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 
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 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 lacune 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 
