Explain the process of pollination in Vallisneria. How is it different in water-lily, which is also an aquatic plant?

Vallisneria is a hydrophtic, sub-merged plant in which male and female flowers borne on separate plants. The female flower reaches on the water surface by the long stalk while the male flower remains under water. When both flowers mature, male or staminate flowers release pollen grains in the water which comes on the water surface and along the water current, reach to female flowers and pollination occurs. Thus pollination is hydrophily.

Water-lily is a free floating, hydrophyte. Its flowers are present above the water. Therefore, pollination is done either by winds or insects.