THE AERATION OF PLANTS 118 
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 
stomata do not exist upon roots. There is thus a necessity 
for a larger reservoir of air than in parts where gaseous 
interchange is more readily effected. 
Besides these comparatively narrow channels we find 
Fig. 75.—Szcrion or Rutzome or Marsilea. 
co.la., lacunz in cortex. 
cases where reservoirs of large size are specially developed. 
Such structures occur in the leaves, rhizomes, and roots 
of aquatic plants which are nearly or entirely submerged. 
Among them conspicuous examples are afforded by the 
leaves of Salvinia and Isoétes (fig. 74), the rhizome of Marsilea, 
(fig. 75), and the leaf stalks of many of the aquatic Phanero- 
gams. These are developed in a similar manner to those 
already described, and they are so prominent in the structure 
8 
