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.
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.
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|