As we have seen, the forester has to exercise considerable forethought—the outcome of long experience—in growing oak so as to obtain long, clean stems. The natural habit of the tree is to form a short, thick bole and a widely spreading crown, the main branches of which come off not far from the ground. To compel the stem to elongate into a long pole he has to plant other trees with it (as we have seen, beech, spruce, etc.), which, while they keep the light off the lower parts of the oaks, do not overtop them. This makes the trees long and spindly at first, as they run up their leaf-crowns higher and higher, and it is part of the forester's art to select the exact time when he may cut away some of the nurse trees and let in just enough, and not too much, light and air, so that the crowns of the oaks shall fill out more and thicken the stems. For it must never be forgotten that the timber is laid on from substance prepared in the leaves.
The natural shape, so to put it, of an oak-tree is that of a wide-spreading, short-stemmed mushroom, and such a shape is realized in the open; the forester compels it to lengthen its stem as much as possible before he lets it extend its crown. Hence he aims at length first, and then lets the tree put on timber in the mass. He does this, of course, by taking advantage of the tree's peculiarities, and one of these is that it grows very rapidly when young. It will be obvious that the skilled forester also has to aim at getting as much timber as possible on the ground in a given time, and in the case