30 NYMPHAEACEAE [CH. 



The floating leaves of the British Waterlilies are typical of 

 swimming leaves in general. The lamina is coriaceous and non- 

 wettable. No leaf which attains to any size can float success- 

 fully unless it be of a strong, leathery texture, since the motion 

 of the water exposes it to tearing, and in heavy rain it is 

 liable to be much more severely battered than an air leaf, which 

 can yield freely in a medium so elastic as the atmosphere 1 . 

 The normal stomates are borne upon the upper surface of the 

 floating leaves, where they are in contact with the air, but water 

 stomates have been observed on the lower surface in two Ameri- 

 can species of Nympkaea 2 . These water pores occur in direct 

 communication with the finest ramifications of the tracheal 

 system. The floating leaves are differentiated from the sub- 

 merged leaves at a very early stage, stomates being developed 

 while the leaf is still in the bud 3 . Floating leaves of an orbicular 

 or peltate form 4 , more or less recalling those of the Nymphaea- 

 ceae, occur both among Monocotyledons and Dicotyledons 

 and appear to be well adjusted to their particular type of habitat. 

 It is clear, in the first place, that a leaf with an entire outline 

 is less easily wetted and submerged than one which is sub- 

 divided. It is obvious, also, that the centre of gravity of a 

 floating leaf which approximates to the circular form, lies at its 

 central point, and that this is therefore the most mechanically 

 economical position for petiolar support 5 . In a peltate leaf, 

 such as that of Victoria regia^ this position is approximately 

 achieved, while, in the orbicular Waterlily leaf with a deep sinus 

 at the base, some approach is made to the same condition. 



All the floating leaves belonging to any associated group of 

 plants, unlike a corresponding series of air leaves, have, without 

 exception, to expand their laminae in one horizontal plane. 

 The competition among the leaves for space is shown by the 

 way in which every available square inch of water surface is 



1 Schenck,H. (1885). 2 Schrenk, J. (1888). 3 Costantin, J. (1886). 

 4 For a mathematical demonstration of the physical advantages accru- 

 ing to a floating leaf from a circular form, see Hiern, W. P. (1872). 

 *Jahn,E.(i8 97 ). 



