THE DIFFERENTIATION OF THE PLANT-BODY 31 
changed, and consequently are most concerned in carrying 
out the vital processes. 
The needs of the protoplasts forming the community of 
the plant embrace, however, as we have seen, something 
more than the arrangements so far described serve to secure 
for them. Hach protoplast must be furnished with a certain 
amount of air, or rather oxygen. Almost all living sub- 
Fic. 84.—Srction or Stem or Potamogeton, SHOWING AIR PASSAGES 
IN THE CORTEX. 
stance must carry on during life the process known as 
respiration. The free-swimming zoospore to which we 
have so often referred obtains a supply of oxygen from the 
water in which it lives, the gas being dissolved therein. 
Aquatic plants also obtain their oxygen from this source, 
but many of them are composed of a large number of cells, 
many of which are situated at some distance from the 
exterior. In such plants large cavities or reservoirs are 
