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. Hippuris vulgaris: 1, submerged leaf; 2, aerial leaf. 130. 
