Pollination refers to the transfer of pollen from dehiscing anthers to the pistil. Unlike animals, plants cannot move to their mates for sexual reproduction. Hence, they need some external device or agency for the transfer of pollen grains from the male parent to the stigma of the female parent. Exceptionally, in Vallisneria, an aquatic plant, complete male flower may be transported to the female flowers, the physical (wind and water) and biological (insects, birds and bats) agencies promote cross-pollination. The dehiscence of anthers and the transfer of pollen are the prime requirements of pollination.