Sam Anderson, age 11, of Omaha, Neb., for his question:
HOW DOES WATER GET TO THE TOP OF A TREE?
A tree has three main parts: a trunk and branches, the leaves and the roots. The branches and leaves together are called the crown. The roots of most trees are hidden in the ground, and they may take up as much space as the trunk and crown do above the ground. Other very important parts of a tree include the fruit and flowers.
Roots of a tree are actually long, underground branches. They have exactly the same layers of tissue as the trunk.
The trunk of a tree is anchored in the ground by roots. But the root's main job is to absorb water with dissolved minerals from the soil.
A tree's main roots branch out into small roots, which in turn branch out into still smaller roots. The main roots of most trees begin to branch out one to two feet under the ground. Some trees have one main root that is larger than the rest. This root goes straight down 15 feet or more and is called the taproot.
Millions of tiny roots grow from the small ones. Each root grows longer at its tip, which is as thin as a thread. As a root tip grows, it pushes its way through particles of soil. Thousands of fine, white root hairs grow just back of the root tip.
When a root's tip comes in contact with drops of water in the soil, the root hairs soak up the water and dissolved mineral. The xylem layer of the roots, trunk and branches then carries the moisture in the form of sap to the leaves. The xylem layer starts in the root and extends to the very top of the tree.
Broadleaf and needleleaf trees are made up of four layers of plant tissue wrapped around one another. From the innermost to the outermost, the layers include the xylem, the cambium, the phloem and the cork. Each layer extends from the roots to the branches.
The woody, central part of the tree is the xylem. It has tiny pipelines that carry water from the roots to the leaves. The cambium's job is to make the trunk, branches and roots grow thicker. The phloem, like the xylem, also has tiny pipelines. The food made by the leaves moves through the phloem to the other parts of the tree.
The main job of the leaves is to make food for the tree. The tissue that surrounds the small veins in the leaves contain tiny green bodies called chloroplasts. Water from the roots passes through the xylem of the trunk, branches and leaves to the chloroplasts, which use the water to make food sugar.
Only a small amount of the water carried to the leaves is used to make sugar. The leaves lose most of the water to the atmosphere.