INFLUENCE OF A CROP ON EVAPORATION 119 



centage of rain water intercepted by the foliage and branches 

 of various kinds of forest was as follows : 



Larch 15 per cent. Spruce Fir 24 per cent. 

 Beech 19 Scotch Pine 30 



The evergreen trees were thus, naturally, the most effective 

 in retaining rain on their leaves. Some compensation for 

 this kind of loss will occur in the case of low growing crops 

 freely exposed to the sky ; these condense the moisture of 

 the air as dew, and a part of this falls to the ground. 



The retention of rain on the leaves is, however, generally 

 a point of minor importance ; the principal loss of water 

 occasioned by vegetation is due to the evaporation of water 

 from the surface of the plant, chiefly through the stomata 

 on the under side of the leaves. This transpiration of water 

 by the plant is a part of its life-functions, and is indeed to 

 a certain extent proportionate to the amount of growth ; the 

 larger is the crop, the greater being the amount of water 

 evaporated by it (p. 52). 



It is often supposed that a soil covered by a crop is moister 

 than a bare soil ; this may be true of the soil surface, which 

 is thus shaded from the sun and protected from wind (p. 1 1 1 ), 

 but it is not true of the soil as a whole 1 . The evaporation 

 of water from the soil particles may be diminished by covering 

 the land with a crop, but the evaporation through the leaves 

 of the crop which takes its place is so much greater that 

 the total evaporation is much increased. A crop in fact 

 dries the soil through its roots, and the greatest part of 



1 The shading of forest soil is recognized as most important to its fertility, 

 and apparently, to the storing of water in it. The influence of this continuous 

 shading is indirect ; it leads to the accumulation of a layer of humus upon 

 the surface of the ground, which acts as a mulch, and also greatly favours 

 the retention of water. 



