1 30 LEAVES OF WATER-PLANTS. [CHAP. 



of Gentians, and Alisma natans, a species allied to 

 the Plantains. In running water, on the contrary, 

 leaves tend to become more or less elongated. 



Subaqueous leaves of fresh-water plants have a 

 great tendency either to become long and grass-like 

 or to be divided into more or less hair-like filaments. 

 I might mention, for instance, Myriophyllum ; Hip- 

 puris, or Mares-tail, a genus which among English 

 plants comes next to Circaea, the enchanter's night- 

 shade, with totally different leaves; Ranunculus 

 aquatilis, a clos2 ally of the Buttercup ; and many 

 others. 



FIG. 2o. Ranunculus aquatilis. 



Some, again, which, when mature, have rounded, 

 floating leaves, have long, narrow ones when young. 

 Thus in Victoria regia the first leaves are filiform, then 

 come one or more which are sagittate, and lastly 

 follow the great orbicular leaves. 



Another interesting case is that in which the same 

 species has two forms of leaf (Fig. 80) namely, more 

 or less rounded ones on the surface, and a second 

 series which are subaqueous and composed of more 

 or less linear or finely-divided segments. 



Mr. Grant Allen has suggested that this tendency 



