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 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 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 
