THE AEKATION OF PLANTS 



115 



by only a very small portion of its surface ; the leaf-stalk 

 of Nymph sea, for example, is always submerged, and only 

 the floating lamina can obtain a direct supply of air. The 

 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 



FIG. 77. POETION OF AEKIAL STEM OF A SPECIES OF Equisetum. 

 a, cortical lacuiia ; b, lacuna iu vascular bundle ; c, chlorophyll-containing cells. 



protoplasts of submerged plants 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 their epidermal cells. 



The air cavities which arise in the stems of terrestrial 

 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 



