HYDROHARMOSE 



127 



tion of them has been found but recently. This explanation has come from 

 the work of E. S. Clements already cited, in which it was found that certain 

 sun plants underwent no material structural change when grown in the 

 shade, and that the same was true also of a few species which grew in two 

 or more habitats of very different water-content. In accordance with this, 

 it is felt that the xerophytic features found in amphibious plants are due to 

 the persistence of stable structures, which were developed when these 

 species were growing in xerophytic situations. When it is called to mind 

 that monocotyledons, and especially the grasses, sedges, and rushes, are 

 peculiarly stable, it may be readily understood how certain ancestral 

 characters have persisted in spite of a striking change of habitat. Such a 

 hypothesis can only be confirmed by the methods of experimental evolution, 

 and a critical study of this sort is now under way. 



171. Hydrophytic types. Hydrophytes permit a fairly sharp division 

 into three groups, based primarily upon the relation of the leaf surface to 

 the two media, air and water. In submerged plants, the leaves are con- 



Fig. 36. HippilHs vulgaris: 1, submerged leaf; 2, aerial leaf. X 130. 



