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.—Szction of PETioLE or WaATER-LILY (Nymphea alba). 
a, c, vascular bundles; 0b, d, air-channels, 
merged leaf ig never cuticularised, and it contains no stomata. 
In many cases large lacune are formed in the substance of 
the tissue, particularly when the lamina is somewhat stout, 
as in Isoétes (fig. 140). a 
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 * 
