hydrophily Vallisneria spiralis sea-grasses exine buoyant hydrophobic Lemnoideae staminate Vallisneria water pollination stigma pollen grasses
Pollination through water (hydrophily) is a much rarer method, occurring in only around 2% of abiotically pollinated flowers. Common examples of this include Calitriche autumnalis, Vallisneria spiralis and some sea-grasses. One characteristic which most species in this group share is a lack of an exine, or protective layer, around the pollen grain. Paul Knuth identified two types of hydrophilous pollination in 1906 and Ernst Schwarzenbach added a third in 1944. Knuth named his two groups 'Hyphydrogamy' and the more common 'Ephydrogamy.' In hyphydrogamy pollination occurs below the surface of the water and so the pollen grains are typically negatively buoyant. For marine plants that exhibit this method the stigmas are usually stiff, while freshwater species have small and feathery stigmas. In ephydrogamy pollination occurs on the surface of the water and so the pollen has a low density to enable floating, though many also use rafts, and are hydrophobic. Marine flowers have floating thread-like stigmas and may have adaptations for the tide, while freshwater species create indentations in the water. The third category, set out by Schwarzenbach, is those flowers which transport pollen above the water through conveyance. This ranges from floating plants, (Lemnoideae), to staminate flowers (Vallisneria). Most species in this group have dry, spherical pollen which sometimes forms into larger masses, and female flowers which form depressions in the water; the method of transport varies.
