Leaf, part of a plant that serves primarily as the plant's food-making organ in a process called photosynthesis. Leaves take part in other plant functions as well, including transpiration and guttation, both of which remove excess water from the plant, and respiration, the process by which a plant obtains oxygen and energy. Leaves also may store food and water and provide structural support.
A leaf is an extension of a plant's stem. Although most leaves are flat, broad, or bladelike, they also may be many other shapes, including round, oval, or feathery. In general, the leaves of trees such as hardwoods tend to be broad and relatively large, and the leaves of conifers, or cone-bearing trees, are usually small and needlelike in shape. In size, leaves range from only several millimeters (a fraction of an inch) long, as in the water plant Elodea, to 15 to 18 m (15 to 60 ft) long, as in some palm trees.
Green leaves derive their color from a green pigment called chlorophyll. The presence of additional pigments causes other leaf colors such as red in coleus and purple in cabbage. In temperate regions of the world, the leaves of some plants change color in autumn. Leaves of most garden plants turn yellow in the autumn, but those of many trees take on brilliant orange or red colors.
Most plants whose leaves change color also lose their leaves in the autumn. Such plants are called deciduous. In other plants, such as laurels and pines, the leaves do not change color and do not fall off in autumn. Such plants are called evergreens.
II THE PARTS OF A TYPICAL LEAF
The typical green leaf is called a foliage leaf. It usually consists of two basic parts: a petiole and a blade.
The petiole is a stalklike structure that supports the leaf blade on the stem. It also serves as a passageway between the stem and the blade for water and nutrients. Another function of the petiole is to move the leaf into the best position for receiving sunlight. Most petioles are long, narrow, and cylindrical.
Many plants, such as grasses and corn, do not have petioles. In these plants the base of the blade is attached directly to the stem—the base encircles the stem as a sheath. Such leaves are called sessile leaves.
The leaf blade is usually a thin, flat structure. Its margins, or edges, may be smooth, as in the dogwood; jagged or toothed, as in the elm; or lobed, as in the oak and maple. The surface of the blade may be smooth, fuzzy, sticky, dull, or shiny. In most plants the leaves have a single blade and are referred to as simple. In other plants, such as clover, the blade is divided into separate leaflets. This kind of leaf is called a compound leaf. Most of the functions carried on by leaves take place in the blade.
The blade consists of an upper and lower epidermis and a spongy layer of tissue, called the mesophyll. Running through the mesophyll is a branching system of veins.
The epidermis is the leaf blade's skin. It is a thin, usually transparent, colorless layer of cells that covers both the upper and lower surfaces of the blade. The epidermis prevents the leaf from losing excessive amounts of water and protects it against injury.
In most plants the epidermis is covered with cutin, a waxy substance secreted by the epidermal cells. The layer of cutin, called the cuticle, is responsible for the glossy appearance of some leaves. The cuticle gives the leaf additional protection by slowing down the rate at which water is lost. Generally, the cuticle is thinner on the epidermis on the underside of the leaf than on the upper epidermis, which is exposed to the sun.
In many kinds of leaves, hairs grow from the epidermis. The soft hairs of plants such as the mullein give the leaves a woolly or feltlike texture. In some plants the epidermal hairs secrete fluids. For example, in geraniums and petunias the hairs secrete a fluid that gives the leaves a clammy texture. The strong-smelling oils of the peppermint and spearmint plants come from epidermal hairs. In other plants, such as the nettle, the epidermal hairs are stiff and contain a poisonous fluid that produces a skin irritation when a person is pricked by them.
B Guard Cells
Scattered throughout the epidermis are pairs of bean-shaped cells, called guard cells. Guard cells contain chloroplasts, which are tiny granules filled with the green pigment chlorophyll. Chlorophyll gives leaves their characteristic green color. Chloroplasts enable leaves to carry on photosynthesis because they are able to absorb carbon dioxide and sunlight, which are required for the food-making process. In response to heat and light, each pair of guard cells pulls apart, and a tiny pore forms between them. The pores, called stomata, open to the outside atmosphere.
When the stomata are open, carbon dioxide and oxygen pass either in or out—when carbon dioxide enters, it takes part in photosynthesis, the food-making process that releases oxygen as a waste product. This oxygen passes out of the leaf. At the same time, oxygen also enters the leaf, where it takes part in respiration, a process that forms carbon dioxide as a waste product. This carbon dioxide passes out through the stomata. Water also passes out of the open stomata in the form of a vapor. This process is called transpiration. Generally, there are more stomata on the under surface of a leaf than on the upper surface. This prevents water from evaporating too quickly or in excessive amounts from the leaf's upper side, which is exposed to the sun. Stomata close at night, providing another level of water conservation.
C Water Pores
In addition to the stomata, many kinds of leaves have large specialized water pores in their epidermis. These pores, called hydathodes, permit guttation, the process by which a plant loses liquid water. Unlike the stomata, hydathodes remain open all the time.
Guttation takes place only when water is being rapidly absorbed by the roots, such as after a heavy rainfall, and when transpiration slows down, as on cool, humid nights. When these conditions occur together, droplets of water can be seen on the leaf early in the morning before they evaporate in the heat of the day. Unlike dew, which condenses on leaves from water vapor in the air and covers the entire leaf surface, guttation droplets form only on the edges and tips of leaves. Generally, the droplets are noticeable only on the leaves of strawberries and a few other kinds of plants.
The mesophyll, sandwiched between the upper and lower epidermis, consists of many thin-walled cells that are usually arranged in two layers. The palisade layer is next to the upper epidermis. It consists of cylindrical cells that are packed closely together. Next to the palisade layer and making up most of the thickness of the leaf blade is the spongy layer. The spongy layer consists of roundish cells that are packed loosely together and have numerous air spaces between them. In most plants the spongy layer extends down to the lower epidermis. However, in certain grasses, irises, and other plants whose leaves grow straight up and down, the spongy layer is wedged between two palisade layers of mesophyll. Like the guard cells, all the cells of the mesophyll contain chloroplasts.
Running through the middle of the mesophyll and branching out to all of its cells are veins. The veins extend into the petiole and connect with other veins in the stem of the plant. A major function of the veins is to help support the leaf blade. Each type of plant has a characteristic pattern of veins forming lines and ridges in the blade.
The veins of a leaf are made up of two specialized tissues, xylem and phloem. Xylem usually forms the upper half of the vein. It consists of tubular open-ended cells that are arranged end to end. The walls of the cells are thick and rigid. Xylem conducts water and dissolved minerals to the leaf blade from the rest of the plant.
Phloem lies on the underside of the vein. It is made up of thin-walled tubular cells with tiny openings at their ends, somewhat like a sieve. These cells are also arranged end to end. Phloem carries food manufactured in the blade to the rest of the plant.
III LEAF GROWTH AND LEAF FALL
A leaf has a limited life span, usually living for only a single growing season in most deciduous plants and seldom more than a few years in evergreen plants. In temperate regions, leaves develop and grow during spring and early summer. In autumn they grow old, change color, and die. In nonwoody plants (low in xylem) the leaves wither and fall away because of decay and various external conditions. Woody plants (rich in xylem) lose their leaves as a result of characteristic changes in the base of the leaf. In tropical regions that have distinct wet and dry seasons, the formation and fall of leaves depend on moisture conditions rather than temperature. Contrary to popular belief, evergreen plants also shed their leaves. However, evergreens are never bare because the old leaves are pushed out only as new leaves develop.
All leaves develop from buds, which are located at the nodes, or joints, of a plant stem and at the end of a plant stem. Contained in the buds are areas of rapidly growing tissue, called the meristem. The meristem gives rise to the first recognizable signs of the leaf. In the spring the buds shed their outer covering and open, exposing the leaves.
As leaves develop, they are arranged on the stem in one of three ways: alternate, opposite, or whorled. The arrangement provides an equal distribution of leaf weight on the stem. It also prevents overlapping so that each leaf can receive adequate sunlight.
B Color Changes
In addition to chlorophyll, leaf cells also may contain other pigments. These pigments account for the color of autumn leaves. Among the pigments found in leaves are yellow xanthophylls, yellowish-orange carotenes, and red and purple anthocyanins. Leaves also may contain tannins, which give them a golden-yellow color in autumn.
Like chlorophyll, xanthophylls and carotenes are contained in tiny granules in some leaf cells. Although these pigments are present throughout the leaf's lifetime, their colors are usually masked by the green of chlorophyll. In the autumn, however, chlorophyll production decreases, and the yellow and orange pigments become visible in the leaves. Eventually all pigment production stops, and the leaves turn brown.
Unlike xanthophylls and carotenes, anthocyanins are not contained in granules but are dissolved in the liquid part of leaf cells. In some plants, such as coleus and red cabbage, anthocyanins are always present, giving the leaves a reddish or purplish color. In other plants, anthocyanins are not present throughout the life of the leaf, but are produced only under certain conditions. In oak and maple leaves, for example, sugar accumulates in autumn. This accumulation is believed to result in the formation of anthocyanins and the production of vivid colors in the leaves.
C Leaf Fall
The leaves of evergreens continue to function and manufacture food throughout the year. In deciduous plants, however, the leaves stop functioning in the autumn and drop off. Leaves may be killed by frost, but changes due to age and growing conditions occur well before then. Decreased day length, reduced light intensity, lower temperatures, lack of water, and decrease of growth-promoting substances in the plant all contribute to the decline of the leaves. The changes start in the weakest part of the petiole, at the base. During autumn the cells in the base of the petiole begin to disintegrate and die. As a result, the leaf blade is supported only by the veins in the petiole. Soon the vascular bundles become plugged, decreasing the flow of water, food, and minerals to and from the leaf blade. When the blade is disturbed, as by wind, it breaks off the plant at the base of the petiole.
Unlike leaf-bearing plants, animals cannot manufacture their own food. For this reason, animals must receive their nourishment, either directly or indirectly, from leaf-bearing plants. For example, cattle, sheep, and horses eat the leaves of grasses and other plants. These animals, in turn, are consumed by various carnivorous animals, including humans. Humans also eat many kinds of leaves directly, including artichoke, cabbage, lettuce, and spinach.
In addition to being a source of food, leaves provide many useful products. For example, the leaves of tea plants are made into a beverage, and the leaves of thyme, sage, and parsley are used for seasoning foods. Tobacco leaves may be smoked or chewed. Drugs are obtained from the leaves of foxglove, witch hazel, senna, and many other plants. Oils extracted from the leaves of geranium and citronella plants are used in manufacturing perfumes and soaps, and oils from mint and wintergreen leaves are made into flavoring extracts. Tannins, chemical substances used in preparing leather, are derived from sumac leaves, and dyes are made from indigo and henna leaves. The leaves of many plants may be used as fertilizer.