Roots 125 



and October i, it has been calculated that 90 per cent. 

 is evaporated from the soil and returned to the air. 

 From the air, however, the soil again absorbs it ; for 

 though leaves do not absorb moisture from the air, 

 the soil does. 



We speak commonly of the air as being 'damp* 

 or ' dry,' as the case may be ; but in point of fact 

 it is never absolutely dry, for nothing could live in 

 it if it were. It always contains some amount of 

 watery vapour, and, whether it be large or small, 

 soil which has been dried during the day regains some 

 degree of moisture by night by the simple process 

 of sucking it from the air. The air sucks it from 

 the soil by day, especially during sunshine or dry 

 wind, and the soil thus dried sucks it back again at 

 night. 



Some soils suck much more moisture from the 

 air than others, and some are also able to keep it 

 much longer than others. We all know that a sandy 

 soil, for instance, is a dry soil : it takes some time 

 to grow really damp, and it dries again very quickly. 

 Indeed, pure quartz-sand seems as if it could not be 

 moistened by anything short of rain or dew, being in- 

 capable of sucking any moisture from the very dampest 

 air. 



A chalk or limestone soil, on the other hand, acts like 

 a sponge, and, though it may dry on the surface, keeps 

 its moisture a long time within. The subsoil of the 

 Kalahari Desert, already mentioned, is limestone ; and 

 this is probably one reason why the grass there is able 

 to remain green so long without rain. Such water as 

 is received is kept for some time, stored up in the sub- 

 soil. 



