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.