THE AERATION OF PLANTS 107 
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 
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. 
Fic. 75.—Secrion oF Ruizome or Marsvlea. 
co.la., lacunee in cortex. 
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 
Phanerogams. These are developed in a similar manner to 
those already described, and they are so prominent in the 
structure that a section shows them separated from each 
other by rows of cells not more than one cell thick (fig. 76). 
In some cases where large cavities of this kind occur 
