THE AEKATION OF PLANTS 113 



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 



co.la. 



FIG. 75. SECTION OF RHIZOME OF Marsilea. 

 co.la., lacunae 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 Isoetes (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 



