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 
1) 
t 
Fig. 84.—Suorion or Svem or Potamogeton, sHowina Aix Passages 
IN THE CORTEX, 
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 lacune is not accurately known, but it pro- 
bably differs somewhat from that of ordinary air, 
These air passages or reservoiry 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. 84). 
