. La biologie d'eau douce. Biologie des eaux douces. L'ENSEMBLE DE LA VÉGÉTATION AQUATIQUE à partir de 189 la fleur-vitre pour servir de corps flottants. Qu'il y a d'espèces terrestres de l'Utricularia qui ont aussi des vessies, il semble fort probable que l'formes aquatiques ont été obtenues à partir des espèces terrestres. Certains auteurs ont suggéré que, étant sans racines et re- quiring azotés plus de nourriture qu'il peut être obtenu à partir de la sub- stances en solution dans l'eau, ces ours ont été mis en place pour assurer l'alimentation des animaux. Il est tout aussi probable que les formes aquatiques sont simplement en utilisant des structures qui sont caractéristiques de leur a

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. Fresh-water biology. Freshwater biology. THE LARGER AQUATIC VEGETATION 189 from the flower-stalk to serve as floaters. As there are land species of Utricularia which also have bladders, it seems quite probable that the aquatic forms have been derived from the land species. Some authors have suggested that, being without roots and re- quiring more nitrogenous food than can be obtained from sub- stances in solution in the water, these bladders have been developed to secure animal food. It is just as probable that the aquatic forms are merely using structures that were characteristic of their ancestors, which were land plants. Why the land species have developed such structures has never been demonstrated. Few, if any, of the flowering water plants depend upon seed repro- duction. Vegetative reproduction by runners, tubers, buds, stem fragments, etc., is particularly prominent among these aquatics. Seed reproduction is, however, common and many are the con- trivances utilized for securing the transfer of pollen and cross pollination. In some few cases, as Ceratophyllum, Naias, and Zannichellia, pollination occurs under water and the pollen is transferred by the water. The wind is an important agent in the transfer of pollen espe- cially for many of the Potamo- getons (Fig. 265). The stamens and pistils of Potamogeton crispus do not mature on the same plant at the same time. As the pistils mature first they must receive pollen from some other plant and by the time the stamens of their own plant.are ready to shed poflen, they have been polHnated and are no longer receptive to pollen. The pollination of Vallisneria spiralis has become a classic illustra- tion of the remarkable capacity for adaptation possessed by some plants. The individuals of this plant are of two kinds — one bearing stamens and the other bearing pistils only. The staminate flower cluster is enclosed in a sac which finally ruptures and the staminate flowers immediately rise to the water surface. Aft

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