CHARACTERS OF AQUATIC PLANTS 479 



but the cells remain capable of absorbing the water in which 

 the plant is living. In the stem this tissue verj^ frequently 

 contains chloroplastids. 



The character of the leaves differs in relation to the 

 habitat. Those which grow in rapid streams are generally- 

 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 are often broader, some- 

 times attaining a considerable size. Their tissue is always 

 very weak, the parenchyma of the mesophyll sometimes being 

 much reduced, so that as the leaf grows old it becomes latticed, 

 as in Ouvarandra. The epidermis is never cuticularised, audit 

 contains no stomata. 



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 characteristic of submerged leaves, but are 

 usually simple and sometimes of considerable size. Those of the 

 Victo7'ia regia are often three feet in diameter, and are turned 

 up at the edges, forming a rim, which helps to preserve the upper 

 surface from being wetted. The upper epidermis of such float- 

 ing leaves is often either strongly cuticularised or impregnated 

 with a waxy secretion, serving the same purpose. The leaves 

 are consequently shiny in appearance, and water will not adhere 

 to them. These floating leaves bear their stomata upon the 

 upper surface only. 



The petioles are long and flexible and possess a peculiar 

 power of adapting themselves to varying depths of water. 

 Should the stream in which they live become shallow, the leaves 

 still remain floating, owing to the power of the petiole to become 

 curved ; should the water rise, the petioles respond by renewing 

 their growth, so as always to keep pace with the increased 

 depth. 



Vegetative reproduction is very common, branches becoming 

 detached from the plant, which speedily put out adventitious 

 roots of their own, and form new plants. 



From their close relation to the watery environment and 

 their power of absorbing liquid through their general epidermis, 

 we can easily explain the absence of the woody tissue. Their 

 transpiration is reduced to a minimum or altogether suppressed, 

 and there is, therefore, no need of a provision for the rapid 

 current of water so essential to the well-being of a terrestrial 



