The IMPORTANCE of PLANTS
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.
No comments:
Post a Comment