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 1% 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 
