Trees can add quite a bit to a garden. Depending on what type you choose to plant, the tree will provide shade, fruit, and beauty for your garden. Placement of a tree is a more difficult task that it would seem at first. One thing to keep in mind is the leaves in autumn; you will not want to put an outdoor wall fountain right next to the tree, or the pump will often become clogged.

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, which 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. The leaves 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.

Stones should be placed in a good bond. This simply means that edges of stones on one course should overlap spaces in the lower courses. Where a stone on an upper course is crooked or does not fit firmly, earth and small stones can be packed in to improve the bond. No vertical crevices should be left.

In planting trees, their mature height and spread must be considered before a selection is made. Tempting as the nursery catalogues are, it is necessary to choose carefully, especially on the average lot, because crowding spoils the growth and appearance of trees, particularly specimen trees. In general, it is wisest and most economical to plant young trees.

Planting a mature tree is difficult and, if done professionally, costly. If, however, a mature tree is badly needed for a terrace or for screening, it may well justify the expense. What you are buying is the time it takes a smaller tree to mature.

Today you can plant trees when in full leaf with the aid of new wilt-proof sprays that seal the leaves against moisture loss until the roots are established. This, however, costs money and entails greater risks than buying your tree and planting it in early spring (the best time) or late fall or winter. If you are planting a tree over 6 feet tall, it will suffer less setback if moved with a burlap root ball.

You must follow the same soil preparation techniques when planting trees as you do for sowing grass seed or for planting flowers. But since the root system must have fertile soil when it is planted, special steps must be taken. Dig a hole 2 feet deep and at least 1 foot wider each way than the full spread of the roots. The bottom of the hole should be broken up with a pitchfork and thoroughly mixed with peat, leaf mold, loam, etc.

Manure should be used sparingly and only on the top of the hole as it burns the roots. The deeper you can cultivate your hole, the better for your tree. Once it is planted, you can cultivate around it but not under the roots. If you strike a subsoil of building rubble or clay, which you are very apt to find anywhere near a house and in which a tree cannot grow, this subsoil must be removed and good soil, or better still, garden humus, substituted for it.