Gayle Savoy, age 14, of St. Augustine, Fla., for her question:
HOW DOES A LEAF PRODUCE FOOD?
The leaf is the main food producing structure of a plant. Almost ail kinds of leaves have the same basic job of serving as a "factory" for making food out of carbon dioxide, light and water.
An upper epidermis or outer skin covers the top surface of a leaf while the lower epidermis covers the underside. You'd need a microscope to see it because it is very thin, but between the upper and lower epidermis is a low roofed "room" that operates as a "factory" that makes food for the plant.
A leaf makes food by building sugars from water and carbon dioxide, when light and a green substance called chlorophyll are present. Most kinds of leaves work best in bright light. The stronger the light, the more sugars they produce.
Water and air are important raw materials for the leaf's food "factory." Water absorbed by the roots moves up through the plant and flows into the leaf through the petiole or stem. Air comes in through countless pores or openings called stomata. Most stomata are in the lower epidermis. Light provides the power for the "factory." It strikes the top of the leaf and filters through the upper epidermis into the cells below.,
Leaves have two kinds of cells that actually make the food. One kind, the palisade cells, lie just beneath the upper epidermis. The other kind, the spongy cells, lie just below the palisade cells.
The cells use light energy to split water molecules into hydrogen and oxygen. The hydrogen is then used to convert carbon dioxide into sugars in a process called photosynthesis.
Stomata are tiny openings that serve as valves that control the flow of air into the leaf. An apple tree leaf may have 47,000 stomata per square inch on its surface while an oak tree leaf may have over 100,000 stomata per square inch.
When the stomata are open, water evaporates from the surface of the spongy leaf cells and escapes through the pores into the air. The process by which water is lost through the leaves is called transpiration. Water lost from the leaf is replaced by more water from the roots.
Sap, a watery solution of mineral salts and food needed by the growing plant, keeps flowing through the plant, thanks to transpiration.
With all its equipment and supplies of raw materials, a leaf could not make food without the chemical compound chlorophyll. Chlorophyll is arranged in tiny packages called chloroplasts, which float inside the palisade cells and spongy cells of the leaf.
Man puts leafy food factories to work for him when he plants vegetable gardens and orchards. It takes about 50 leaves to make one medium sized apple. Thirty leaves make the food stored in an average sized peach. Fifteen leaves can make the food packed into a cluster of grapes. It takes about a dozen banana leaves to make a bunch of bananas.
The food making process in leaves goes on at ordinary temperature and presures when chlorophyll, water, carbon dioxide and light are present.