106 VEGETABLE PHYSIOLOGY 
the degree of the illumination and the capacity of the 
cavities formed. Light is, however, not the only factor, 
Fic. 73.—Srction or Lear 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 
Fie. 74.—Srcrion or Lear or Isoétes. 
u, lacunar cavities; b, 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 
