CHLOROPHYLL

Chlorophyll is the green pigment in plants that absorbs light energy for use in photosynthesis. Chlorophyll also is found in simple organisms called algae and in some bacteria. Most plant cells do not produce chlorophyll unless the plant is exposed to light. This is why plants kept away from light are white or yellow rather than green. Chlorophyll is located in disk-shaped membranes called thylakoids within cells. In most plants, thylakoids are contained in tiny cell bodies called chloroplasts. The chloroplasts in the leaves of plants carry out all the essential processes of photosynthesis. Light energy absorbed by chlorophyll is channeled to specialized reaction centers in the thylakoids. The reaction centers, along with electron-carrier molecules, convert the light energy to chemical energy. Oxygen is released in the process.

Chemical energy is needed for taking carbon dioxide from the air, eventually leading to the production of sugars and such other food substances as starch, fat, protein, and vitamins.

There are several forms of chlorophyll. The most common forms in plants are chlorophyll a and chlorophyll b. They absorb most of the long wavelengths (red rays) and the short wavelengths (blue-violet rays) of visible light. They absorb the middle wavelengths (green rays) least effectively. Some bacteria, like plants, make their own food by photosynthesis. These bacteria have special chlorophylls that can absorb longer wavelengths called infrared rays, which lie beyond the visible light spectrum. When dried, chlorophyll looks like blue or green-black powder.

Chloroplast, is a specialized structure within the cells of plants. Chloroplasts serve as the site of photosynthesis. They contain chlorophyll, the green pigment that absorbs energy from sunlight for use in photosynthesis. Chlorophyll also gives green plants their color. In the fall, the production of chlorophyll in woody plants ceases. The colors of yellow pigments in the chloroplasts then become visible.

The chloroplasts of most plants are shaped like disks or lenses. Under a microscope, they can be suspended in the part of a cell called the cytoplasm. Except for the cell nucleus, chloroplasts are the most visible structures in a plant cell. Chloroplasts are one of several types of specialized plant-cell structures called plastids. Other plastids contain yellow, orange, or red pigments, and provide the colors of many flowers and fruits. Plastids also store oil, protein, and starch. 

Definition of Photosynthesis

Definiton of Photosynthesis, is a food-making process that occurs in green plants. Photosynthesis is the chief function of leaves. The word photosynthesis means putting together with light.




Green plants use energy from light to combine carbon dioxide and water to make food. All our food comes from this important energy-converting activity of green plants. Light energy is converted to chemical energy and is stored in the food that is made by green plants. Animals eat the plants, and we eat animal products as well as plants. The light used in photosynthesis is absorbed by a green pigment called chlorophyll. Each food-making cell in a plant leaf contains chlorophyll in small bodies called chloroplasts.

In chloroplasts, light energy causes water drawn from the soil to split into molecules of hydrogen and oxygen. In a series of complicated steps, the hydrogen combines with carbon dioxide from the air, forming a simple sugar.

 

Oxygen from the water molecules is given off in the process. From sugar--together with 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 chemical energy needed to produce these compounds.

Certain bacteria and algae can also capture light energy and use it to make food. For example, photosynthetic bacteria contain chlorophyll in tiny bodies called chromatophores. In chromatophores, compounds other than water are combined with carbon dioxide to form sugar. No oxygen is released.

Green plants convert carbon dioxide and water into food and oxygen. Plants and animals, in turn, "burn" the food by combining it with oxygen to release energy for growth and other activities of life. This process, which is called respiration, is the reverse of photosynthesis. Oxygen is used up and carbon dioxide and water are given off. Plants then use the carbon dioxide and water to produce more food and oxygen. The cycle of photosynthesis and respiration maintains the earth's natural balance of carbon dioxide and oxygen.

How a Leaf Makes Food

A green leaf is a marvelous food-making factory. Using only the energy of the sun, it takes simple materials and turns them into energy-rich food. This section describes how a leaf obtains the raw materials needed to make food. It then provides a simple explanation of how the leaf produces food through photosynthesis. Finally, this section discusses transpiration, a process of water loss that plays a key role in the operation of the leaf food factory.

Obtaining the raw materials.

A leaf needs three things to make food. They are (1) carbon dioxide, (2) water, and (3) light. The carbon dioxide and water serve as the raw materials of photosynthesis. The light, which is normally sunlight, provides the energy that powers photosynthesis.

Carbon dioxide enters a leaf from the air. The epidermis (outer surface) of the leaf has many tiny pores. These openings, called stomata, enable carbon dioxide to enter the leaf. Each pore is surrounded by two curved, bean-shaped guard cells that can swell and relax. When they swell, the pore is opened wide, and carbon dioxide enters the leaf. When the guard cells relax, the pore closes. In most plants, the stomata open during the day and close at night. A leaf has many stomata. For example, a cottonwood leaf may have 1 million stomata, and a sunflower leaf nearly 2 million. However, the pores are so small that they make up less than 1 percent of the leaf's surface. In most plants that grow in full sun, the majority of the stomata are in the shaded lower epidermis of the leaves. In many other plants, the stomata are about equally divided between the upper and lower epidermis.

Water. A leaf obtains water that has been absorbed by the plant's roots. This water travels up the stem and enters the leaf through the petiole. Tiny tubes in the leaf's veins carry the water throughout the blade. These tubes make up the vein's xylem (water-transporting tissue). The inside of the blade is very humid. The epidermis is covered by a waxy coating called the cuticle, which helps keep the leaf from drying out. Nevertheless, a leaf does lose much water. Most of it escapes as vapor through the stomata by the process of transpiration.

Light. Leaves cannot make food without light. But most leaves work best when the sunlight is at a certain level of brightness. If the light is too dim, the leaf will not make enough food. But if the light is too bright, it can damage the food-making cells. The leaves of many plants that grow in bright sunlight have an extremely thick cuticle, which helps filter out strong light and guards against excess water loss. The leaves may also have many threadlike structures called hairs growing out of the epidermis. These structures are not true hairs, which grow only on mammals, but they resemble hairs. Epidermal hairs further reduce the intensity of bright light. Such plants as geraniums and white poplar trees have so many epidermal hairs that they feel fuzzy.

Some plants, including the herbs, ferns, and shrubs of the forest floor, thrive in shade. The leaves of most of these plants have a thin cuticle and few epidermal hairs. These features allow as much of the dim light as possible to enter the leaves.

Photosynthesis occurs inside the leaf blade in two kinds of food-making tissues-palisade tissue and spongy tissue. The tall, slender cells of the palisade tissue are the chief food producers. They form one to three layers beneath the upper epidermis. The broad, irregularly shaped cells of the spongy tissue lie between the palisade tissue and the lower epidermis. Floating within both kinds of cells are numerous small green bodies known as chloroplasts. Each chloroplast contains many molecules of the green pigment chlorophyll.

Partly surrounding each cell of the palisade and spongy tissue is an air space filled with carbon dioxide, water vapor, and other gases. The cells absorb carbon dioxide from this air space. When light strikes the chloroplasts, photosynthesis begins. The chlorophyll absorbs energy from the light. This energy splits the water molecules into molecules of hydrogen and oxygen. The hydrogen then combines with carbon dioxide to produce a simple sugar. This process is extremely complicated and involves many steps. The oxygen that is left over from the splitting of the water molecules enters the air through the stomata.

The sugar produced by photosynthesis is carried in special tubelike cells that make up the vein's phloem (food-transporting tissue). The sugar moves through the petiole to the stem and all other parts of the plant. In the plant cells, the sugar may be burned and thus release energy for growth or other activities. Or the sugar may be chemically altered and form fats and starches. In addition, the sugar may be combined with various minerals, and so produce proteins, vitamins, and other vital substances. The minerals enter the plant dissolved in the water absorbed by the roots.

Transpiration occurs as the sun warms the water inside the blade. The warming changes much of the water into water vapor. This gas can then escape through the stomata. Transpiration helps cool the inside of the leaf because the escaping vapor has absorbed heat.

Transpiration also helps to keep water flowing up from the roots. Water forms a continuous column as it flows through the roots, up the stem, and into the leaves. The molecules of water in this column stick to one another. As the molecules at the top of the column are lost through transpiration, the entire column of water is pulled upward. This pulling force is strong enough to draw water to the tops of the tallest trees. In addition, transpiration ensures a steady supply of dissolved minerals from the soil. A plant may lose much water through transpiration. A corn plant, for example, loses about 4 quarts (3.8 liters) of water on a hot day. If the roots cannot replace this water, the leaves wilt and photosynthesis stops.  Next >>>

SPECIALIZED LEAVES

Some leaves have special functions along with or instead of food making. Such specialized leaves include (1) protective leaves, (2) storage leaves, (3) tendrils, (4) bracts, and (5) insect-capturing leaves.

Protective leaves include bud scales, prickles, and spines. As described earlier, bud scales are specialized leaves that protect the young, undeveloped tissues of the bud. Bud scales are short and broad, and they overlap like roof shingles. In many plants, the bud scales have an outer layer of waterproof cells. Prickles and spines are sharp leaf structures that protect the plant from being eaten. For instance, prickles cover the leaves of the Canada thistle. The prickles protect the plant from grazing animals. Many cactuses have clusters of spines. In many species of cactuses, the pointed spines replace the leaves on the mature plants. In these plants, the green stem has the job of photosynthesis.

Storage leaves. Most plants store food in their roots or stems. However, some plants have special leaves that hold extra food. Onion and tulip bulbs, for example, consist mainly of short, fat storage leaves called bulb scales. These leaves cannot make food. Their job is to store food underground during the winter months.

BULB. Many plants that grow in dry places have thick leaves that store water. The mosslike stonecrop plants that grow on rocky cliffs in the Southwestern United States have such leaves.

Tendrils are slender, whiplike structures that help hold climbing plants in place. They wrap around twigs, wires, and other solid objects. Among many climbing plants, specialized leaves serve as tendrils. For example, climbing garden peas have compound leaves in which the upper leaflets are threadlike tendrils. In one kind of sweet pea, a garden flower, the entire leaf blade becomes a tendril. The plant's stipules enlarge and take over the food-making job. In the greenbrier vine, the stipules form long, curving tendrils.





Bracts grow just below the blossoms of certain plants. Most bracts are smaller and simpler in shape than a plant's regular leaves. Many members of the daisy family-including daisies, goldenrods, marigolds, and sunflowers-have bracts. These bracts form a cup beneath the plant's cluster of flowers. A few kinds of plants, such as the flowering dogwood and poinsettia, have large, showy bracts. These bracts look like part of the flower, but they are not.

Insect-capturing leaves. Carnivorous (meat-eating) plants, such as the butterwort, pitcher plant, sundew, and Venus's-flytrap, have leaves that capture insects. These leaves, like other leaves, can make food using sunlight. But they also have features that attract, trap, and then digest insects. Plants with insect-capturing leaves grow in wetlands, where the soil contains little nitrogen. They obtain this necessary nutrient from the captured insects. For a description of these plants and their leaves.  << --- Next>>>

The leaf becomes fully grown

Leaves complete their growth within one week to several weeks, depending on the kinds of plants that produce them. At first, the unfolding leaf must get all its food from older leaves or from food stored by the plant. Soon, however, the young leaf turns a deeper green and begins to make its own food. Gradually, the leaf produces extra food, which is sent to the rest of the plant. During the growing season, the color of the leaf changes from bright green to a duller green. The leaf also becomes tougher because its cells develop thicker walls. During this time, a special change occurs in the leaves of deciduous trees and shrubs. A corky layer of cells known as the abscission zone develops where the stalk of the leaf joins the stem. This zone breaks down in autumn, causing the leaf to separate from the stem.

The leaf changes color

The leaf is green because it contains a green pigment (coloring matter) called chlorophyll. This pigment plays a major role in photosynthesis. The leaf also has other colors, but they are hidden by the chlorophyll. As autumn approaches, however, the shorter days and cooler nights cause the chlorophyll in deciduous broad leaves to break down. The hidden colors of the leaf appear as the chlorophyll breaks down. The leaf may then show the yellow color of the pigment xanthophyll or the orange-red tones of the carotene pigments. In addition, a group of red and purple pigments called anthocyanins forms in the dying leaf. The color of the autumn leaf depends on which of the pigments is most plentiful in the leaf.

The leaf dies

After the chlorophyll breaks down, the leaf can no longer make food. The tiny pipelines between the leaf and the stem become plugged. These pipelines carried water to the leaf and food from it. The cells in the abscission zone separate or dissolve, and the dying leaf hangs from the stem by only a few strands. These strands dry and twist in the wind. When the strands break, the dead leaf floats to the ground. After the leaf falls, a mark remains on the twig where the leafstalk had been attached. This mark is called a leaf scar. The broken ends of the water and food pipelines can be seen within the leaf scar. On the ground, the dead leaf becomes food for bacteria and fungi. They break the leaf down into simple substances, which then sink into the soil. There, these substances will be absorbed by plant roots and provide nourishment for new plant growth.
The leaves in the winter buds stop growing during the summer and remain dormant (inactive) throughout the winter. During the winter months, the buds are protected from drying out by special outer leaves called bud scales. In spring, warmth and moisture cause the dormant leaves to become active. The bud scales drop off, and the leaves unfold.

The parts of a leaf

Most leaves have two main parts: (1) the blade and (2) the petiole, or leafstalk. The leaves of some kinds of plants also have a third part, called the stipules.

The blade, or lamina, is the broad, flat part of the leaf. Photosynthesis occurs in the blade, which has many green food-making cells. Leaf blades differ from one another in several ways. The chief differences are in: (1) the types of edges, (2) the patterns of the veins, and (3) the number of blades per leaf. The types of edges. Almost all narrow, grasslike leaves and needle leaves have a blade with a smooth edge. But the edge of broadleaf blades varies greatly among the different types of plants. Many broadleaf plants, particularly those that are native to warm climates, have smooth-edged leaf blades. The rubber plant, a common house plant, is a good example of such a plant.

The leaves of many temperate broadleaf plants have small, jagged points called teeth along the blade edge. Birch and elm trees have such leaves. In many plants, the teeth contain hydathodes, tiny valvelike structures that can release excess water from the leaf. The teeth of young leaves on many plants, including cottonwood and pin cherry trees, bear tiny glands. These glands produce liquids that protect the leaf from plant-eating insects. Some temperate broadleaf plants-including sassafras trees and certain mulberry and oak trees-have lobed leaves. The edge of such a leaf looks as if large bites have been taken out of it. The lobing helps heat escape from the leaf.

The patterns of the veins. Veins are pipelines that carry food and water in a leaf. If you hold a leaf blade up to light, you can see the pattern of its veins. In most broad leaves, the veins form a netlike pattern, with several large veins connected by many smaller ones. The smallest veins supply every part of the blade with water. They also collect the food made by the green cells. There are two main types of net-vein patterns-pinnate (featherlike) and palmate (palmlike or handlike). Pinnately net-veined leaves have one large central vein, called the midrib, which extends from the base of the blade to its tip. Other large veins branch off on each side of the midrib. The leaves of beech, birch, and elm trees have such a vein pattern. A palmately net-veined leaf has several main veins of about equal size, all of which extend from a common point at the base of the blade. The vein patterns of maple, sweet gum, and sycamore leaves are palmate.

Narrow leaves and needle leaves are not net-veined. Narrow leaves have a parallel-vein pattern. Several large veins run alongside one another from the base of the blade to the tip. Small crossveins connect the large veins like steps on a ladder. Needle leaves are so small that they have only one or two veins. These veins run through the center of the blade. Leaf veins do more than carry water and food. They also support the blade, much as the metal ribs support the fabric of an open umbrella. The veins are tougher and stronger than the green tissue around them. They help the leaf keep its shape and prevent it from collapsing or tearing. The number of blades per leaf. A leaf may have one or more blades. A leaf that has only one blade is called a simple leaf. Apple and oak trees, grasses, and many other kinds of plants have simple leaves. A leaf that has more than one blade is known as a compound leaf. The blades of a compound leaf are called leaflets.

The petiole is the stemlike part of the leaf. It joins the blade to the stem. Within a petiole are tiny tubes, bound together tightly like a bundle of drinking straws. These tubes are a continuation of the midrib veins. Some of the tubes carry water into the leaf. Others carry away food that the leaf has made. The leaves of many plants have petioles that grow extra long if the blades are shaded. For example, white clover plants growing among unmowed grass may have petioles up to 6 inches (15 centimeters) long. These long petioles lift the clover leaflets into the sunlight. In a lawn where the grass is kept short, the clover petioles may measure less than 1 inch (2.5 centimeters) long. In many trees and shrubs, the petioles bend in such a way that the blades are turned to receive the most sunlight. As a result of this bending, few of the leaves are shaded by other leaves. The petiole also provides a flexible "handle" that enables the blade to twist in the wind and so avoid damage. In some plants, the petioles are much larger than the stems to which they are attached. For example, the parts we eat of celery and rhubarb plants are petioles. In contrast, the leaves of some soft-stemmed plants, particularly grasses, have no petioles.
The leaflets in a compound leaf may be arranged in a pinnate or palmate pattern. In pinnately compound leaves, the leaflets grow in two rows, one on each side of a central stalk, called the rachis. Plants with pinnately compound leaves include ash and walnut trees and garden peas. The leaflets in a palmately compound leaf are attached at the tip of the leafstalk. Clover, horsechestnut trees, and many other plants have palmately compound leaves. A few plants-including carrots, honey locust trees, and Kentucky coffeetrees-have double compound leaves. In double compound leaves, each leaflet is divided into a number of still smaller leaflets. One double compound leaf looks more like a group of twigs and leaves than like a single leaf.

The stipules are two small flaps that grow at the base of the petiole of some plants. Many stipules look like tiny green leaf blades. In some plants, the stipules grow quickly, enclosing and protecting the young blade as it develops. Some stipules, such as those of willows and certain cherry trees, produce substances that prevent insects from attacking the developing leaf. In many plants, the stipules drop off after the blade has developed. But garden peas and a few other kinds of plants have large stipules that persist and become an extra food-producing part of the leaf.

The Importance of Leaves

The chief job of leaves is to make food for plants. This food-making activity, called photosynthesis, occurs mostly in fully grown leaves. But young leaves also are important. They wrap tightly around the tips of growing stems. They thus keep the delicate tips moist and help protect them from insects, cold, and other dangers.

Leaves are also vital to animals. Animals cannot make their own food. They depend on plants for their basic supply of food. Many animals eat leaves. For example, antelope, sheep, and other grazing animals eat grass leaves. People also eat leaves, such as those of cabbage, lettuce, and spinach plants. But even when people and animals eat the fruits, roots, seeds, and stems of plants, they are obtaining food made by leaves.

In the same way, eggs, meat, milk, and all other animal foods can be traced back to food made by photosynthesis.
Leaves help make the air breathable. They release oxygen during photosynthesis. People and animals must have oxygen to live. Without the activities of leaves, the earth's supply of breathable oxygen would probably soon be used up.

People obtain many products from leaves in addition to food. For instance, we use the leaves of the tea plant to make tea. Peppermint and spearmint leaves contain oils used to flavor candy and chewing gum. Such leaves as bay, sage, and thyme are used in cooking to flavor foods. Some drugs come from leaves. For example, the drug digitalis, which is used to treat certain heart diseases, comes from the leaves of the purple foxglove, a common garden flower. Leaves of abaca and sisalana plants provide fiber used in making rope. Finally, the leaves of the tobacco plant are used to make cigarettes, cigars, and other tobacco products.

The life story of a leaf

A leaf begins its life in a bud. Buds are the growing areas of a stem. They form along the sides of the stem, at the point just above where a fully grown leaf is attached. A bud also grows at the tip of the stem. A leaf bud contains undeveloped leaf and stem tissues. Within the bud is a mound slightly larger than the head of a pin. Each leaf starts out as a tiny bump on the side of this mound. The mature bud contains a tightly packed group of tiny leaves. In most soft-stemmed plants, the buds are hard to see. A new leaf becomes noticeable only after it begins to unfold. Most soft-stemmed plants continue to form new leaves until the plants flower or until cold weather sets in. In temperate regions, which have warm summers and cold winters, the aboveground parts of many soft-stemmed plants die after the first hard frost, but the roots live through the winter. Other soft-stemmed plants die completely after the cold weather arrives.

Woody plants, on the other hand, may live many years. They grow several sets of leaves during their lifetime. Most needleleaf trees and shrubs shed old leaves and grow new ones continuously throughout the year. So do most broadleaf trees in the tropics. But in temperate regions, most broadleaf trees and shrubs are deciduous. Deciduous plants of temperate regions shed all their leaves each fall and grow a new set each spring. Deciduous trees and shrubs start growing the next year's leaves even before the present year's leaves have fallen. The new leaves are enclosed in winter buds.

The leaves in the winter buds stop growing during the summer and remain dormant (inactive) throughout the winter. During the winter months, the buds are protected from drying out by special outer leaves called bud scales. In spring, warmth and moisture cause the dormant leaves to become active. The bud scales drop off, and the leaves unfold.

Leaves of Plants

Leaf is the main food-making part of almost all plants. Garden flowers, grasses, shrubs, and trees depend on their leaves to make food for the rest of the plant. So do many other plants, including ferns, vegetables, vines, and weeds. Each leaf is a little food factory. It captures energy from sunlight and uses it to make sugar out of water from the soil and carbon dioxide, a gas in the air. This sugar is changed to many other chemical substances.

These substances become the food that provides plants with energy to grow, to produce flowers and seeds, and to carry on all their other activities. Plants store the food made by leaves in their fruits, roots, seeds, stems, and even in the leaves themselves. Without this food, plants could not live. In addition, all the food that people and animals eat comes either from plants or from animals that eat plants.Leaves vary in appearance among plants. Many are oval, but others are shaped like arrowheads, feathers, hands, hearts, or any number of other objects.

However, most leaves can be divided into three groups according to their basic shape. (1) Broad leaves are the type of leaf that most plants have. These leaves are fairly wide and flat. Plants that have such leaves include maple and oak trees, pea plants, and rosebushes. (2) Narrow leaves are long and slender. Narrow leaves grow on grasses. Grasses include not only lawn grasses but also barley, corn, oats, wheat, and other cereal grasses. Lilies, onions, and certain other plants also have narrow leaves. (3) Needle leaves grow on firs, pines, spruces, and most other cone-bearing trees and shrubs. Needle leaves resemble short, thick sewing needles. A few other kinds of cone-bearing plants, including certain cedars and junipers, have scalelike leaves.

Most leaves grow from 1 to 12 inches (2.5 to 30 centimeters) in length. Some plants, however, have huge leaves. The largest leaves grow on the African raffia palm. The leaves of this tree measure up to 65 feet (20 meters) long. The giant water lily of South America has round, floating leaves that grow up to 6 feet (1.8 meters) across. In contrast, some plants have extremely small leaves. The true leaves of asparagus plants, for example, are so tiny that they are hard to see without a magnifying glass. In these plants, the stems, rather than the leaves, produce food.

The number of leaves on plants ranges from several to thousands. Most soft-stemmed plants have few leaves. For instance, a barley or wheat plant produces only 8 to 10 leaves each season. But trees and shrubs have an enormous number of leaves. A fully grown elm or pine tree produces thousands of leaves. Some simple plants that manufacture their own food do not have leaves. For example, liverworts and mosses are simple food-making plants that lack true leaves. In some of these simple plants, however, the green food-making tissues look like tiny leaves.

PARTS OF PLANTS

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.

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.

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.

SEED. In conifers, the reproductive parts are in the cones. A conifer has two kinds of cones. The pollen, or male, cone is the smaller and softer of the two. It also is simpler in structure. Seed, or female, cones are larger and harder than the male cones. A pollen cone has many tiny sporangia that produce pollen grains. Each of the scales that make up a seed cone has two ovules on its surface. Every ovule produces a spore that grows into a female gametophyte. This tiny plant produces egg cells. The wind carries pollen grains from the pollen cone to the seed cone. A pollen grain sticks to an adhesive substance near an ovule. It usually enters the pollen chamber of the ovule through an opening called the micropyle. The pollen grain then begins to form a pollen tube. Two sperm cells develop in the tube. After the pollen tube reaches the egg cell, one of the sperm cells fertilizes the egg. The second sperm cell disintegrates. The fertilized egg develops into a sporophyte embryo, and the ovule containing the embryo becomes a seed. The seed falls to the ground and, if conditions are favorable, a new sporophyte begins to grow.

 

In ferns and mosses, the sporophyte and gametophyte generations consist of two greatly different plants. Among ferns, the sporophytes have leaves and are much larger than the gametophytes. Clusters of sporangia called sori form on the edges or underside of each leaf. Spores develop in the sporangia. After the spores ripen, they fall to the ground and grow into barely visible, heart-shaped gametophytes. A fern gametophyte produces both male and female sex cells. If enough moisture is present, a sperm cell swims to an egg cell and unites with it. The fertilized egg then grows into an adult sporophyte. Among mosses, a sporophyte consists of a long, erect stalk with a podlike spore-producing container at the end. The sporophyte extends from the top of a soft, leafy, green gametophyte. It depends on the gametophyte for food and water. The gametophyte is the part of the plant community recognized as moss.

Vegetative propagation. Plants can spread without sexual reproduction. Through vegetative propagation, a part of a plant may grow into a complete new plant. Vegetative propagation can take place because the pieces of the plant form the missing parts by a process called regeneration. Any part of a plant-a root, stem, leaf, or flower-may be propagated into a new plant. A plant may even grow from a single cell of another plant. Propagation occurs most often in plants with stems that run horizontally just above or below the ground. The strawberry plant, for example, sends out long, thin stems called runners that grow along the surface of the soil. The runners, at points where they touch the ground, send out roots that produce plantlets (new leaves and stems). These plantlets are actually part of the parent plant. New plants form only when the plantlets are separated from the parent plant. Ferns, irises, many kinds of grasses, blueberries and some other shrubs, and some species of trees propagate from underground stems.

Many plants that grow as weeds are able to spread rapidly by vegetative propagation. These plants are sometimes difficult to kill because they often can regrow their lost parts by regeneration. For example, a dandelion will regrow new stems and leaves even if only part of its roots are left in the soil. Farmers use vegetative propagation to raise many valuable food crops, such as apples, bananas, oranges, and white potatoes. For example, they cut potatoes into many parts, making sure that each part has at least one eye (bud). Each piece of potato will grow into a new potato plant. Propagation by this method produces new potato plants more quickly than do the seeds of a potato plant. Vegetative propagation is also widely used in gardening. Many plants, including gladioli, irises, lilies, and tulips, are propagated from bulbs or corms. These plants take longer to reach the flowering stage when grown from seeds.

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

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

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

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.

Factors Affecting Plant Growth

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

Hereditary factors. Within the nucleus of all plant cells are tiny bodies called chromosomes that contain hereditary units called genes. These bodies contain "instructions" that direct the growth of the plant. As the cells divide and multiply, the "instructions" are passed on to each new cell. Substances made within a plant also play a part in regulating plant growth. These substances, called hormones, control such activities as the growing of roots and the production of flowers and fruit.

Botanists do not know exactly how all plant hormones work. But they have learned that certain hormones, called auxins, affect the growth of buds, leaves, roots, and stems. Other growth hormones, called gibberellins, make plants grow larger, cause blossoming, and speed seed germination. Still other hormones called cytokinins make plant cells divide.

Environmental factors. All plants need light, a suitable climate, and an ample supply of water and minerals from the soil. But some species grow best in the sun, and others thrive in the shade. Plants also differ in the amount of water they require and in the temperatures they can survive. Such environmental factors affect the rate of growth, the size, and the reproduction of all plants. The growth of plants also is affected by the length of the periods of light and dark they receive. Some plants, including lettuce and spinach, bloom only when the photoperiod (period of daylight) is long. Such plants are called long-day plants. On the other hand, asters, chrysanthemums, and poinsettias are short-day plants. They bloom only when the dark period is long. Still other plants, among them marigolds and tomatoes, are not affected by the length of the photoperiod. They are called day-neutral plants.

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

However, roots demonstrate positive geotropism. Even if a seed or bulb is planted upside down, its roots grow downward-toward the source of gravity. The stem of the same bulb shows negative geotropism by growing upward-away from the source of gravity. Hydrotropism occurs chiefly in roots and is almost always positive.

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

How Plants Grow

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

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

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

The embryo of a seed has all the parts needed to produce a young plant. It may have either one or more cotyledons, which digest food from the endosperm for the growing seedling. The seed absorbs water, which makes it swell. The swelling splits the seed coat, and a tiny seedling appears. The lower part of the seedling, called the hypocotyl, develops into the primary root. This root anchors the seedling in the ground and develops a root system that supplies water and minerals. Next, the upper part of the seedling, called the epicotyl, begins to grow upward. At the tip of the epicotyl is the plumule, the bud that produces the first leaves. In some plants, such as the many kinds of beans, the growth of the epicotyl carries the cotyledons above ground. In corn and other plants, cotyledons remain underground, within the seed. After a seedling has developed its own roots and leaves, it can make its own food. It no longer needs cotyledons to supply nourishment.
Most plants grow in length only at the tips of their roots and branches. The cells in these areas are called meristematic cells. They divide and grow rapidly and develop into the various tissues that make up an adult plant. In trees and other plants that increase in thickness, new layers of cells form between the bark and wood. This area is the cambium. New layers of cells are made as the cambium grows each year. These layers form the woody rings that enable people to tell the age of a tree.
Some kinds of plants, called perennial plants, live for many years. Most perennials produce seeds yearly. Annual plants live only about one year. Biennial plants live for two years. Most annuals and biennials produce seeds only once.

Water movement. Plants must have a continuous supply of water. Each individual plant cell contains a large amount of water. Without this water, the cells could not carry on the many processes that take place within a plant. Water also carries important materials from one part of a plant to another. Most water enters a plant through the roots. Tiny root hairs absorb moisture and certain minerals from the soil by a process called osmosis. In many plants, fungi that grow on the roots help the plants absorb water and minerals. In vascular plants-that is, plants with special conducting tissues-these materials are transported through the xylem tissue of the roots and stems to the leaves. There, water and minerals are used in making food. Water also carries this food through the phloem tissue to other parts of the plant.

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

Photosynthesis is the process by which plants make food. The word photosynthesis means putting together with light. In green plants, sunlight captured by chlorophyll enables carbon dioxide from the air to unite with water and minerals from the soil and create food. This process also releases oxygen into the air. People and animals must have this oxygen to breathe. Most photosynthesis takes place in small bodies called chloroplasts within the cells of plant leaves. These chloroplasts contain chlorophyll, which absorbs sunlight. Energy from the sun splits water molecules into hydrogen and oxygen. The hydrogen joins with carbon from the carbon dioxide to produce sugar. The sugar helps a plant make the fat, protein, starch, vitamins, and other materials that it needs to survive.
Some plants, called parasites and saprophytes, have little or no chlorophyll and cannot produce their own food through photosynthesis. These plants must rely on outside sources for food. Parasites attach to living plants and take the nutrients they need from these plants. Saprophytes grow on dead and decaying organisms, or use organic substances produced by living organisms for food. Mistletoe and dodder are common parasites found in many parts of the world. Mistletoe grows on the trunks and branches of many trees. It is called a partial parasite because it also makes some of its own food. Indian pipe is a saprophyte that grows near fungi. It uses organic materials produced by fungi for food. A plant called giant rafflesia is a parasite that grows on the roots and stems of other plants. It bears the largest flower of any known plant. Rafflesia flowers may grow over 3 feet (91 centimeters) wide.

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

Populations and Communities

A population is a group of the same species that lives in an area at the same time. For example, all the moose on Isle Royale make up a population, as do all the spruce trees. Ecologists determine and analyze the number and growth of populations and the relationships between each species and the environmental conditions.

Factors that control populations. The size of any population depends upon the interaction of two basic forces. One is the rate at which the population would grow under ideal conditions. The second is the combined effect of all the less-than-ideal environmental factors that limit growth. Such limiting factors may include low food supply, predators, competition with organisms of the same or different species, climate, and disease.

The largest size of a particular population that can be supported by a particular environment has been called the environment's carrying capacity for that species. Real populations normally are much smaller than their environment's carrying capacity for them because of the effects of adverse weather, a poor breeding season, hunting by predators, or other factors.

Factors that change populations. Population levels of a species can change considerably over time. Sometimes these changes result from natural events. For example, a change in rainfall may cause some populations to increase and others to decrease. Or the introduction of a new disease can severely decrease the population of a plant or animal species. In other cases, changes may result from human activities. For example, power plants and automobiles release acidic gases into the atmosphere, where they may mix with clouds and fall to earth as acid rain. In some regions that receive large amounts of acid rain, fish populations have declined dramatically.

Communities

A community is a group of animal and plant populations living together in the same environment. Wolves, moose, beavers, and spruce and birch trees are some of the populations that make up the forest community of Isle Royale. Ecologists study the roles different species play in their communities. They also study the different types of communities, and how they change. Some communities, such as an isolated forest or meadow, can be identified easily. Others are more difficult to define.

A community of plants and animals that covers a large geographical area is called a biome. The boundaries of different biomes are determined mainly by climate. The major biomes include deserts, forests, grasslands, tundra, and several types of aquatic biomes. 

The role of a species in its community is called its ecological niche. A niche consists of all the ways that a species interacts with its environment. It includes such factors as what the species eats or uses for energy; what predators it has; the amounts of heat, light, or moisture it needs; and the conditions under which it reproduces. Ecologists have long noted that many species occupy a highly specialized niche in a given community. Various explanations have been proposed for this. Some ecologists feel that it results from competition-that if two species try to "fill" the same "niche," then competition for limited resources will force one of the species out. Other ecologists maintain that a species that occupies a highly specialized niche does so because of the rigid physiological demands of that particular role in the community. In other words, only one species occupies the niche not because it has out-competed other species, but because it is the only member of the community physiologically capable of playing that role.

Changes in communities occur over time in a process called ecological succession. This process occurs as a series of slow, generally predictable changes in the number and kinds of organisms in an area take place. Differences in the intensity of sunlight, protection from wind, and changes in the soil may alter the kinds of organisms that live in an area. These changes may also alter the number of populations that make up the community. Then, as the number and kinds of species change, the physical and chemical characteristics of the area undergo further changes. The area may reach a relatively stable condition called the climax community, which may last hundreds or even thousands of years. 

Ecologists distinguish two types of succession-primary and secondary. In primary succession, organisms begin to inhabit an area that had no life, such as a new island formed by a volcanic eruption. Secondary succession takes place after an existing community suffers a major disruption-for example, after a climax forest community is destroyed by fire. In this example, a meadow community of wildflowers and grasses will grow first, followed by a community of shrubs. Finally trees will reappear, and the area will eventually become a forest once more, until it is disturbed again. Thus, the forces of nature ultimately cause even climax communities to change. Increasingly, ecologists view fires and other large natural disturbances as acceptable and even desirable.

 

Ecology

Ecology, is the branch of science that deals with the relationships living things have to each other and to their environment. Scientists who study these relationships are called ecologists.

The world includes a tremendous variety of living things, from complex plants and animals to simpler organisms, such as fungi, amebas, and bacteria. But whether large or small, simple or complex, no organism lives alone. Each depends in some way upon other living and nonliving things in its surroundings. For example, a moose must have certain plants for food. If the plants in its environment were destroyed, the moose would have to move to another area or starve to death. In turn, plants depend upon such animals as moose for the nutrients (nourishing substances) they need to live. Animal wastes and the decay of dead animals and plants provide many of the nutrients plants need.

The study of ecology is important because our survival and well-being depend on ecological relationships around the world. Even changes in distant parts of the world and its atmosphere affect us and our own environment.

Although ecology usually is considered a branch of biology, ecologists must employ such disciplines as chemistry, physics, and computer science. They also rely on such fields as geology, meteorology, and oceanography to study air, land, and water environments and their interactions. This multidisciplinary approach helps ecologists understand how physical environments affect living things. It also helps them assess the impact of environmental problems, such as acid rain or the greenhouse effect.

Ecologists study the organization of the natural world on three main levels: (1) populations, (2) communities, and (3) ecosystems. They analyze the structures, activities, and changes that take place within and among these levels. Ecologists normally work out of doors, studying the operations of the natural world. 

Ecology, They often conduct field work in isolated areas, such as islands, where the relationships among the plants and animals may be simpler and easier to understand. For example, the ecology of Isle Royale, an island in Lake Superior, has been studied extensively. Ecology, Many ecological studies focus on solving practical problems. For example, ecologists search for ways to curb the harmful effects of air and water pollution on living things.
 

The history of forests

The first forests developed in marshlands about 365 million years ago, toward the end of the Devonian Period. They consisted of tree-sized club mosses and ferns, some of which had trunks nearly 40 feet (12 meters) tall and about 3 feet (1 meter) thick. These forests became the home of early amphibians and insects.

By the beginning of the Carboniferous Period-about 360 million years ago-vast swamps covered much of North America. Forests of giant club mosses and horsetails up to 125 feet (38 meters) tall grew in these warm swamps. Ferns about 10 feet (3 meters) tall formed a thick undergrowth that sheltered huge cockroaches, dragonflies, scorpions, and spiders. In time, seed ferns and primitive conifers developed in the swamp forests.

When plants of the swamp forests died, they fell into the mud and water that covered the forest floor. The mud and water did not contain enough oxygen to support decomposers. As a result, the plants did not decay but became buried under layer after layer of mud. Over millions of years, the weight and pressure on the plants turned them into great coal deposits.

Later forests. As the Mesozoic Era began, about 248 million years ago, severe changes in climate and in the earth's surface wiped out the swamp forests. In the new, drier environment, gymnosperm trees became dominant. Gymnosperms are plants whose seeds are not enclosed in a fruit or seedcase. Such trees included seed ferns and primitive conifers like those that grew in the swamp forests. They also included cycad and ginkgo trees, which became widespread. Gymnosperm trees formed forests that covered much of the earth. Amphibians, insects, and large reptiles lived in these forests.

 

The first flowering plants appeared during the early Cretaceous Period, sometime after 145 million years ago. Flowering plants, which are called angiosperms, produce seeds enclosed in a fruit or seedcase. Many angiosperm trees became prominent in the forests. They included magnolias, maples, poplars, and willows. Flowering shrubs and herbs became common undergrowth plants.

At the start of the Cenozoic Era, about 65 million years ago, the earth's climate turned cooler. Magnificent temperate forests then spread across North America, Europe, and Asia. The forests included a wealth of flowering broadleaf trees and needleleaf conifers. Many birds and mammals lived in these forests.

Modern forests. The earth's climate continued to turn colder. By about 2.4 million years ago, the first of several great waves of glaciers had begun to advance over much of North America, Europe, and Asia. By the time the last of these glaciers had retreated-about 11,500 years ago-the ice sheets had destroyed large areas of the temperate forests in North America and Europe. Only the temperate forests of southeastern Asia remained largely untouched. 

The forests of the world took on their modern distribution after the last of the glaciers retreated. For example, the great boreal forests developed across northern Europe and North America. But the world's forest regions are not permanent. Today, for instance, temperate forests are invading the southern edge of the boreal region. Another ice age or other dramatic environmental changes could greatly alter the world's forests.

Deforestation

Human activities have had tremendous impact on modern forests. Since agriculture began about 11,000 years ago, large forest areas have been cleared for farms and cities. Beginning in the 1800's, great expanses of forest have also been eliminated because of logging and industrial pollution. The destruction and degrading of forests is called deforestation.

Severe deforestation now occurs around the world, even in the most remote rain forests and boreal forests. Until the late 1940's, rain forests covered about 8.7 million square miles (22.5 million square kilometers) of the earth's land. Today, they cover less than half that area. Millions of acres or hectares of rain forests are destroyed each year.

Since 1800, huge areas of temperate forests have also been cleared. Many parts of eastern North America, for example, have less than 2 percent of even degraded forests remaining.

Industrial pollution is a chief cause of deforestation. Factories often release poisonous gases into the air and dangerous wastes into lakes and rivers. Air pollutants may combine with rain or other precipitation and fall to earth as acid rain. Acid rain and polluted bodies of water can restrict plant growth or even kill most plants in a forest.

Massive deforestation has made many remaining forest tracts small, isolated islands. As forests become smaller, their ability to sustain the full variety of plant species decreases. Many forests are so seriously degraded by logging activities that they fail to regenerate replacement forests.

Loss of forests has helped create many ecological problems. For example, rain water normally trapped by the forests is causing more floods around the world. In addition, as forest areas decrease or degrade, the production of oxygen from photosynthesis also decreases. Oxygen renewal is vital to the survival of oxygen-breathing organisms. At the same time, as less carbon dioxide is taken up by photosynthesis, the amounts of carbon dioxide released into the air increases. Thus more heat from the sun is trapped near the earth's surface instead of being reflected back into space. Many scientists believe that this greenhouse effect is causing a steady warming that could lead to threatening climatic conditions. 

The destruction of forest ecosystems also destroys the habitats of many living creatures. Countless species of animals and plants have been wiped out by deforestation, and more are killed each year at an increasing rate.

To combat these problems, people and governments have been seeking out and protecting old growth forests that remain undisturbed by humans. Such protection enables scientists to conduct long-term research on how old growth forests sustain the variety of plants and animals that live there.

Forest succession

In forests and other natural areas, a series of orderly changes may occur in the kinds of plants and animals that live in the area. This series of changes is called ecological succession. Areas undergoing succession pass through one or more intermediate stages until a final climax stage is reached. Forests exist in intermediate or climax stages of ecological succession in a great number of places.

To illustrate how a forest develops and succession occurs, let us imagine an area of abandoned farmland in the Southeastern United States. The abandoned land will first support communities of low-growing weeds, insects, and mice. The land then gradually becomes a meadow as grasses and larger herbs and shrubs begin to appear. At the same time, rabbits, snakes, and ground-nesting birds begin to move into the area.

In a few years, young pine trees stand throughout the meadow. As the trees mature, the meadow becomes an intermediate forest of pines. The meadow herbs and shrubs die and are replaced by plants that grow better in the shade of the pine canopy. As the meadow plants disappear, so do the food chains based on them. New herbivores and predators enter the area, forming food chains based on the plant life of the pine forest.

Years pass, and the pines grow old and large. But few young pines grow beneath them because pine seedlings need direct sunlight. Instead, broadleaf trees-particularly oaks-form the understory. As the old pines die, oaks fill the openings in the canopy. Gradually, a mixed deciduous-evergreen forest develops.

But the succession is still not complete. Young oaks grow well in the shade of the canopy, but pines do not. Therefore, a climax oak forest may eventually replace the mixed forest. However, pine wood is more valuable than oak wood. For this reason, foresters in the Southeastern United States use controlled fires to check the growth of oaks and so prevent climax forests from developing. 

Different successional series occur in different areas. In southern boreal regions, for instance, balsam fir and white spruce dominate the climax forests. If fire, disease, or windstorms destroy a coniferous forest, an intermediate forest of trembling aspen and white birch may develop in its place. These deciduous trees grow better in direct sunlight and on unprotected, bare ground than do fir and spruce.

The aspen-birch forest provides the protection young boreal conifers need, and soon spruce and fir seedlings make up most of the understory. In time, these conifers grow taller than the aspen and birch trees. Deciduous species cannot reproduce in the shade of the new canopy, and eventually the climax forest of fir and spruce trees is reestablished.

The Structure of Forests

Every forest has various strata (layers) of plants. The five basic forest strata, from highest to lowest, are (1) the canopy, (2) the understory, (3) the shrub layer, (4) the herb layer, and (5) the forest floor.

The canopy consists mainly of the crowns (branches and leaves) of the tallest trees. The most common trees in the canopy are called the dominant trees of the forest. Certain plants, especially climbing vines and epiphytes, may grow in the canopy. Epiphytes are plants that grow on other plants for support but absorb from the air the water and other materials they need to make food.

The canopy receives full sunlight. As a result, it produces more food than does any other layer.

In some forests, the canopy is so dense it almost forms a roof over the forest. Fruit-eating birds, and insects and mammals that eat leaves or fruit, live in the canopy.

The understory is made up of trees shorter than those of the canopy. Some of these trees are smaller species that grow well in the shade of the canopy. Others are young trees that may in time join the canopy layer. Because the understory grows in shade, it is not as productive as the canopy. However, the understory provides food and shelter for many forest animals.

The shrub layer consists mainly of shrubs. Shrubs, like trees, have woody stems. But unlike trees, they have more than one stem, and none of the stems grows as tall as a tree. Forests with a dense canopy and understory may have only a spotty shrub layer. The trees in such forests filter out so much light that few shrubs can grow beneath them. Most forests with a more open canopy and understory have heavy shrub growth. Many birds and insects live in the shrub layer.

The herb layer consists of ferns, grasses, wildflowers, and other soft-stemmed plants. Tree seedlings also make up part of this layer. Like the shrub layer, the herb layer grows thickest in forests with a more open canopy and understory. Yet even in forests with dense tree layers, enough sunlight reaches the ground to support some herb growth. The herb layer is the home of forest animals that live on the ground. They include such small animals as insects, mice, snakes, turtles, and ground-nesting birds and such large animals as bears and deer. 

The forest floor is covered with mats of moss and with various objects that have fallen from the upper layers. Leaves, twigs, and animal droppings-as well as dead animals and plants-build up on the forest floor. Among these objects, an incredible number of small organisms can be found. They include earthworms, fungi, insects, and spiders, plus countless bacteria and other microscopic life. All these organisms break down the waste materials into the basic chemical elements necessary for new plant growth.

 

The life of the forest

Forests are filled with an incredible variety of plant and animal life. For example, scientists recorded nearly 10,500 kinds of organisms in a deciduous forest in Switzerland. The number of individual plants and animals in a forest is enormous.

All life in the forest is part of a complex ecosystem, which also includes the physical environment. Ecologists study forest life by examining the ways in which the organisms interact with one another and their environment. Such interactions involve (1) the flow of energy through the ecosystem, (2) the cycling of essential chemicals within the ecosystem, and (3) competition and cooperation among the organisms.

The flow of energy. All organisms need energy to stay alive. In forests, as in most other ecosystems, life depends on energy from the sun. However, only the green plants in the forest can use the sun's energy directly. Through a process called photosynthesis, they use sunlight to produce food.

All other forest organisms rely on green plants to capture the energy of sunlight. Green plants are thus the primary producers in the forest. Animals that eat plants are known as primary consumers or herbivores. Animals that eat herbivores are called secondary consumers or predators. Secondary consumers themselves may fall prey to other predators, called tertiary (third) consumers. This series of primary producers and various levels of consumers is known as a food chain.

In a typical forest food chain, tree leaves (primary producers) are eaten by caterpillars (primary consumers). The caterpillars, in turn, are eaten by shrews (secondary consumers), which are then eaten by owls (tertiary consumers). Energy, in the form of food, passes from one level of the food chain to the next. But much energy is lost at each level. Therefore, a forest ecosystem can support, in terms of weight, far more green plants than herbivores and far more herbivores than predators.

The cycling of chemicals. All living things are made up of certain basic chemical elements. The supply of these chemicals is limited, and so they must be recycled for life to continue.

The decomposers of the forest floor promote chemical recycling. Decomposers include bacteria, earthworms, fungi, and some insects. They obtain food by breaking down dead plants and the wastes and dead bodies of animals into their basic chemicals. The elements pass into the soil, where they are absorbed by the roots of growing plants. Without decomposition, the supply of such essential elements as nitrogen, phosphorus, and potassium would soon be exhausted.

Some chemical recycling does not involve decomposers. Green plants, for example, release oxygen during photosynthesis. Animals-and plants as well-need this chemical to oxidize (burn) food and so release energy. In the oxidation process, animals and plants give off carbon dioxide, which the green plants need for photosynthesis. Thus the cycling of oxygen and carbon dioxide works together and maintains a steady supply of the two chemicals.

Competition and cooperation. Every forest animal and plant must compete with individuals of its own and similar species for such necessities as nutrients, space, and water. For example, red squirrels in a boreal forest must compete with one another-and with certain other herbivores-for conifer seeds, their chief food.

Similarly, the conifers compete with one another and with other types of plants for water and sunlight. This competition helps ensure that the organisms best adapted to the forest will survive and reproduce.

Cooperation among the organisms of the forest is common. For many species, cooperation is necessary for survival. For example, birds and mammals that eat fruit rely on plants for food. But the plants, in turn, may depend on these animals to help spread their seeds. Similarly, certain microscopic fungi grow on roots of trees. The fungi obtain food from the tree, but they also help the tree absorb needed water and nutrients.