THE AEEATION 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. 



FIG. 75. SECTION OF KHIZOME OF Marsilea. 

 co.la, lacunae in cortex. 



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 

 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 



