Mary Lynn Gatte, age 14, of Youngstown, Ohio, for her question:

HOW DOES A TREE USE FOOD AND WATER?

Trees show little change from day to day, but they are constantly growing and changing. They don't have nervous systems but they certainly respond to stimuli. The leaves of a tree follow the sun. Where there's a strong wind, trees grow more on one side than the other. They often develop twisted limbs and contorted bases where roots cling in the ground. In swampy land, trees often grow a network of props to help anchor the trunk.

Sap in a tree is much like the blood system in a person. The sap flows upward from the roots to the uppermost branches, and then back down again to the ground. Trees do not have hearts to pump the sap but they do have a pumping system. It is a suction pump, not a forcing pump as we find in humans.

Millions of hairlike rootlets constantly absorb water from the soil. Each hair is a tentacle made of a fragile membrane which sucks in water and minerals that have dissolved in the ground. The material becomes sap  and, once inside the membrane, is pulled up from the hair roots into the small roots, then to larger roots and finally into the trunk of the tree. It then enters a steady upward flowing stream.

A tree's trunk is filled with threadlike tubes that run from the roots to the last leaf on the smallest twig. As moisture in the leaves is evaporated by the sun and air through openings called stomata, it is replaced by the moisture blow it. In this way,. a steady suction is maintained.

Not all of a tree's sap turns to vapor and is evaporated by the stomata. Some, charged with elements that feed the tree which it picked up in the leaf cells, seeps back down through cells of bark and wood. As it moves downward, it leaves food elements which allow the tree to live and continue to grow the tree's leaves, through a process called photosynthesis, make food elements that keep a tree alive and growing. A green substance called chlorophyll helps to trap energy from the rays of the sun. Then it uses this energy to break down molecules of carbon dioxide and water, putting the atoms tgoether again in the form of a food element, sugar. The light energy that was trapped by the chlorophyll then exists as chemical energy in the sugar. A tree's leaf in the sunlight takes in carbon dioxide from the air by way of the stomata. It removes oxygen and hydrogen, as vapor, from the sap that has come up from the roots.

Nothing else goes into a tree except a very small quantity of minerals from the water in the earth. Rain does not enter through the stomata or pores. It gets into a tree only by being absorbed by the rootlets.