594 



ECOLOGY 



854-857). Another group of amphibious plants, represented by Alisma, 

 Castalia, and Potamogeton natans, have narrow, thin, submersed leaves 

 and broad, thick, aerial, or floating leaves, while in Sagittaria, similar 

 differences are supplemented by the development of basal lobes (figs. 

 848-853). 



Not all amphibious plants, even in the above genera, are equally plastic. For 

 example, Radicula palustris has pinnately compound leaves in all habitats, and 

 Sagittaria graminea and S. lancifolia develop air leaves without basal lobes. Few 

 plastic species are equally variable at all times, the early leaves of Proserpinaca and 

 Sium being much divided, regardless of the habitat, while the latest leaves react 

 much less readily than do the plastic intermediate leaves. After Sagittaria has 

 commenced to produce sagittate air leaves, submergence results commonly in the 

 development of new air leaves with long petioles rather than in a renewed develop- 

 ment of water leaves. However, Limnophila seems to be about equally plastic at 

 all times, and in Myriophyllum heterophyllum, even the emersed flowering branches 

 develop water leaves upon submergence. 



The causes of leaf variation in amphibious plants. If a Proserpinaca 

 plant is removed from the water, entire leaves at once begin to develop 

 instead of dissected leaves; on the other hand, dissected leaves soon 

 appear if an air-grown specimen is placed in water (fig. 865). Alter- 

 nations of the two leaf forms may be produced by alternating the con- 

 ditions (fig. 866) . Such experiments show the extreme plasticity of the 

 plant, and the close relation existing between leaf form and environ- 

 ment. Among the factors suggested as responsible for these form changes 

 is nutrition. In Sagittaria and Castalia the smallest, narrowest, and 

 lightest leaves are developed in the deepest water, where but little light 

 penetrates, and where, therefore, the manufacture of carbohydrates 

 necessarily is slight. Even in Sium and Proserpinaca, the water leaves, 

 though larger than the air leaves, contain less structural material. In 

 such plants as Castalia and Alisma each new leaf is larger than the 

 preceding, as though the food supply increases with the increasing ex- 

 panse of foliage, involving a constant increase in food-making power. 

 Furthermore, each new leaf usually is better placed for light reception 

 than is the preceding leaf. The nutrition theory is favored further by 

 the fact that the removal of roots or leaves is followed by the renewed 

 development of small water leaves. 



The nutrition theory is inadequate, because it accounts for differences 

 in size rather than in form, and because nutrition is not a simple 

 factor, but a complex of many factors. Light has been regarded as 



