148 PLANT LIFE 



definitely modified in relation to conditions 

 of drought that they have become extremely 

 intolerant of moisture, even in quantities such 

 as would barely suffice to keep an ordinary 

 mesophyte alive. Plants such as these 

 stand at one extreme end of the scale of 

 vegetation, the other end being occupied by 

 the genuine aquatics or hydrophytes which 

 also are unable to endure mesophytic con- 

 ditions, because they lose water too readily, 

 Different as are these extreme examples from 

 one another, they yet agree in this respect, 

 namely that the chemical processes character- 

 istic of their vital functions are incapable of 

 becoming so modified as to produce the kind 

 of structure suited to average mesophytic 

 conditions. In the case of aquatics the 

 general nature of this defect is clearer than 

 in the xerophytes, and mainly depends on 

 the inability to form a suitable cuticle, added 

 to which the functions of water conduction 

 and mechanical support are often inadequate 

 for a terrestrial habitat. 



