INFLUENCE OF ENVIRONMENT ON PLANTS 333 



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

 surface from being wetted. The upper epidermis of such 

 lioating 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 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 

 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 parenchyma 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 epider- 

 mis 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 super- 

 ficial investment, such conduction is not called for for 

 nutritive purposes. Their transpiration moreover is re- 

 duced 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 prominent. Their food materials reach them 

 dissolved in the water in which they live, and hence they 

 have no need of the complicated root system with its 

 absorbent root-hairs, which is so characteristic of a plant 



