THE ABRATION OF PLANTS 109 
stomata are placed upon the upper surface, and afford its 
only means of entrance. The stems and roots are also cut 
off from air by being placed either in water or in mud. The 
protoplasts of such a plant are almost entirely dependent 
upon the reservoir of air which the body of the plant can 
contain, a small quantity only entering by diffusion from 
the water into its epidermal cells. 
The air cavities which arise in the stems of terrestrial 
Fic. 77,—Portion oF AERIAL STEM OF Equisetum, 
d, cortical lacuna; b, lacuna in vascular bundle; v, chlorophyll-containing cells. 
plants, such as the grasses, are probably not primarily 
developed with a view to the aeration of the plant, but are 
rather intended to economise the material used in construc- 
tion. The hollow stems with a rigid periphery, strength- 
ened at intervals by diaphragms, such as occur at the 
nodes of these organs, are especially adapted to maintam 
