Hydrophily

Hydrophily is a fairly uncommon form of pollination whereby pollen is distributed by the flow of waters, particularly in rivers and streams. Hydrophilous species fall into two categories: (i) Those that distribute their pollen to the surface of water. e.g. Vallisneria's male flower or pollen grain are released on the surface of water, which are passively carried away by water currents; some of them eventually reach the female flower (ii) Those that distribute it beneath the surface. e.g. seagrasses in which female flower remain submergered in water and pollen grains are released inside the water.

Vallisneria spiralis is an example of hydrophily. Female flowers reach the water's surface temporarily to ensure pollination.

Surface pollination

Surface pollination is more frequent,[1] and appears to be a transitional phase between wind pollination and true hydrophily. In these the pollen floats on the surface and reaches the stigmas of the female flowers as in Hydrilla, Callitriche, Ruppia, Zostera, Elodea. In Vallisneria the male flowers become detached and float on the surface of the water; the anthers are thus brought in contact with the stigmas of the female flowers.[1] Surface hydrophily has been observed in several species of Potamogeton as well as some marine species.

Submerged pollination

Species exhibiting true submerged hydrophily include Najas, where the pollen grains are heavier than water, and sinking down are caught by the stigmas of the extremely simple female flowers,[1] Posidonia australis orZostera marina[2] and Hydrilla.

NOTE

1. Chisholm 1911, p. 3
2. Cox 1988, pp. 261–279.

References

• Cox, P.A. (1988). Hydrophilous pollination. Annual Review of Ecology and Systematics. 19. pp. 261–279.CS1 maint: ref=harv (link)
•  This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). "Pollination". Encyclopædia Britannica. 22 (11th ed.). Cambridge University Press. pp. 2–5.