INFLUENCE OF ENVIRONMENT ON PLANTS 339 



attain a considerable size. The cell-walls of the former 

 are often thickened, but in the latter the tissue is always 

 very weak, the parenchyma of the mesophyll sometimes 

 being greatly reduced. In Ouvirandra as the leaf becomes 

 fully developed this tissue disappears, only the veins re- 

 maining, so that it presents the appearance of a coarse 

 grating or piece of lattice-work. The epidermis of a sub- 



FIG. 141. SECTION OF PETIOLE OF WATEK-LILY (Nymphcea alba). 

 a, c, vascular bundles ; 6, d, air-channels. 



merged leaf is never cuticularised, and it contains no stomata. 

 In many cases large lacuneB are formed in the substance of 

 the tissue, particularly when the lamina is somewhat stout, 

 as in Isoetes (fig. 140). 



In plants with floating leaves the roots and stems are 

 similar in character to those of the first class. The leaves, 

 however, which lie upon the top of the water, are usually 

 tough and thick, their undersides being sometimes deeply 



22* 



