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. Inthe second place, these larger 
and larger quantities of water require corresponding 
increase in the sectional area of the pipes or water con- 
duits—1z.e. the vessels of the wood—through which they 
have to pass in order to reach the leaves. This is ensured 
by the increage 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 
