332 



VEGETABLE PHYSIOLOGY 



being greatly reduced. In Ouvirandra as the leaf becomes 

 fully developed this tissue disappears, only the veins 

 remaining, so that it presents the appearance of a coarse 

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

 merged leaf is never cuticularised, and it contains no 

 stomata. In many cases large lacunae are formed in the 



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

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



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 

 rugose. They have not the much-divided outline character- 

 istic of submerged leaves, but are usually simple and some- 

 times of considerable size. Those of the Victoria regia 

 are often three feet in diameter, and are turned up at the 



