THE DIFFEKENTIATION OF THE PLANT-BODY 31 



the parts in which the protoplasts are comparatively little 

 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. Each protoplast must be furnished with a certain 



FIG. 34. SECTION OF STEM OF Potamogeton, SHOWING AIR PASSAGES 

 IN THE CORTEX. 



amount of air, or rather oxygen. Almost all living sub- 

 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; 

 manv of which are situated at some distance from the 



