44 



NATUEAL HISTOEY BULLETIN 



Horal habits are still essentially land plants. It must not be 

 assumed, however, that they have retained unmodified their 

 former habits of pollination. While some of them are possibly 

 anemophilous in the simpler sense of the term, others have 

 specialized flowers and employ the surface film of water in most 

 ingenious ways to aid in the transfer of pollen. Nor should the 

 members of the hydro-anemophilous group be looked upon as 

 transitional to the subsurface habit of pollination. On the con- 

 trary they probably represent a distinct specialization, with 

 structures and habits intimately related to pollination at the 

 surface of the water. Conspicuous examples are seen in Vallis- 

 neria and Elodea, while less highly specialized forms are found 

 in certain species of Myriophyllum and Potamogefon. 



The submersed plant that brings its flowers to the surface of 

 the water for pollination accomplishes this in one of three ways, 

 or, in the dioecious forms, there may he a combination of two of 

 these methods; — 



1. By elongation of the floral axis. This is the simplest and 

 probably in general the most primitive plan since it might be 

 developed gradually as plants pushed out into deeper water. 

 Examples are seen in Myriopliyllum spicatum L., several species 

 of Potamogeton, and in the pistillate flower of VaUisneria spi- 

 ralis L. 



2. By detachment of flowers from the plant. This leaves the 

 flowers free to float to the surface of the water. Obviously this 

 plan may be employed for staminate flowers only, and is there- 

 fore always associated with some other plan for the pistillate 

 flower which must retain connection with the plant until the 

 seeds are matured. Well known examples are the staminate 

 flowers of Elodea and VaUisneria spiralis L. 



3. By elongation of the flowers. This plan is the most highly 

 specialized of the three, involving as it does the radical modifica- 

 tion of the floral parts in order to bring the stamens and stigmas 

 to the surface of the water. In Elodea the epigynous pistillate 

 flower may be one thousand times as long as wide. The perfect 

 flowers of Heteranthera duhia (Jacq.) MacM., though hypogyn- 

 ous, attain to a marked degree of elongation in their upward 

 stretch toward the surface of the water. 



Another alternative suggests itself, namely, self-pollination be- 

 neath the surface of the water, but this seems seldom to occur. 



