Trees can be broken down into three main parts: the roots, the leaves and the woody structure between them. The roots' function is to bring raw materials-water and mineral salt dissolved in water-to the tree. The leaves absorb carbon dioxide from the air and use the sun's light energy to combine this gas with the moisture from the roots, thus making the simple sugars that are the basic nutrients of the tree.
The trunk, limbs, branches and twigs hold the leaves in position to receive the life-giving sunlight and air; they also act as transportation, carrying raw materials between roots and leaves. The materials absorbed by the roots are pulled up by capillary attraction and the osmotic action induced by evaporation of water from the leaves. Loss of water through the leaves is called transpiration.
On a summer day, a single birch tree may transpire 700 to 900 gallons of water. It is this enormous flow of water that causes a continuous flow of sap from the roots to the topmost twigs.
In planting or transplanting a tree, and in building on a lot where you wish to preserve the trees, the gardener's chief consideration must be to protect the root structure of the tree. The big roots near the stem anchor the tree to the ground, while the fine root hairs at the ends of the rootlets absorb the water from the soil.
The stem or trunk of a tree has three parts: the bark, the wood and the pith. The pith is the central part and around it is the wood. Between wood and bark is the cambium, a thin layer that produces new wood and bark. When the cambium ring is severed, as by a wire cable, the tree is killed, and since the cambium protects against insects and disease, anything driven into it can wound the tree severely.
Outside of man himself, trees have countless enemies.
There are 200,000 known kinds of insects that attack trees, in addition to diseases such as blight, rust and rot, storms and droughts. Luckily, birds help to keep caterpillars, borers, beetles and other insects in check.
