112 VEGETABLE PHYSIOLOGY 
occupy as much space as the cells themselves (fig. 78). 
Light appears to influence their development somewhat, 
Sta 
A 
ne 
Po) 
| 
a) 
° 
cK 
Fia. 73,—Srcrion or LEAF SHOWING THE LaraEe INTERCELLULAR 
Spacks 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.—Snuction or Luar or Isoétes, 
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 
