Teresa Foley, age 11, of St. Catharines, Ontario, Canada, for her question:
WHAT ARE LEAVES?
Some trees live for thousands of years, but each year the life giving part the leaf is replaced. A leaf is fresh and young most of the time. Long before it is time for a tree's leaves to fall, the next year's crop of leaves has started to form. And just as soon as a new leaf is fully spread out, a bud for the following year's growth appears above the stem.
Almost all leaves have one basic job: to produce food for the plant to which they are attached. Each leaf is much like a factory as it makes food out of carbon dioxide, light and water.
As a leaf makes food, it is stored in the plant's stems, roots, fruit and seeds. The food provides energy for the seeds to sprout, for the flowers to bloom and for the fruits to form. Without this food from the leaves, plants could not live.
The number of leaves on a plant can vary from a single leaf to more than a million leaves. They come in all sizes and almost every possible shape.
A banana leaf can grow up to 10 feet in length, but that isn't the record holder. A relative of the banana in Madagascar, called the traveler's tree, has leaves 20 feet long. And a kind of South American palm tree has leaves more than 26 feet long. But topping them all is a type of African raffia palm with leaves up to 50 feet long.
When light and a green substance called chlorophyll are present, a leaf will make food by building sugars from water and carbon dioxide. Most leaves work best in bright light although the dim light of shady places is best for others, such as pine trees. water and air are important raw materials for the leaf's food making job. Water absorbed by the roots of the tree or plant move into the leaf after traveling up the stem and branch. Air enters the leaf through pores called stomata.
Light provides the power for a leaf's food making job. Light strikes the top of the leaf and then filters through the upper cells to those below.
Air, in the form of carbon dioxide, is most important for food production. Lots of stomata on the leaves open in the day to let air in and close at night. An apple tree leaf may have as many as 47,000 stomata per square inch and on an oak tree leaf there will be more than 100,000 per square inch.
When the stomata of a leaf are open, water evaporates from the surface on the spongy leaf cells and escapes through the pores into the air. Water lost from the leaf is replaced by more water from the roots.
The movement of water through a leaf, called transpiration, helps to keep a stream of sap flowing through the plant.