THE AEKATION 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 



FIG. 77. PORTION OF AERIAL STEM OF Equisetum. 

 a, cortical lacuna ; 6, lacuna in vascular bundle ; c, 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 maintain 



