. Fresh-water biology. Freshwater biology. 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.
. Fresh-water biology. Freshwater biology. 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 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 of
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectfreshwa, bookyear1918
