VEGETABLE PHYSIOLOGY 



oocupy as much space as the cells themselves (fig. 73). 

 Light appears to influence their development somewhat, 



FIG. 73. SECTION OF LEAF SHOWING THE LARGE INTERCELLULAR 

 SPACES OF THE MESOPHYLL. 



though no definite relation can be shown to exist between 

 the degree of the illumination and the capacity of the 



FIG. 74. SECTION OF LEAF OF Isoetes. 

 a, lacunar cavities ; b, vascular bundle. 



cavities formed. Light is, however, not the only factor, 

 and probably not the most important one, in determining 

 their extent, for they are usually prominent in the cortex 

 of roots, which receive but little illumination. The 



