32 NYMPHAEACEAE [ch. 



takes the form of a slender stem occupying a slanting position 

 in the water. The petioles arise obliquely from the flexible axis, 

 to which they have a very pliable attachment. If the stem be 

 pushed to and fro in the water, the leaves follow all its move- 

 ments while yet retaining their position on the surface. In the 

 case of such a hydrophyte as Hydrocharis, on the other hand, 

 in which not only the leaves but the rosette as a whole floats 

 freely, the entire plant responds to every movement of the water. 

 In spite, however, of a form and structure suited, up to a certain 

 point, to their environment, floating leaves still remain liable to 

 serious risks of wetting and submergence; this is proved by the 

 fact that plants bearing such leaves are quite unable to colonise 

 windy and exposed surfaces where the water is liable to be 



rough^. 



In the summer, in addition to the floating leaves of Castalia 

 alba, others may be seen which rise well above the water surface 

 and are typical air leaves in appearance. The White Waterlily 

 is even able, on occasion, to develop a terrestrial form which can 

 vegetate for an entire summer without submergence^. The 

 leaves of this land form are described as generally being 

 short-stalked, with their lower surfaces almost on the ground. 

 Eighteen centimetres is the greatest diameter recorded: the 

 margins are inrolled towards the upper side. 



We showed that in the Alismaceae it is possible to arrange 

 the species in a series beginning with those in which the leaves 

 are extremely simple and concluding with those in which they 

 are highly differentiated, such as Sagittaria sagittijolia. We also 

 pointed out that in the Arrowhead the successive juvenile leaves 

 epitomised the series recalling the various mature forms of leaf 

 characteristic of the less highly differentiated species. In both 

 respects the Nymphaeoideae run strictly parallel with the Alis- 

 maceae. Victoria regia may be regarded as occupying the same 

 position among the Nymphaeoideae as Sagittaria sagittijolia 



1 See pp. 288, 289. 



2 Bachmann, H. (1896). A land form oi Nuphar pumilum (Nymphaea 

 pumila, Hoffm.) was obtained experimentally by Mer, (1882^). 



