THE LARGER AQUATIC VEGETATION 



183 



nature seem to be able to bring forth either floating or submerged 

 leaves or both as the conditions imposed seem to require. If grow- 

 ing shoots of Ranunculus aquatilis are not allowed to reach the 

 surface of the water only the segmented leaves develop. If speci- 

 mens of Potamogeton heterophylhis are suddenly left stranded by 

 receding water thp floating leaves may persist and 

 be succeeded by more floating leaves, thus enabling 

 the plant to live for a considerable time, often 

 persisting until the rising water returns. In such a 

 case the submerged leaves soon die from exposure, 

 but the floating leaves have, on the upper surface, 

 stomata which, in cooperation with the thick 

 cuticle, are able to regulate the loss of water. 



Some of the amphibious species, such as Sagit- 

 taria natans, are especially variable in leaf form. 

 The early seedling leaves are bladeless and ribbon- 

 like, while the later leaves which rise above the 

 surface have a distinct blade and stalk (Figs. 258 

 and 259). From the evident plasticity of these 

 plants it may be supposed that the form of leaf to 

 be produced is not predetermined but depends 

 upon conditions. Wachter has experimented with 

 Sagittaria natans and 

 finds that plants hav- 

 ing the ribbon-Hke 

 leaves may be pre- 

 vented from later pro- 

 ducing bladed leaves 

 either by reducing the 

 intensity of light or 

 by partial starvation. 

 Plants which have already formed bladed leaves may be induced 

 in like manner to bring forth the ribbon form. In view of such 

 results it is not unreasonable to suppose that both the floating 

 and the submerged leaves may easily have developed during the 

 past from aerial leaves and that both kinds are useful to many 

 species. 



Fig. 258. Sagittaria 

 natans. Transition from 

 ribbon-like to bladed 

 leaves. § natural size. 

 (After Wachter.) 



Fig. 259. Sagittaria chinensis. Transition 

 from bladed to ribbon-like leaves. The 

 reversion has been produced by cutting 

 off the roots repeatedly. § natural size. 

 (After Wachter.) 



