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. 
