90 THE OAK 



the total area of the leaf-surface exposed to the air and 

 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 ot 

 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 con- 

 duits i.e. the vessels of the wood through which they 

 have to pass in order to reach the leaves. This is ensured 

 by the increase in diameter of the stem and main root 

 and their chief branches, a larger number of vessels, &c., 

 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 corre- 

 spondingly 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 thick- 

 ening 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 



