THE LOSS OF WATER BY THE PLANT 



175 



Quite different from these are the thin-leaved xerophytes. 



To this type belong such 



plants as Alhagi Camelorum, 



the wild alfalfa of the steppes, 



some representatives of the 



Cucurbitaceae, as Citrullus colo- 



cynthis, while the species of 



sagebrush, Artemisia, found in 



semideserts, closely approach 



this group. These plants 



have thin tender leaves, which 



wilt rapidly when a branch is 



cut from the plant. The in- 

 tensity of transpiration is very 



high in plants of this type, 



especially in sunlight. Even 



on very hot dry days they 



keep their stomata wide open, 



losing considerable amounts of 



water, but assimilating ener- 

 getically. As a result of this 



vigorous transpiration, xero- 

 phytes of this type are able 



to lower considerably the 



temperature of their leaves. In the hot air and soil of the desert 



this protection against over- 

 heating is of great importance. 

 The considerable loss of water 

 is replaced from the lower 

 layer of the soil, as xerophytes 

 of this type possess an ex- 

 tremely well-developed root 

 system. Where the water 

 table is not too deep, the 

 plants are able to reach it 

 with their long roots and may 

 stand, therefore, the greatest 

 heat. Where water cannot be 

 Fig. 77. -Sedum acre. reached, the root system 



spreads over a very great volume of soil. During the driest 



BgBE 



Fig. 76. — Gigantic cacti in an Arizona 

 desert. 



