32 BOTANY OF THE LIVING PLANT 



which it affords to the defenceless protoplast. Moreover, it has made 

 possible the building up of large mechanically stable plant-bodies, 

 whether buoyed up in water or supporting themselves in air. The 

 presence of cell-wall may appear to have complicated the problem of 

 physiological interchange by interposing barriers between the proto- 

 plasts. But this difficulty has been surmounted by those threads 

 of protoplasm, which traverse the walls, and link up the protoplasts 

 into a continuous living system. 



