114 VEGETABLE PHYSIOLOGY 



that a section shows them separated from each other by 

 rows of cells not more than one cell thick (tig. 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 



FIG.* 76. SECTION OF STEM OF Potamogeton, SHOWING AIR 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 Equisetum (tig. 77), 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 



