CELLULAR STRUCTURE 29 



which is essential to the exercise of its vital 

 functions. In the absence of sufficient water- 

 content the protoplasm may actually die. 

 Even if it is able to tolerate comparative 

 desiccation, it passes into a state in which 

 all vital reactions are slowed down until they 

 are practically brought to a standstill. 



We see clearly, therefore, that an adequate 

 water supply must be regarded as a primary 

 condition of all active life prior in import- 

 ance even to the provision for photosynthesis 

 in the green plant. For when water is cut 

 off, the building up of complex carbon com- 

 pounds is ipso facto arrested, however com- 

 pletely all other conditions of photosynthesis 

 may be satisfied. 



It is clear that plants which pass the whole 

 of their lives in a watery medium are not 

 confronted with the risk of water starvation, 

 but this danger may, and often does, become 

 acute as soon as they exchange a purely 

 aquatic for a terrestrial habitat. It is no 

 exaggeration to assert that the most salient 

 features in the structure, and the behaviour 

 of green vegetation in general, is mainly 

 connected with a solution of the problems pre- 

 sented by water requirements on the one hand, 

 and by those of photosynthesis on the other. 

 It is in relation to these two overwhelmingly 

 important functions that vegetation has 

 assumed much of its present form, for any 

 plant failing to achieve success in these two 

 directions must either suffer extinction, or 



