Page:The Oak.djvu/108

90 sunlight is larger each successive summer than it was the previous one. Several very important consequences follow from this. In the first place, the larger area of leaf-surface evaporates more water than before, and as this water is derived from the soil the absorbing surface of the roots has to increase, or the larger supplies needed could not be obtained. In the second place, these larger and larger quantities of water require corresponding increase in the sectional area of the pipes or water conduits—i.e., the vessels of the wood—through which they have to pass in order to reach the leaves. This is insured by the increase in diameter of the stem and main root and their chief branches, a larger number of vessels, etc., being added each season. In the third place, as the leaf-crown enlarges its weight increases, and the surface it exposes to the swaying action of the wind is correspondingly greater; consequently the necessity arises for more strength and rigidity in the supporting stem, and for a larger hold on the soil on the part of the root-system, which has to withstand the lever action of the swaying tree. These needs, again, are met by the thickening of the woody parts of the shoot-axis and roots, and by the greater spread and increased number of points of contact in the soil of the latter.

Correlated with these phenomena we have the increased leaf-surface playing the part of an enlarging manufactory, which turns out increased supplies of constructive materials each summer; for it is in the leaves