INFLUENCE OF ENVIEONMENT ON PLANTS 333 



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 

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

 surface from being wetted. The upper epidermis of such 

 floating leaves is often either strongly cuticularised, or 



FIG. 143. SECTION OF PETIOLE OF WATER-LILY (Nymphcea alba). 

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



impregnated with a waxy secretion serving the same pur- 

 pose. 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 pecu- 

 liar 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 



