by pollen carried by water. The pollen may be transported on the water surface, as in tasselweeds (Ruppia) and water starworts (Callitriche), in which case it needs to be light enough to float and water repellent and the stigmas must be exposed at the water surface. Movement of such pollen grains may be enhanced by a natural outer coating of oil, which alters the surface tension of the water. In tape grass (Vallisneria spiralis) the whole male flower is released and attaches itself to the female flower at the surface of the water.
Pollen may also be transported through water. The mechanisms for this are very variable and are thought to be derived from entomophilic mechanisms. In naiads (Najas), pollen grains are heavy and sink due to gravity onto the female flowers underneath. In eel grasses (Zostera) pollen grains transform into structures that resemble a pollen tube, which wrap themselves around the stigma. Compare entomophily