36 VEGETABLE PHYSIOLOGY 



water in which it lives, the gas being dissolved therein. 

 Aquatic plants also obtain their oxygen from this source, 

 but many of them are composed of a large number of cells, 

 most of which are situated at some distance from the 

 exterior. In such plants large cavities or reservoirs are 

 constructed, in which a quantity of air is slowly accumulated 



FIG. 34. SECTION OF STEM OF Potamogeton, SHOWING Am PASSAGES 



IN THE COBTEX. 



and into which the respiratory products can be discharged. 

 From such reservoirs the oxygen which the cells require 

 is obtained. The composition of the atmosphere in these 

 chambers or lacunae is not accurately known, but it pro- 

 bably differs somewhat from that of ordinary air. 



These air passages or reservoirs are very conspicuous in 

 the stalks of floating leaves such as those of the water-lily, 

 and in the submerged stems of most aquatic plants (fig. 34). 



