n] FEATURES OF ENVIRONMENT 35 



plateau, hotter rocks, and nowhere a shade-giving 

 tree. Before sunrise the traveller shivers in his 

 blankets near the much needed camp fire. 



Paradoxical as it may seem, there is often a heavy 

 dew. This is not so much falling dew, moisture 

 condensed out of the atmosphere by the cold ; it is 

 rather the direct evaporation from the subsoil and 

 this vapour is condensed near the ground before it gets 

 well into the dry desert air. In many sandy deserts 

 there is plenty of subsoil water, but the difficulty is 

 to get at it in drinkable quantities. 



Animals and plants must have water. In the 

 desert they must be economical with it, not to pers- 

 pire but to have provisions for holding it, or to catch 

 even the smallest quantities. The roots of trees and 

 shrubs often go down to astonishing depths ; fre- 

 quently there is much more of such plants below 

 than above the ground. Many plants are furnished 

 with woolly hairs ; others have very narrow, slender 

 leaves, standing together like bunches of wire ; this 

 arrangement enhances the radiation, and the re- 

 sulting coolness causes deposition of the dew upon 

 them and this is what these plants want. Striking 

 examples are the Tillandsias, related to the Bromelias, 

 but instead of making a scroll, or rain-catching nest 

 of their leaves as they do in tropical forests, these 

 plants in arid regions grow upon the most exposed 

 branches of the trees and are transformed into small 



32 



