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. After a 



Potamogeton crispus. Pollen distribution by 

 the wind. (After Kerner.) 



