WATER POLLINATION 379 



flowers, which are only i mm. in diameter, are formed in 

 hundreds in a single inflorescence. Each consists of two 

 stamens enclosed in a perianth of two large segments and 

 a small one. They are detached under water and rise 

 slowly to the surface, where they open, the perianth segments 

 curving back, and supporting the flower on a little tripod 

 which is very stable. They are drifted about on the water. 

 In the inmiediate neighbourhood of a female flower they 

 are drawn to it by surface tension, and cluster round it, 

 tipping over so that the stamens touch the stigma. If the 

 female flower is momentarily submerged, as often happens 

 with small waves, the attendant males are inverted over it 

 in an air-bubble, and the chances of pollination are improved. 

 After fertilisation the peduncle coils up and the female 

 flower is drawn once more under water, where the fruit 

 ripens. 



In Elodea canadensis, also investigated by Wyllie (1904), 

 the sohtary male flowers are detached under water and shoot 

 to the surface, where they burst open, scattering pollen on 

 the water, where it drifts to the stigmas. 



Finally, we have the case of completely submerged 

 flowers like those of Zostera and Zannichellia. The pollen 

 is shed under water and drifts, submerged, to the filamentous 

 stigmas. We have already noted the tendency to elongation 

 in such pollen grains ; this culminates in the thread-like 

 pollen of Zostera, and is correlated with the absence of the 

 extine. The secondary nature of the aquatic habit in flower- 

 ing plants is nowhere more evident than in the very general 

 retention of flowers with sub-aerial pollination, and the small 

 number of cases of completely submerged flowers. 



§ 12. Pollination — Floral Mechanisms 



We have been concerned so far with the agencies effecting 

 pollination, without entering, in most cases dealt with, into 

 the exact method of pollen transfer. Of wind- and water- 

 pollinated flowers not much more need be said — pollination 

 is random. In entomophilous plants, on the other hand. 



