114 VEGETABLE PHYSIOLOGY 
that a section shows them separated from each other by 
rows of cells not more than one cell thick (fig. 76). 
In some cases where large cavities of this kind occur 
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 
Fic. 76.—SEcrion or Stem or Potamogeton, sHow1ng ATR PASSAGES 
IN THE CoRTEX, 
cells of which it is composed are gradually destroyed, leaving 
a cavity of some size. Instances of this mode of formation 
are afforded by the stems of Hquisetum (tig. 77), the haulms 
of grasses, and the hollow stems of the Umbelliferes 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 
