Hydrophily - Cross-pollination
All hydrophytes are not necessarily pollinated by water. In fact most of the aquatic plant are anemophilous, e.g. Alisma, Nymphaea. Like anemophilous plants floral envelops are highly reduced or absent in hydrophilous plants. Hydrophily may involve underwater pollination referred to as hyphydrophily, e.g., Ceratophyllum, Majus, and Zostera. A unique example of this type is Zostera mariana (a submerged marine perennial) in which pollen grains are long (up to 250pm) and needle-like resembling pollen tubes. Because of the specific gravity of this pollen they freely float at any depth, and when they come in contact with the stigma they coil around it. In some taxa ephydrophily operates. In these plants pollination is brought about at the surface of water.
The classical example is the submerged dioecious plant, Vallisneria. The male and female flowers are produced under water but on maturity the males get detached from the stalk and float on the surface while the female flowers attached to thin, spirally coiled, long, slender stalks are brought to the surface at the time of pollination. Pollination is achieved through water currents when male flowers come in contact with pistillate flowers. After pollination the flowers are dragged down to the bottom by the recoiling of the stalk. The fruits thus develop under water.