Hydrophily

Transfer of pollen grains from the anther to the stigma through water is known as hydrophily.

When pollination of flowers occurs below water, it is said to be hypo-hydrophily.

When pollination of flowers occurs at the surface of water, it is called epi-hydrophily. 

To ensure successful pollination through water, hydrophilous plants have evolved unique structural features:

Hydrophily is classified into two types based on the level at which pollination occurs:

 

Ceratophyllum (Hornwort) — Freshwater

• A submerged freshwater plant with separate male and female flowers.
• Each male flower contains 30–45 stamens.
• On maturity, the stamens detach from the male flower and float upward to the surface.
• The anthers rupture during the upward journey, releasing pollen grains.
• These pollen grains are round and without an exine (outer protective wall).
• The pollen grains sink downward and are trapped by the sticky lower surface of the style (which serves as stigma) of the female flowers.
• Pollination is thus achieved entirely underwater.

Zostera marina (Sea Grass) — Marine

• A marine (saltwater) flowering plant.
• The female flowers remain permanently submerged in water.
• Pollen grains are long, ribbon-like, and without exine, adapted for floating in water.
• Pollen grains are released inside the water and carried passively by water currents.
• When they reach the stigma, the pollen coils around it, achieving pollination.
• Pollen is protected from wetting by a mucilagenous covering.

Vallisneria spiralis (Ribbon Weed / Eel Grass)

Vallisneria spiralis is a dioecious (having separate male and female plants), submerged freshwater aquatic plant.

Step-by-step mechanism:

Pollination by water in Vallisneria

• Meaning - Hydrophily is pollination by water; rare in flowering plants, limited to about 30 genera, mostly monocotyledons (e.g., Vallisneria, Hydrilla, Zostera).
• Pollen Characteristics - Pollen grains are long and ribbon-like; protected from wetting by a mucilaginous covering.
• Vallisneria Mechanism - Female flower reaches the water surface via a long stalk; male flowers/pollen are released onto the surface and carried passively by water currents.
• Seagrasses Mechanism - Female flowers remain submerged; pollen grains are released inside the water (e.g., Zostera).
• Water Hyacinth & Water Lily - Flowers emerge above water level and are pollinated by insects or wind, not water.
• Colour & Nectar - Flowers are not colourful and do not produce nectar; produce large amounts of pollen due to the chance factor of reaching the stigma.