204 DRAINAGE BY ROOTS OF TREES. 
humid than that of cleared land. This agrees with experience. 
The soil of the natural forest is always moist, except in the ex- 
tremest droughts, and it is exceedingly rare that a primitive 
wood suffers from want of humidity. How far this accumula- 
tion of water affects the condition of neighboring grounds by 
lateral infiltration, we do not know, but we shall see, in a 
subsequent chapter, that water is conveyed to great distances 
by this process, and we may hence infer that the influence in 
question is an important one. 
It is undoubtedly true that loose soils, stripped of vegetation 
ud broken up by the plough or other processes of cultivation, 
may, until again carpeted by grasses or other plants, absorb 
more rain and snow-water than when they were covered by a 
natural growth; but it is also true that the evaporation from 
such soils is augmented in a still greater proportion. Rain 
scarcely penetrates beneath the sod of grass-ground, but runs 
off over the surface ; and after the heaviest showers a ploughed 
field will often be dried by evaporation before the water can 
be carried off by infiltration, while the soil of a neighboring 
grove will remain half saturated for weeks together. Sandy 
soils frequently rest on a tenacious subsoil, at a moderate depth, 
as is usually seen in the pine plains of the United States, where 
pools of rain-water collect in slight depressions on the surface 
of earth the upper stratum of which is as porous as a sponge. 
In the open grounds such pools are very soon dried up by the 
sun and wind; in the woods they remain unevaporated long 
enough for the water to diffuse itself laterally until it finds, in 
the subsoil, crevices through which it may escape, or slopes 
which it may follow to their outcrop or descend along them to 
lower strata. 
Drainage by Roots of Trees. 
Becquerel notices a special function of the forest to which I 
have already alluded, but to which sufficient importance has 
not, until very recently, been generally ascribed. Lrefer to the 
