monocots grow from seeds with one seed leaf, called a cotyledon (see Cotyledon). All other angiosperms, including the eudicots, have seeds with two cotyledons.
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 grow in Antarctic areas, for example, where the temperature seldom rises above 32 °F (0 °C). Many desert plants flourish in areas where the temperature may rise well above 100 °F (38 °C).
Each kind of plant needs a particular kind of environment to thrive. For example, cattails live only in such damp places as marshes and swamps. Cactuses, on the other hand, grow chiefly in deserts. The type of environment a plant lives in is called its habitat.
Many elements make up a plant’s environment. One of the most important is climate—the typical temperature, amount of sunlight, and precipitation in an area. The environment of a plant also includes the soil and the plants, animals, and other organisms that live in the same area. All these elements are important parts of the plant’s ecosystem.
Botanists divide the world into biomes—natural communities of plants, animals, and other organisms. Important land biomes include (1) the tundra, (2) the taiga << TY guh >> , also known as boreal << BAWR ee uhl >> forests, (3) temperate coniferous << kuh NIHF uhr uhs >> forests, (4) temperate deciduous << dih SIHJ yoo uhs >> forests, (5) chaparrals << `chap` uh RALZ >> , (6) deserts, (7) grasslands, (8) savannas, (9) tropical rain forests, and (10) tropical dry forests. In addition, many plants live in aquatic regions.
Human beings have greatly affected biomes. In North America, for example, great forests once extended from the Atlantic Ocean to the Mississippi River. Advancing settlers cleared most of the trees, and cities and farms replaced the forests. In other parts of the world, irrigation and the use of fertilizers have enabled plants to grow on once-barren land. This section describes the natural plant life in the important land biomes and in aquatic regions.
The tundra
is a cold, treeless area that surrounds the Arctic Ocean, near the North Pole. It extends across the uppermost parts of Asia, Europe, and North America. The land in these regions remains frozen most of the year. 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 ground 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.
Boreal forests,
also called taiga, grow in regions that have a short summer and a long, cold winter. The growing season may last less than three months. Boreal forests cover many northern parts of Asia, Europe, and North America. They also grow on the high mountains of these continents. Trees 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 in boreal forests is often sandy and unable to support many types of small plants.
Temperate coniferous forests
grow in western North America and in part of South America. They stand along coasts or on mountains. They grow in areas that have mild, wet winters and warm, dry summers. The redwood forests of northern California and the kauri forests of New Zealand are both examples of temperate coniferous forests. Major trees of temperate coniferous forests include cedars, Douglas-firs, giant sequoias, hemlocks, pines, and redwoods. 
Temperate deciduous forests
cover large areas of North America, central Europe, and eastern Asia. In the United States, temperate deciduous forests grow mostly east of the Mississippi River and extend northward into Canada. Most of these areas have cold winters and warm, wet summers. 
Most of the trees in temperate deciduous forests are broadleaf trees. Such trees have broad, flat leaves. They also are deciduous—that is, they lose their leaves every fall and grow new ones in the spring. Trees in temperate deciduous forests include ashes, basswoods, beeches, birches, hickories, maples, oaks, poplars, yellow-poplars, and walnuts. A thick growth of wildflowers, seedlings, and shrubs typically covers the forest floor.
Chaparrals
consist of thick growths of shrubs and small trees. Cork and scrub oaks, manzanitas, and many unusual herbs grow on chaparrals. Chaparrals occur in areas with hot, dry summers and cool, wet winters. Such areas exist in western North America, southern Europe, the Middle East, and northern Africa. They also occur in southern parts of Africa and Australia. 
During the dry summer season, fires often break out on chaparrals. But these fires actually help to maintain plant life. Many of the plants that grow on chaparrals are either resistant to fire or 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 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.
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 deserts 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 Sonoran Desert 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 despite these harsh conditions, many plants live in desert regions. These plants—sometimes called xerophytes << ZIHR uh fyts >> —include acacias, cactuses, creosote bushes, Joshua trees, sagebrush, and yuccas. Wildflowers also bloom in the desert. See Flower (Flowers of the desert).
Desert plants do not grow close together. By spreading out, each plant can collect water and minerals from a large area. The roots of most desert plants extend over large areas to capture as much rain water as possible. Cactuses and other succulent (fleshy) plants store water in their thick leaves and stems. See Cactus; Desert.
Grasslands
are open areas where grasses are the most plentiful plants. 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, people use almost all prairie land to raise food crops and livestock. Farmers and ranchers grow such grains as barley, corn, oats, and wheat where bluestem, buffalo, and grama grasses once covered the land. See Grassland.
Savannas
are grasslands with widely spaced trees. Some savannas exist in regions that receive little rain. Others are in tropical regions, such as the Llanos 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.
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 cinnamon, mahogany, teak, and cacao, from which chocolate is made. The trees grow so close together that little sunlight can reach the ground. As a result, only ferns and other plants that require little light can grow on the forest floor. Many plants, including orchids and vines, grow high on the trees.
The topsoil in tropical rain forests is of poor quality because heavy rainfall can wash away nutrients. But the hot, humid conditions and the activities of such organisms as fungi and termites rapidly break down fallen leaves and other plant matter. Breaking down this material releases nutrients back into the soil. Living plants then quickly absorb the nutrients, enabling lush growth. See Rain forest.
Tropical dry forests,
also called tropical seasonal forests or seasonally dry tropical forests, grow in warm areas that have wet and dry seasons. They are found in tropical to subtropical areas in parts of Florida, Mexico, Central America, South America, Africa, India, Southeast Asia, and Australia. They also grow on islands in the Caribbean Sea and the Indian and Pacific oceans. Average yearly rainfall can be as high as in some tropical rain forests. But most of the rain falls in the wet season. Little rain falls in the dry season, which can last for months.
Most trees in tropical dry forests are broadleaf trees. Despite the warm temperatures, the stress of the long dry season causes nonevergreen trees to lose their leaves, much as do temperate deciduous forests in the winter. The fallen leaves return nutrients to the soil. The bareness of the trees in the dry season allows increased light to pass, leading to the growth of dense underbrush. Many kinds of trees grow in tropical dry forests, including acacias, baobabs, kapoks, and kola trees.
Aquatic regions
are bodies of fresh or salt water. Freshwater areas include lakes, rivers, and swamps. Coastal marshes and oceans are saltwater regions. Most aquatic plants, which also are 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 underwater. Other aquatic plants, such as duckweeds, float 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 become flooded, leaving the plants that live in these areas completely covered by water. As a result, only a limited number of plant species survive. Common freshwater plants include cattails, duckweeds, pondweeds, sedges, and water lilies. Such trees as baldcypresses, blackgums, and willows also grow in fresh water. Saltwater plants include cordgrass, eelgrass, and sedges. See Water plant.
Mangrove forests are dense growths of mangrove trees. They grow along ocean coastlines throughout the tropics and subtropics. Mangrove plants have adaptations to living in salty, waterlogged conditions. These adaptations can include stilt roots that prop the plants above the water, pores in the bark that enable the plants to breathe, leaves that can remove excess salt, and seeds that float. Mangrove forests are important because they stabilize coastlines. Their complex structures also provide places for living things to hide from predators (hunting animals). Mangrove forests provide habitat for many animals. These animals include crabs, fish, turtles, shrimp, snails, cats such as panthers and tigers, and shore birds.
Shallow ocean waters may support underwater meadows of sea grass. Sea grasses are flowering plants distinct from seaweeds, which are actually types of algae. Sea grasses live around the world in shallow coastal waters, where there is enough light for photosynthesis. Sea grass beds provide food and shelter for many organisms, including algae, birds, crabs, fish, manatees, mollusks, sea urchins, turtles, and worms. Mangrove forests and sea grass meadows are among the most threatened of all ecosystems.
Parts of plants
Plants—like all living things—are made up of cells. In plants, there are many kinds of cells that have special jobs. 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 make up a tissue. Plants include many types of complex tissues. All plants, except nonvascular plants—that is, hornworts, liverworts, and mosses—have tissue specialized to carry water, minerals, and other nutrients throughout the body. This tissue is called vascular tissue. It consists of two specialized tissues called xylem << ZY luhm >> and phloem << FLOH uhm >> . Xylem carries water and minerals from the roots of the plant to the leaves. Phloem carries food made through photosynthesis in the leaves to the other parts of the plant.
A plant consists 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 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. Plants with food-storing 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, called the taproot, that is larger than the rest. Carrots and radishes have taproots. Taproots grow straight down, some as deep as 15 feet (5 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 thimble-shaped root cap. The root cap protects the root’s delicate tip as it pushes through the soil. Threadlike root hairs grow farther back on the root. Few of these structures reach 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 nearly all plants have a special relationship with fungi. In this relationship, known as mycorrhiza << `my` kuh RY zuh >> , fungi cover or penetrate the growing tips of a plant’s roots. Water and nutrients enter the roots through the fungi. The presence of fungi extends the root system and improves its ability to absorb water and minerals. See Root.
Stems
differ greatly among various plant 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 parts of plants. The stems hold the leaves up 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 stems. Aerial stems are either woody or herbaceous (nonwoody). Plants with woody stems include trees and shrubs. These plants are rigid because they contain large amounts of woody xylem. Most herbaceous stems are soft and green because they contain only small amounts of xylem.
In nearly all plants, a stem grows longer from the end, called the apex. The cells that form this growth area are together known as the apical meristem. An apical meristem grows the plant by producing 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 forms 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 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 photosynthesis. In this process, a green pigment called chlorophyll in the leaves absorbs light energy from the sun. The plant’s cells use this energy to combine water and carbon dioxide, making sugar. From sugar, plants can make starch, fat, protein, vitamins, and other complex compounds essential for life. The plant uses the food made by photosynthesis for growth and repair or stores it in the stems or roots. 
Leaves differ greatly in size and shape. Some leaves are 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, also called margins, of these leaves may be smooth, toothed, or wavy. Grasses 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 particular 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. Most leaves develop two main parts—the blade and the petiole << PEHT ee ohl >> . 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 a thin leafstalk that grows between the base of the blade and the stem. It carries water and food to and from the blades. The leaves of some plants also have a pair of parts called stipules << STIHP yoolz >> . The stipules are two 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 << STOH muh tuh >> . Carbon dioxide, oxygen, water vapor, and other gases pass in and out 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 daisies and dandelions, have many tiny flowers that form a single, flowerlike head. Loading the player...
Flower blooming
Most flowers have four main parts: (1) the calyx << KAY lihks >> , (2) the corolla, (3) the stamens, and (4) the carpels.
The calyx consists of small, usually green, leaflike structures called sepals. The sepals protect the flower bud. Inside the calyx are the petals. The petals are the largest, most colorful part of most flowers. All the petals of a flower make up the corolla. The flower’s reproductive organs—the stamens and carpels—are attached to the stem inside the sepals and the petals. In many flowers, the stamens and petals are fused (joined). 
The stamen is a male reproductive organ, and the carpel is a female reproductive organ. Each stamen has an enlarged part called an anther on the end of a long, narrow stalk called the filament. Pollen grains, which develop sperm (male sex cells), are produced in the anther.
Carpels of most flowers have three main parts: (1) a 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 after sperm cells fertilize the egg cells. The term pistil is sometimes used to describe the female reproductive organ. It can refer to a single carpel, many unfused carpels, or many carpels fused together. In this article, the section The reproduction of plants tells how sperm cells unite with 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 (20 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 are only 1/16 inch (1.6 millimeters) long. 
There are two main types of seeds—naked and enclosed. Gymnosperms have naked seeds. The seeds of these plants develop on the upper side of the scales that form their cones. All flowering plants, on the other hand, 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 others 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 is the outer skin. It protects the embryo, which contains all the parts needed to form a new plant. The embryo also contains one or more cotyledons, also called 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 << `mehg` uh guh MEE tuh `fyt` >> stores food.
Seeds have many features that help them spread. The wind carries many seeds, including the winglike ones of the maple tree and the fluffy seeds of dandelion and milkweed. Some seeds, such as those of the coconut, may float on water from one area to another.
Animals also help distribute seeds. Some plants have burs and sticky substances that cling to the fur or feathers of animals. Many kinds of animals eat berries and fruits but do not digest the seeds. The seeds are passed with 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. See Seed.
The reproduction of plants
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 carry genetic (hereditary) material, including genes. Genes determine the characteristics that living things inherit from their parents. A plant that is produced by sexual reproduction inherits genes from both parents. The plant is a unique individual and has traits that may differ from those of either parent.
Asexual reproduction does not involve separate sperm and egg cells. It 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 may be called vegetative propagation. Many plants reproduce both sexually and by vegetative propagation.
Sexual reproduction
in plants occurs as a complex cycle called alternation of generations. It involves two distinct generations, also called phases. During one phase of the life cycle, the plant is called a gametophyte << guh MEE tuh `fyt` >> , or gamete-bearing plant. In seed plants, the gametophyte is barely visible and is rarely noticed by people. It produces gametes << GAM eets >> —that is, the sperm and egg cells. It may produce sperm cells or egg cells, or both, depending on the species of plant. After 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 << SPAWR uh fyt >> , or spore-bearing plant. When people see a plant it is usually the sporophyte phase. Sporophytes produce tiny structures called spores. The spores form in closed capsules called sporangia << spuh RAN jee uh >> . Gametophytes develop from the spores, and the life cycle begins again.
In nonseed plants, such as 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 << SAWR ee >> form on the edges or underside of each leaf. Spores develop in the sori. After the spores ripen, they fall to the ground. There, they 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 container that produces spores at the end. The sporophyte extends from the top of a soft, leafy, green gametophyte. The sporophyte depends on the gametophyte for food and water. The gametophyte is the part of the plant recognized as moss.
In seed plants, which include flowering plants 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. The male and female reproductive organs of a plant produce spores. The spores grow into gametophytes, which remain inside the reproductive organs.
In flowering plants, the reproductive parts are in the flowers. A plant’s stamens are its male reproductive organs. Each stamen has a filament with an enlarged tip called an anther. The carpels are the plant’s female reproductive organs. The ovary, which forms the round base of the carpels, contains the ovules. The anthers hold structures called microsporangia. 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, something must first transfer a pollen grain from the anther to the carpels. This transfer is called pollination. If pollen from a flower reaches the carpels of the same flower, or the carpels of another flower on the same plant, the fertilization process is called self-pollination. When pollen from a flower reaches the carpels of another plant, the fertilization process is called cross-pollination.
In many cross-pollinated plants, such animals as birds and insects carry the pollen grains from flower to flower. In other cross-pollinated plants, the wind transports the pollen. Many cross-pollinated plants have large, showy flowers, a sweet scent, and sweet nectar. These features attract such insects as ants, bees, beetles, butterflies, and moths. They also may attract larger animals, such as bats, hummingbirds, and small rodents. 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. Wind may carry pollen as far as 100 miles (160 kilometers). Some airborne pollen causes hay fever and other allergies. In aquatic plants, water carries the pollen.
If a pollen grain reaches a carpel 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 forms a pollen tube. Two sperm cells develop in the tube. After the pollen tube reaches the egg, one sperm cell fertilizes the egg. The second sperm cell disintegrates. The fertilized egg develops into a sporophyte embryo. The ovule containing the embryo becomes a seed. It can take 6 to 24 months after pollination for conifer seeds to mature.
Asexual reproduction.
Plants also can spread through asexual reproduction. Any plant produced through asexual reproduction will be a clone, or an individual that is genetically identical to its parent. Plants that are capable of asexual reproduction also are capable of sexual reproduction. Under certain conditions, asexual reproduction offers advantages over sexual reproduction. For example, it may enable plants to colonize new areas more quickly than they could otherwise. However, clones lack the diversity of plants produced by sexual reproduction between plants. As a result, they may be more vulnerable to diseases and other problems.
Among plants, asexual reproduction is also called apomixis << `ap` uh MIHK sihs >> . Apomixis can occur through vegetative propagation or seed production. Vegetative propagation occurs when a piece of a plant regrows missing parts by a process called regeneration. Any part of a plant—a root, stem, leaf, or flower—may grow into a new plant. A plant may even grow from a single cell.
Vegetative propagation occurs most often in plants with stems that run just above or below the ground. The strawberry plant, for example, sends out long stems called runners that grow along the surface of the soil. At points where they touch the ground, the runners send out roots that produce plantlets (new leaves and stems). These plantlets are actually part of the parent plant. They become new plants only when separated from the parent. Ferns, irises, many kinds of grasses, blueberries and some other shrubs, and some species of trees propagate naturally from underground stems. 
Many plants that grow as weeds are able to spread rapidly by vegetative propagation. These plants may be difficult to kill because they can regrow lost parts by regeneration. For example, a dandelion will regrow new stems and leaves even if only its roots are left in the soil.
Farmers use vegetative propagation to raise many valuable crops, such as apples, mangos, taro, bananas, oranges, and white potatoes. For example, they cut potatoes into many parts, making sure that each part has an eye (bud). Each piece of potato will grow into a new potato plant. This method produces new potato plants more quickly than planting the seeds of the plant. Vegetative propagation can also be used to reproduce plants that have been bred to bear seedless fruit.
Vegetative propagation is also widely used in gardening. Many plants, including gladioli, irises, lilies, and tulips, are grown from bulbs or corms. These plants take longer to flower when grown from seeds.
Asexual reproduction through seed production can occur through many different mechanisms. For example, some plants can produce seeds without fertilization. Asexual seed production enables a plant to spread clones of itself on a broader scale than is possible with vegetative propagation, because seeds can travel farther. This type of reproduction is more common in certain groups of plants, such as grasses and roses. Dandelions, mangos, and oranges and their relatives all commonly produce seeds asexually.
The growth of plants
Four major processes take place in the growth of most kinds of green plants. These processes are (1) germination, (2) the movement of water, (3) photosynthesis, and (4) respiration. A plant’s growth is shaped by its heredity and environment.
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. After spring arrives, the seeds start to germinate.
Loading the player...The growth of radish plants
Seeds need three things to grow: (1) proper temperature, (2) moisture, and (3) oxygen. Most seeds, like most kinds of plants, grow best between 65 and 85 °F (18 and 29 °C). The seeds of plants that live in cold climates may germinate at lower temperatures. Those of tropical regions may sprout at higher temperatures. Seeds get the moisture they need from the ground. The moisture softens the seed coat, enabling the growing parts to break through. Moisture also prepares certain materials in the seed for their role in seed growth. If a seed receives too much water, it may begin to rot. If it gets too little water, 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 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 << `hy` puh KAHT uhl >> , 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 << `ehp` uh KAHT uhl >> , begins to grow upward. At the tip of the epicotyl is the plumule << PLOO myool >> , 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 << `mehr` uh stuh MAT ihk >> 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 growing season. Biennial plants live for two growing seasons. Most annuals and biennials produce seeds only once.
Movement of water.
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. In many plants, fungi that grow on the roots help the plants absorb water and minerals. These materials are transported through the xylem 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 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.
Many species of plants have developed special methods for collecting and storing water that enable them to survive with little rainfall. For example, some cactuses have roots that spread over large areas just below the surface. These roots quickly absorb water from the light rains and sudden floods that occur in the desert. Cactuses store water in their fleshy stems. The leaves of cactuses are 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.
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 hairy. In addition, tundra plants grow close to the ground, where snow covers them and protects them from strong winds.
Photosynthesis
is the process by which plants make food. The word photosynthesis means putting together using light. Plants that make their own food are called autotrophs << AW tuh trohfs >> . Most photosynthesis takes place in small bodies called chloroplasts within the cells of plant leaves. These chloroplasts contain chlorophyll, which enables them to capture the energy in sunlight. Energy from the sunlight splits water molecules into hydrogen and oxygen. The hydrogen joins with carbon from the carbon dioxide to produce sugar. From sugar—in addition to nitrogen, sulfur, and phosphorus from the soil—green plants can make starch, fat, protein, vitamins, and other complex compounds essential for life. Photosynthesis provides the energy needed to make these compounds. Photosynthesis also releases oxygen into the air. Animals must have oxygen to breathe.
Some plants, called heterotrophs << HEHT uh roh trohfs >> , have little or no chlorophyll and cannot make their own food. These plants must rely on outside sources for food. Indian pipe is a heterotroph that grows near fungi. It feeds off organic materials produced by the fungi.
Many heterotrophs are parasites that attach to other plants and take the nutrients they need from these plants. Mistletoe and dodder are common parasites. Mistletoe grows on the trunks and branches of trees. It is called a partial parasite because it also makes some of its own 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 (90 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. It decreases as winter approaches.
Effects of heredity and environment.
Both a plant’s heredity and its environment shape its growth. Heredity, for example, determines such traits 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.
All plant cells contain tiny bodies called chromosomes, which include the genes. The genes provide “instructions” that direct the growth of the plant. As the cells divide and multiply, the chromosomes are passed on to each new cell.
Substances made within a plant also play a part in regulating plant growth. These substances, called hormones, control such activities as the growth of roots and the production of flowers and fruit. Botanists have learned that certain hormones, called auxins << AWK sihns >> , affect the growth of buds, leaves, roots, and stems. Other growth hormones, called gibberellins << `jihb` uh REHL ihnz >> , make plants grow larger, cause blossoming, and speed seed germination. Hormones called cytokinins << `sy` toh KY nuhnz >> make plant cells divide.
Environmental factors.
All plants need light, a suitable climate, and an ample supply of water and minerals. 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 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 period of darkness is long. Still other plants, among them marigolds and tomatoes, are not affected by the length of the day. 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, something outside the plant causes it 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 thing that causes the reaction. Tropisms are named according to the things that cause them. Phototropism << foh TAHT ruh pihz uhm >> is bending caused by light. Geotropism << jee AHT ruh pihz uhm >> is a response to gravity. Hydrotropism << hy DRAHT ruh pihz uhm >> is caused by water.
A plant placed in a window exhibits positive phototropism if its stems and leaves grow toward the source of light. Roots 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.
Some plants are affected by being touched. When the sensitive plant 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 touch is removed, the plant’s branches and leaflets return to their original position.
Plants may be affected by chemical poisoning when the soil contains too much of certain substances. For example, most plants require small amounts of copper, iron, and zinc. But people may introduce excessive amounts of these metals into the soil during the mining and smelting of ore. Such contamination kills large numbers of plants. 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 stretches of barren land where few plants survive.
Plants also may be affected by nutrient deficiencies, when the soil lacks sufficient amounts of certain substances. Nutrient deficiencies are harmful to plants in a number of ways. They may cause changes in leaf color, reduction in leaf size, dead spots, reduced growth, and wilting. Each symptom often can be linked to lack of a specific nutrient, usually nitrogen or potassium.
Some plants have developed unusual adaptations to survive on soil deficient in nutrients. Insect-eating plants obtain nitrogen and other nutrients by trapping and digesting insects in their leaves. These carnivorous plants break down the insects for nutrients but make their own food by photosynthesis. Insect-eating plants include the pitcher plant, the sundew, and the Venus flytrap.
Pitcher plants have tube-shaped leaves that collect rain water. Sweet substances around the rim of each tube attract insects. After an insect enters the tube, downward-pointing hairs keep the struggling victim from escaping. In time, the insect becomes exhausted, slides into the water, and drowns. The plant then digests it by means of a fluid secreted by glands in the leaves.
The leaves of the sundew plant grow hairs that give off a sticky substance. When an insect gets stuck on this substance, the hairs wrap around it. More fluid covers and suffocates the insect. The plant then gradually digests the insect.
The Venus flytrap captures insects with its hinged leaves. The inside of each leaf has hairs, and bristles edge the rim. When an insect lands on the hairs, the two halves of the leaf close like a trap, with the bristles interlocking. After the plant digests the insect, the leaves open again.
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Venus flytrap
Plant diseases and pests
Diseases and pests attack and injure almost all species of plants. They cause serious, widespread damage to agricultural, garden, and ornamental plants—many of which have lost the natural defenses of wild plants.
Diseases.
Many kinds of microorganisms cause diseases in plants. They include bacteria, fungi, viruses, and tiny worms called nematodes. Fungi and viruses cause more plant diseases than the other microorganisms do.
Environmental conditions can weaken plants so that they are more easily infected. Such conditions include air pollution, unusual extremes in temperature, nutrient deficiencies, and low levels of light or oxygen.
Diseases may affect every part of the plant. Many diseases interfere with the plant’s ability to carry out photosynthesis. These diseases may damage leaves or block the flow of water or nutrients to stems and leaves. Bacteria, fungi, or viruses may invade plant tissues and kill cells in a small area. For example, dead spots on leaves and fruits indicate places where microorganisms have killed plant cells. Infection also may cause yellowing and death of leaves at the edges. 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 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 carried to plants in spores. Insects, animals, rain, or wind may spread these fungal spores. Some bacteria and viruses spread in the same way. Nematodes not only cause certain diseases but also transmit viruses from diseased to uninfected plants. Some bacteria and fungi live on plant refuse in the soil and infect healthy plants. Others are carried on seeds.
Some plant infections cause serious illness when the plants are eaten by human beings and animals. For example, a fungus called ergot infects wheat, barley, and rye. It produces chemicals that can cause the illness ergotism. This illness afflicts people who eat bread made from the infected grain. Other fungi produce poisons called mycotoxins that can harm people or animals if consumed in food.
Widespread outbreaks of plant disease can cause famine. During the 1840’s, about 1 million people in Ireland died after a fungal disease destroyed the nation’s potato crop. Other diseases have killed large numbers of certain plant species. For example, a fungal disease called chestnut blight has destroyed the chestnut tree throughout North America.
Pests.
Insects damage or destroy plants in a number of ways. Some insects feed on flowers and fruit. Insects with chewing mouthparts, such as beetles and grasshoppers, eat holes in leaves and stems. The destruction of leaves affects the growth of crops by reducing photosynthesis. Swarms of grasshoppers have destroyed entire crops of alfalfa, cotton, and corn. Spongy moth caterpillars also eat leaves. They have damaged many forests. Other insects have piercing mouthparts with which they pierce plants and suck the juices. The wounds made by the insects provide places for microorganisms 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 they may give leaves a “burned” appearance. Other insects interrupt the flow of food and water in plants by feeding on vascular tissue.
Rabbits, rodents, and other animals also feed on plants. Some kinds of rodents burrow into the soil and feed on the roots, seeds, and bulbs of plants.
How plants protect themselves.
Many plant species have developed physical and chemical defenses to avoid being eaten. Many plants also protect themselves through timing when they produce flowers and fruits.
Physical defenses of plants include such structures as prickles, spines, and thorns. Most of these structures are modified leaves or branches. They prevent attacks by large plant-eating animals. Other plants produce heavy coatings of wax or dense, stiff hairs. These 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.
Plants have a wide variety of chemical defenses against animals. The leaves and fruits of citrus plants produce sticky, strong-smelling oils. These oils discourage many insects. Many plants contain chemicals that taste bad or are poisonous. Such plants include foxglove, nightshade, and yew.
Insects can quickly become immune to the chemicals plants produce. In some cases, insects develop a means of breaking down the toxins made by plants. As a result, plants continually develop new toxins by altering existing ones. Some scientists describe this process as a biological “arms race” between plants and their predators. In some 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. These poisons prevent most insects from eating the plants. However, caterpillars of monarch butterflies eat milkweed plants and store the poison in their bodies. The poison makes monarch butterflies distasteful and so protects them from many of their own predators.
Certain plant species obtain protection from animals through a relationship called mutualism. In this relationship, the plant provides a special type of food for a particular insect species. The insects, in turn, protect the plant from other animals. One example of plant-insect mutualism is the relationship between ants and acacia trees. Ants live in hollow spines on the acacia trees. 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 land on the trees. The fierce ants can protect the trees even from such large animals as elephants. 
Many plants try to ensure the survival of their seeds through the timing of when they produce flowers and fruits. Some plants produce flowers and fruits early in the growing season, when insects are few. 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. Some acorns survive to grow into new oak trees. In other years, oak trees produce fewer acorns, preventing the 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.
Disease and pest control.
People fight plant diseases and pest damage by various means. Chemicals make up the largest part of most programs to control plant diseases and pests. These chemicals include bactericides, fungicides, insecticides, nematocides, and rodenticides. Growers also cross resistant plants with other varieties of the same species to develop new varieties that combine resistance with high productivity. Such efforts have resulted in the development of high-yield, disease-resistant wheats, for example.
Some farmers control diseases and pests without the use of synthetic (artificial) chemicals, a practice known as organic agriculture. For example, organic farmers may use pesticides from natural rather than synthetic sources. They also may use biological pest control, in which natural predators control pests. For example, farmers may introduce mites known to feed on insect pests. An approach called integrated pest management combines a limited use of chemical pesticides with natural control methods.
The domestication of plants
People began to alter plants more than 10,000 years ago, when they learned to raise food by farming. Prehistoric people noticed that some plants grew better than others, were easier to harvest, or provided a better source of food. They saved seeds from these plants to grow new ones. This process marked the beginning of the domestication of plants. Over time, plant domestication turned wild plants into the crops grown today. Some of the first domesticated crops included barley, peas, squash, and wheat.
How crops differ from wild plants.
Domesticated crops differ from their wild ancestors because people have bred crops to have desirable traits. These traits include bearing larger or more numerous fruits, being easier to harvest, and having better taste or lower amounts of toxic substances. For example, people domesticated corn from a wild plant called teosinte. Teosinte grows a small cob with about 5 to 10 hard kernels. By comparison, modern corn produces a much larger cob with hundreds of soft kernels.
Wild wheat seeds are smaller than those of domesticated wheat. Wild seeds fall off the plant before they can be harvested and are enclosed by tough husks. Domestication produced brittle husks that are easier to remove and larger seeds that remain on the plant for harvest.
Domesticated papayas, tomatoes, and other fruits can be several times as large as their wild relatives. Domesticated potatoes contain only low levels of the poisons that protect their wild relatives. Domesticated plants can also vary more markedly than their wild relatives. For example, broccoli, Brussels sprouts, cabbage, and kale are domesticated varieties of the same plant species.
Industrialized agriculture.
In the 1800’s and 1900’s, farmers in the United States and other developed countries achieved dramatic increases in the productivity of crops. These improvements came through the industrialization of agriculture. This transformation involved a number of developments, including (1) the invention of powered machines, (2) the development of electric power, (3) improvements in crop breeding, through the study of plant genetics, (4) the use of artificial fertilizers, and (5) the development of chemical pesticides.
Industrialized agriculture did not spread to less developed countries in Asia, Africa, and Central and South America until much later. Farmers in such countries continued to practice traditional agriculture. They often could not produce enough food to feed their growing populations. However, many developing countries adopted techniques from industrialized agriculture between the 1940’s and 1970’s. The dramatic improvements in crop production that resulted are known as the Green Revolution. The Green Revolution saved millions of lives by reducing the danger of famine. Much of this success came through efforts to breed better crops. Scientists bred new varieties of corn, rice, and wheat that grew quickly and produced large grains.
Breeding and genetics.
The scientific study of plants has greatly aided attempts to make plants more productive and attractive. Plant breeding involves manipulating plants to introduce desirable traits. These traits include disease resistance or larger seeds. Most improvements in crops have been achieved using such methods. For instance, these methods produced the improved crops of the Green Revolution.
Beginning in the 1980’s, scientists have introduced desirable traits into crops by artificially transferring genes between organisms. This process creates genetically modified crops, often called GM crops. In many cases, genes are exchanged between organisms that could never be bred traditionally. For example, in so-called Bt crops, scientists have inserted a gene from bacteria that causes the crops to produce their own insecticide.
The most commonly grown GM crops are resistant to herbicides or insects. These crops include GM corn, cotton, and soybeans. A smaller number of crops have been modified for disease resistance. These include papaya and squash. Others have been modified to be more nutritious. For example, golden rice has been modified to produce the nutrient beta carotene. The human body can use this nutrient to make vitamin A. Vitamin A deficiency is a major cause of blindness in parts of Africa and India, among other areas. However, farmers have not planted golden rice widely because of controversy over its safety and effectiveness. The use of GM crops is controversial, and many countries restrict their cultivation and sale.
Conservation of plants
Human beings have caused rapid environmental changes around the world. As a result, many plant species have declined greatly in numbers. Some have already become extinct. Many others are at risk of extinction. In fact, more than 20 percent of plant species may be threatened, according to the International Union for the Conservation of Nature and Natural Resources (IUCN). Plants form the foundation of nearly all food chains on land, and they are vital to many ecosystems. As a result, the health and survival of plants is critical for the survival of other living things. Major threats to plants include habitat destruction, loss of biodiversity (the variety of living things), invasive species, pollution, and global warming.
Habitat destruction
may be the greatest threat to plants. People have destroyed vast areas of wilderness to make room for farms and cities. People also have caused great damage by harvesting timber, minerals, or other natural resources. As the population of human beings has grown, people have caused ever more damage. For example, people have cut down about half of the native forests that once covered the world. Much of the forest that remains has been damaged. People also have destroyed vast areas of grasslands, wetlands, and other habitats. Such habitats support a tremendous variety of animals and other living things. As a result, habitat destruction poses a great threat to biodiversity.
Plants and other living things also are threatened by habitat fragmentation. Habitat fragmentation occurs as natural areas become smaller and distances between them increase. Habitat fragmentation can cut off plants from potential pollinators. It may also make it difficult for plants to disperse their seeds to suitable habitats. 
Loss of biodiversity.
Habitat destruction can lead to loss of genetic diversity within a species. Loss of diversity can make plants more vulnerable to other threats, such as diseases or climate change. Habitat fragmentation also reduces biodiversity, both within a species and among different species. Plants and other living things in an ecosystem depend on one another in a complex web of relationships. Diverse ecosystems are more robust than those with less diversity—because the loss of individual species will less likely cause additional damage. For example, if several kinds of insects pollinate a plant, the loss of any one kind will less likely harm the plant.
Loss of biodiversity also threatens crops. The majority of the food people eat comes from three major crops: corn, rice, and wheat. These crops are often grown in monocultures—that is, a single crop grown over a large area. This practice can enable the rapid spread of disease or pests because the crops are genetically similar—so a disease that kills one plant can wipe them all out. Monocultures also threaten the biodiversity of other organisms because they support far fewer species than do natural areas. People have developed many different varieties of crops over thousands of years. In many cases, however, the wild relatives of crops have become less diverse. This loss of diversity reduces the traits available to breeders. For example, it may limit breeders’ access to genes for resistance to disease or drought found among a crop’s wild relatives.
Invasive species.
An invasive species is an organism introduced into an area that spreads quickly and harms native wildlife. The increase in travel and trade among distant parts of the world has resulted in the introduction of huge numbers of invasive species. Some of these species were released accidentally. However, people introduced many invasive plants intentionally, often as ornamental plants. Notable invasive plants include Australian acacias, common buckthorn, erect prickly pear, Japanese knotweed, kudzu, purple loosestrife, strawberry guava, and water hyacinth.
Invasive plant species often outcompete native plants by growing and reproducing rapidly. They may produce more seeds than native plants. Some invasive plants produce toxins that make it difficult for native plants to grow. Invasive plants can damage ecosystems and habitats. They also can be major pests in agriculture. In addition, many plant diseases and pests are invasive species. These include various fungi, viruses, and insects.
Pollution
also harms plants. Most pollution comes from agricultural runoff, automobile exhaust, industry, and wastewater. Pollution damages plants in a variety of ways. Harmful chemicals can pollute the soil and water, poisoning plants. Air pollution can cause smog, which blocks the sunlight plants need for photosynthesis. However, some plant species remove pollution from the air, soil, or water. They may be planted to help restore polluted areas.
Global warming.
Plants also are threatened by global warming, an increase in the average temperature at Earth’s surface. Climate scientists estimate that Earth’s average surface temperature rose by about 1.4 Fahrenheit degrees (0.76 Celsius degree) from the mid-1800’s to the early 2000’s. This warming has caused other climate changes, such as changing patterns of precipitation. An increase in carbon dioxide in the atmosphere causes most global warming. This increase comes chiefly from burning coal, oil, and natural gas.
Plants are sensitive to changes in climate. Plants are fixed in place and cannot migrate. As a result, individual plants are limited in their ability to respond to changes in climate. Instead, plants rely on seed dispersal to gradually spread to new areas. Scientists have found evidence that the distribution of plants has already begun to change because of global warming. On mountains, plant species are spreading to higher elevations. In the Northern Hemisphere, plant species are shifting northward, as their favored climates shift north. Also, many plants have begun to bloom earlier in the spring, in response to warming temperatures.
Global warming also affects interactions among plants and other living things. Many animals time their migrations to take advantage of plant growth. As the timing of plant development in the spring changes, animals risk arriving too late to take advantage of the growth. Many plants rely on animals for pollination, and animals rely on plants for their fruit, nectar, or seeds. Global warming threatens to disrupt some of these relationships.
Climate scientists predict that Earth’s surface temperature will rise an additional 2.0 to 11.5 Fahrenheit degrees (1.1 to 6.4 Celsius degrees) by 2100. Many scientists worry that rapid global warming will cause many species to become extinct. Plant species may not be able to spread to suitable habitats. This problem is made worse by habitat destruction and fragmentation.
Plants play an important role in limiting global warming by taking up carbon dioxide for use in photosynthesis. They store the carbon in their tissues. In this way, plants can help reduce global warming. However, when a plant is killed, its carbon returns to the atmosphere, especially if the plant is burned. As a result, deforestation and other habitat destruction can increase the rate of global warming. Thus, many people argue that forests and other ecosystems should be preserved to reduce global warming.
Conservation efforts.
Many people are working to save plant species and ecosystems. Governments have established national parks to protect natural habitats. People also attempt to renew and restore damaged habitats. For example, people may try to remove invasive species and replant native species.
Other efforts focus on collecting seeds from wild plants. Scientists also collect the seeds of crop varieties and their wild relatives. They store the seeds at cold temperatures in seed banks, where they can be used in restoration projects or for breeding. If a species becomes extinct in the wild, it may be possible to reintroduce it through plants that have been grown from seeds in seed banks. 
The evolution of plants
Plants on land are thought to have evolved (developed over many generations) from green algae that lived in fresh water. Most scientists think that all plants on land developed from the same green algae ancestor. Based on evidence from fossils of spores, plants may have begun making the transition to land about 470 million years ago. They had become well established on land by 430 million years ago. Some genetic studies suggest that plants may have appeared on land 500 million years ago or even much earlier. Before this transition, land was mostly rocky and barren, supporting only microbial life. The spread of plants transformed the land, providing habitat for animals and other organisms.
Plants and other living things evolved into many new forms as they colonized the land. The move to land required plants to evolve adaptations to prevent drying out, to obtain water and nutrients from the soil, and to support themselves for growing upright. Cooperation with fungi may have helped early plants adapt to the stresses of life on land. Such relationships with fungi remain important for plants today.
Nonvascular plants were the first to spread to land. They grew mostly in wet areas. The earliest known vascular plants, called rhyniophytes << RY nee uh fyts >> , lived from about 420 million to 380 million years ago. They did not have leaves or roots. Instead, they had only branched stems with sporangia. These plants were small, with the largest species probably growing only 6 to 8 inches (15 to 20 centimeters) tall.
Larger plants called trimerophytes << TRIHM uhr uh fyts >> may have developed from the rhyniophytes. The trimerophytes had a more complex plant body with numerous stems and branches. But they lacked leaves, and only some of them may have had simple roots. Other small, leafless vascular plants, called zosterophylls << `zahs` tuhr uh FIHLZ >> or zosterophyllophytes << `zahs` tuhr uh FIHL uh fyts >> , appeared shortly after the rhyniophytes and also may have descended from them. Some botanists believe trimerophytes and zosterophylls are the ancestors of all vascular plants that live today. They believe that ferns and related plants evolved from trimerophytes during the Devonian Period, from about 420 million to 360 million years ago. Early forms of seed plants also may have evolved from trimerophytes at this time. Scientists believe lycophytes, the first plants to have leaves, evolved from zosterophylls.
During the Carboniferous Period, about 360 million to 300 million years ago, more complex and larger vascular plants evolved. Great forests of ferns, horsetails, lycophytes, and early seed plants covered Earth. Plant matter from these forests accumulated in vast swamps. This plant matter later formed large coal deposits.
Gymnosperms became the most plentiful plants during the Mesozoic Era, which began about 250 million years ago. Conifers, cycads, and ginkgoes were among the most important plants. They served as food for the dinosaurs that roamed the land during this time. Many now-extinct types of gymnosperms also flourished. 
The oldest known fossils of flowering plants are about 130 million years old. However, some scientists believe they may have evolved tens of millions of years earlier. Scientists are not certain which plants became the ancestors of these angiosperms, but their closest living relatives are gymnosperms. Among the first angiosperms were magnolias, sycamores, water lilies, and willows. Angiosperms diversified rapidly in the Cretaceous Period. Insects also diversified during this time. The evolution of the two groups was linked, due to their dependence on each other through pollination. The process by which living things that interact with one another evolve together is known as coevolution. Angiosperms became increasingly common during the Cretaceous Period. But it was not until after the dinosaurs became extinct, about 65 million years ago, that angiosperms came to dominate the land.
During the Cenozoic Era, beginning about 65 million years ago, angiosperms dominated landscapes and took on major ecological roles. Forests of angiosperms covered much of Earth. Both angiosperms and insects continued to diversify, through further coevolution. Angiosperms also influenced the evolution of other animals by providing a rich abundance of food. Late in the Cenozoic Era, grasslands and large grazing animals began to appear. 
The Pleistocene Epoch, which lasted from about 2.6 million to 11,500 years ago, included the last major ice ages. During the ice ages, great sheets of ice covered large areas of land. These ice ages influenced plant evolution and distribution. The ice sheets that covered northern portions of the globe forced plant species to move southward. During the warmer periods between ice ages, woodlands covered much of the land. During cooler periods, grasses became more abundant. During ice ages, tundra plants flourished.
The end of the Pleistocene changed the makeup and distribution of biomes. As the ice sheets retreated, forests replaced large areas of tundra, and the distribution of grasslands changed. These biomes are shifting farther in response to global warming. Habitat destruction and agriculture also have brought great changes to the distribution of plants. However, angiosperms remain by far the largest and most diverse group of plants.
Classification of plants
Botanists classify plants by grouping them according to common descent. Groups that share a common ancestor also tend to share certain characteristics.
Botanists may determine which plants are closely related by comparing their traits, including their overall appearance, their internal structure, and the form of their reproductive organs. Relationships can also be verified through genetic testing. However, not all botanists agree on how plants should be divided. As a result, there are a number of different classification systems for the plant kingdom. One classification system is described in the table with this article, A classification of the plant kingdom. This system classifies plants into several large groups and a number of divisions. A division is the same grouping as a phylum in the animal kingdom.
One major group consists of nonvascular plants. These plants lack xylem and phloem that carry water and food from one part of the plant to another. The other major group consists of vascular plants that contain these specialized tissues. Within these groups are several smaller groupings, including divisions and classes.
