32 



VEGETABLE PHYSIOLOGY 



exterior. In such plants large cavities or reservoirs are 

 constructed, in which a quantity of air is slowly accumulated 

 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). 



A somewhat simi- 

 lar mechanism is 

 provided in the case 

 of terrestrial plants. 

 At the time of their first formation, all the cells are 

 in close approximation to each other at all points of 

 their surface. This condition is, however, only tempo- 



FIG. 35. CORTEX OF ROOT, SHOWING INTER- 

 CELLULAR PASSAGES BETWEEN THE CELLS. 



FIG. 36. SECTION OF LEAF SHOWING THE LARGE INTERCELLULAR SPACES 

 OF THE MESOPHYLL. 



rary ; during the early stages of growth the cell-walls 

 split apart at particular places, usually the angles of the 

 cells. A system of intercellular spaces is thus formed 



