236 FLOWERS OF AQUATICS [CH. 



contraction does not actually bring it to the bottom of the water, 

 but that the last stages in the descent are accomplished by its 

 own weight, when it is ripe 1 . Other Hydrocharitaceae, e.g. 

 the marine genus Enhalus^^ possess a pollination mechanism 

 resembling that of Vallisneria. Others again, e.g. Elodea calli- 

 trichoides*, have, by a further modification, arrived at a type 

 of pollination which is strictly hydrophilous, for the pollen, 

 instead of being rubbed off against the stigmas, is shed explo- 

 sively and falls on to the surface film, reaching the stigmas by 

 flotation. The ultimate stage in the series of the Hydrocharita- 

 ceae is reached by the marine genus Halophila, in which neither 

 male nor female flowers emerge from the water, and the process 

 of pollination takes place in complete submergence 4 . The 

 stigmas are thread-like and the pollen-grains, being united into 

 strings, adhere readily to the stigmas, which present elongated 

 receptive surfaces. 



The family Hydrocharitaceae is, indeed, of unique interest 

 from the standpoint of the evolution of submerged pollination, 

 since it includes within itself all stages in the transition from 

 entomophily to hydrophily 5 . It contains insect-pollinated 

 flowers, such as Hydrocharis Morsus-ranae and Elodea densa, with 

 attractive perianths, and, sometimes, nectaries ; flowers in which 

 the unwetted pollen is conveyed over the water by the ' boat 

 mechanism,' e.g. Fallisneria-, flowers in which the pollen floats 

 on the surface of the water, e.g. Elodea callitrichoides; and, finally, 

 flowers with entirely submerged pollination, such as Halophila. 



Callitriche*, among the Dicotyledons, provides another group 

 of species in which the transition from aerial to aquatic pollina- 

 tion can be followed. The genus is subdivided into two sections : 

 Eu-callitriche, to which the ordinary amphibious species of 

 Water Starwort belong, and of which C. verna is the type, and 



1 Royer, C. (1881-1883). 2 Delpino, F. and Ascherson, P. (1871). 

 3 Hauman-Merck, L. (19132). See p. 55. 4 See p. 130. 



5 See pp. 55-57. 



6 Hegelmaier, F. (1864), Jonsson, B. (1883-1884), and Schenck, H. 

 (1885). 



