rNTLUENCE OF ENVIRONMENT 271 



this, however, occur in a few floating forms, e.g. Sal- 

 vinia, Pistia, which are covered with hairs, but what 

 the significance of this is, is hard to see. 



Xbeophytes 



Very different from the plants we have been con- 

 sidering are the "Xerophytes," or dry-region plants, 

 in which are developed all manner of devices for pre- 

 venting loss of water, and thus resisting the effects of 

 excessive dryness. This has been so successful that 

 very few regions are so dry as to be absolutely desti- 

 tute of vegetation. 



The surface exposed to the air in these xerophytes 

 is much reduced, the leaves being either extremely 

 small or entirely absent, and the green assimilating 

 tissue is confined mainly to the stem and branches, 

 which may in some cases become flattened and leaf- 

 like, as in many Acacias. Where the leaves are present 

 they are either provided with very thick outer cells, so 

 that they are hard and leathery in texture, like the 

 oleander or manzanita, or they are covered with a 

 dense felt of hairs, which forms a most efficient pre- 

 ventive against loss of water, and also acts as a shield 

 against the too powerful rays of the sun. Many desert 

 plants show this covering of hairs, which gives them 

 their characteristic gray color. 



The xerophytes are of course most perfectly devel- 

 oped in hot deserts such as those of the southwest- 

 ern United States and northern Mexico, the Sahara, 

 and many parts of Australia. The traveller passing 

 through Arizona and New Mexico will find the vegeta- 



