Dana Marie Torisky, age 12, of Monroeville, Pennsylvania, for her question:

How does a tree grow?

This question pops up on Arbor Day when citizens for the Green Revolution plant youthful trees to justify their faith in the future. Your young tree may need watering and other tender loving care through its first, season or so. But if all goes well, it' will thrive and grow to keep its part of the bargain. Recent experiments suggest that in some mysterious way it is sensitive to our reactions. But let's not expect it to explain how and where it adds new growth to become bigger.

You can observe how a tree adds its growth, but the project drags through ten or 20 years. So let's use an imaginary camera to speed up the procedure. The record starts with you, planting a young tree with a topmost twip that just happens to be level with the top of your head. When the tender job of planting is done, you cut two shallow marks on its trunk    not deep enough or long enough to cause severe wounds. One is level with your shoulders, the other is level with your knees.

The next view jumps 10 years ahead. Both of you have grown taller, but the top of the tree is now way above your head. Its trunk has grown thicker, its branches spread wider and there are more of them. Now look for your mark on the trunk. Your knees and shoulders are now higher, so you would expect the old markers to be higher. But they are not. They still mark the 10 year age level of your knees and shoulders.

These observations reveal the growth pattern of the tree. Every year it adds new growth to the top of its crown and to the tips of its twigs and branches. Every year it adds a new circle of woody cells around the outside of its trunk, just blow the bark. It grows taller by reaching its top toward the sky. It grows sturdier by adding outer layers to thicken its trunk and branches. It grows wider by lengthening the tips of its twiggy fingers.

Christmas tree evergreens start out with their lower branches brushing the ground and become forest giants with their lowest boughs held high above the ground. They do this by adding new growth at the top and sprouting new branches higher up the trunk. Then one by one they discard their low growing Christmas tree boughs. These fall off and the wounds are sealed with oozing gobs of weatherproof resin.

Forest trees are left to grow as they please. But sometimes we need to plan the future for the shade and ornamental trees in our parks and gardens. This is when we need to know their basic growth patterns. For example, in most non evergreens, the low branches stay at that level and grow thicker through the years. If we want a tall tree with a straight trunk.and lofty spreaking boughs, we must have the low young branches carefully severed. These wounds are sealed and the tree sprouts new branches higher up its taller trunk, just where we want them to spread their shady boughs.