90 THE OAK. 



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 need- 

 ed could not be obtained. In the second place, these 

 larger and larger quantities of water require correspond- 

 ing 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 ne- 

 cessity arises for more strength and rigidity in the sup- 

 porting 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 in- 

 creased leaf-surface playing the part of an enlarging 

 manufactory, which turns out increased supplies of con- 

 structive materials each summer ; for it is in the leaves 



