332 VEGETABLE PHYSIOLOGY 



either long and thin, or are very much, and finely, divided, so 

 that they offer, in either case, no resistance to the force of 

 the current. In more sluggish water they may be long 

 and ribbon-like, but are frequently broader, and sometimes 

 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 



FIG. 142. SECTION OF LEAF OF Isoetes. 

 a, lacunar cavities ; 6, vascular bundle. 



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 

 substance of the tissue, particularly when the lamina is 

 somewhat stout, as in Isoetes (fig. 142). 



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 



