Leaf | Research & Encyclopedia Articles

Leaf

The leaf is the principal energy-capturing and food-producing organ of plants. Leaves are attached to and supported by the plant stem, which also provides the leaf with water and inorganic nutrients from the soil. They usually have a flattened surface, called a blade (or lamina) which absorbs radiant energy from the sun, and a slender stalk called the petiole, which supports the blade and connects it to the stem. Leaves provide nourishment to the plant by converting light energy into chemical energy used for growth and development. The conversion process is called photosynthesis and requires uptake of carbon dioxide gas from the atmosphere and the subsequent release of oxygen. Tiny pores, called stomata, in the surface of the leaf facilitate this exchange of gases. The stomata can open and close as needed to support rapid movement of gases into and out of the leaf. The extensive surface area of the leaf tends to result in extensive water loss into the atmosphere. To minimize this loss, and reduce the possibility of wilting, the leaf is covered with a tight outer layer of cells called an epidermis. The epidermis is covered with a waxy coating called a cuticle. Because the epidermis and cuticle impede gas exchange, stomata are crucial for photosynthesis. When the stomata are open, they also allow water to escape to the atmosphere. To minimize water loss, stomata tend to close at night when photosynthesis is not possible, and open only during the day when rapid gas exchange is needed. Under unusually dry conditions when soil moisture is inadequate the plant may become water-stressed. The stomata may then close during the day the to prevent wilting. Photosynthesis is then impeded also, and plant growth is reduced.

The main light-absorbing compound in leaves is called chlorophyll.

It is this pigment that gives plants their characteristic green color. It is found in organelles called chloroplasts in the mesophyll cells in the interior of the leaf. With the aid of internal enzyme systems, chlorophyll transfers absorbed light energy to sugars, starch, and other energy storing carbon-based compounds. These compounds are then used to support plant growth and development. The oxygen produced in photosynthesis is released into the atmosphere where it replaces oxygen used by animal and plant respiration and by combustion. In the autumn chlorophyll is bleached and disappears from leaves. Leaves then change color to yellow, orange or red, as other pigments such as carotenoids and anthocyanins, previously masked by high concentrations of chlorophyll, are revealed.

Leaves originate in the apical (top) bud of the growing plant, together with the tissues of the developing stem. They vary greatly in form, structure, and arrangement. These differences are useful in differentiating among plant species. Leaves of monocotyledons tend to have internal conducting tissue (veins) arranged in parallel patterns, whereas dicotylendons are more apt to have a net-like pattern. Leaves may be arranged on the stem in several different ways. Some plants have leaves set opposite one another on the stem, others have three or more leaves in a whorl at each node, and still others may have an alternating or spiral arrangement. Leaves of dicotyledons may be simple or compound. Simple leaves contain a single undivided blade, while compound leaves are divided into leaflets. Some leaves issue directly from the main stem (lacking a connecting support or petiole) and are said to be sessile. The nature of the margin of the leaf blade is highly variable. Some leaves have a smooth edge; others are toothed, and still others deeply lobed. In some species, leaves may be reduced to spines as in the Japanese barberry, or to the scale-like food containing structures such as those found in onion bulbs.