IX. 



DESERTS 



From what has been already said, it is evident that 

 every tree, every plant, every spire of grass indeed, is 

 a pumping apparatus on a larger or smaller scale, by 

 which a portion at least of the water which descends 

 from the clouds begins to mount up again almost as 

 soon as it has fallen. 



Plants give up to the air, chiefly by transpiration 

 through their leaf-pores, but partly also by evaporation 

 from their whole surface, nearly as much water as is 

 taken up by their roots — nearly, but not quite — for, as 

 long as they are growing, they need some water for 

 the formation of new shoots and leaves. The quantity 

 is not much in itself, though water makes up a large 

 part of the weight of most plants. But it is quite clear 

 that, without water, they cannot grow at all. 



Provided a plant has a plentiful supply of water, 

 enough, that is, to make up for what it loses, it does 

 not seem to matter how much it transpires. Some 

 plants thrive perfectly well in dry air — where they give 

 off moisture constantly and rapidly — if only their roots 

 be kept in damp soil ; and others thrive equally well in 

 comparatively dry soil, provided the air be damp enough 



