106 



VEGETABLE PHYSIOLOGY 



the degree of the illumination and the capacity of the 

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



FIG. 73. SECTION OF LEAF SHOWING THE LARGE INTERCELLULAR 

 SPACES OF THE MESOPHYLL. 



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 



FIG. 74. SECTION OF LEAF OF Isoetes. 

 a, lacunar cavities ; Z>, vascular bundle. 



explanation of the relatively large development in this 

 region may lie in the fact that the intercellular cavities 

 there have very little communication with the outer air, as 



