108 VEGETABLE PHYSIOLOGY 



the mode of formation is different. A mass of tissue lying 

 in the position of the subsequent cavity does not keep 

 pace in its development with the growth of the cells sur- 

 rounding it, and consequently becomes ruptured, and the 

 cells of which it is composed are gradually destroyed, leav- 

 ing a cavity of some size. Instances of this mode of 

 formation are afforded by the stems of Equisetum (fig. 77), 



FIG. 76. SECTION OF STEM OP Potamogeton, SHOWING Am PASSAGES 



IN THE COKTEX. 



the haulms of grasses, and the hollow stems of the 

 Umbelliferae and other plants. 



The occurrence of these large air-containing cavities in 

 partially submerged plants may be explained by a considera- 

 tion of their habitat. The plant is in contact with the air 

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

 Nymphaea, for example, is always submerged, and only the 

 floating lamina can obtain a direct supply of air. The 



