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 Hquisetum (fig. 77), 
ay) RS Y 
0) 
acess 
a 
Fic. 76.—Srction or Stem or Potamogeton, sHowinc AIR PassaGEs 
IN THE CORTEX, 
the haulms of grasses; and the hollow stems of the 
Umbelliferee 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 Nymphiea, for example, is always submerged, and only 
the floating lamina can obtain a direct supply of air. The 
