THE AERATION OF PLANTS 115 
by only a very small portion of its surface; the leaf-stalk 
of Nympheza, 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 
caarait 
geek: 
i: 
AT 
\? ce Pye ae 
we 
MOY a 
Oaaes: 
ny 
Fic. 77.—Portion or AuriaL Svem or A Spucius or Lquisetum. 
a, cortical lacuna; 0, lacuna in 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 
8 * 
