340 VEGETABLE PHYSIOLOGY 



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 

 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 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 resuming their growth, so as always to keep 

 pace with the increased depth. Their structure resembles 

 that of the stem in that they are composed of turgid paren- 

 chyma and have little or no development of woody tissue. 

 They also contain conspicuous lacunae or air-channels 

 (fig.' 141). 



Vegetative reproduction is very common, branches 

 becoming detached from the plant, which speedily put out 

 adventitious roots of their own and form new plants. 



Their watery environment explains the peculiarity of 

 their structure. From the nature of their surroundings 

 and their power of absorbing liquid through their epidermis 

 we can easily explain the absence of the woody tissue, 

 which we have seen to be, when present, especially devoted 

 to the conducting of water from the roots throughout the 

 plant. Their absorbing tissue being their whole superficial 

 investment, such conduction is not called for, for nutritive 

 purposes. Their transpiration, moreover, is reduced to a 

 minimum, and there is therefore no need of a provision for 

 the rapid current of water which is so essential to the well- 

 being of a terrestrial plant, in which this function is so 



