Environment and Adaptation 199 



by no means confined to very dry regions, but may 

 be found almost everywhere, even in regions of 

 heavy rainfall. The character of the soil and the 

 exposure may be such as to allow most of the water 

 that falls to escape, and plants growing under such 

 conditions must provide for this. Thus a plant 

 growing on a steep gravelly hillside, or in the 

 crevice of a rocky cliff, can use only a very small 

 part of the rain that falls upon it, and consequently 

 such plants will show a more or less pronounced 

 xerophytic habit. 



Desert Plants. Of course it is in the more arid 

 parts of the world that the xerophytes abound, 

 and it is these desert plants that offer the most 

 striking examples of xerophytic adaptation. The 

 simplest method of checking the loss of water is by 

 reducing the number and size of the leaves, and in- 

 creasing the thickness of the epidermis. A plant 

 growing on the dry hillside, contrasted with the 

 same species in the moist valley below, will show this 

 very clearly. In the more pronounced xerophytes 

 the leaves may be entirely lost, as in the Spanish 

 broom or in many cacti and some euphorbias. In 

 such xerophytes the photosynthetic function is taken 

 over by the superficial tissues of the stem. 



This desert vegetation is very strikingly devel- 

 oped in the hot, arid regions of Northern Mexico, 

 and the adjacent deserts of Southern California and 

 Arizona. Among the most striking xerophytes of 

 this region are the innumerable cacti, some like the 



