CHAPTER 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 moist- 

 ure 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 to check 

 transpiration and allow them to retain most of the moisture 

 they draw up. But when once a plant has thoroughly 

 flagged, the case is different. Then nothing short of 



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