Double Fertilisation

To determine which aquatic plant does not show pollination by water, let's explore the pollination mechanisms of each provided option:

1. Vallisneria: This is an aquatic plant that is known to exhibit hydrophily, where pollination occurs through water. Male flowers release pollen that floats on the water surface to reach female flowers.
2. Hydrilla: Similar to Vallisneria, Hydrilla also relies on hydrophilous pollination. Pollen is transported by water currents to pollinate female flowers.
3. Water hyacinth: This plant does not use water for pollination. Instead, water hyacinth primarily relies on entomophily (pollination by insects). It has beautiful, fragrant flowers that attract insects for pollination.
4. Zostera: Also known as eelgrass, Zostera is a marine plant that carries out pollination through water (hydrophily), like most seagrasses.

Based on the above explanations, Water hyacinth is the aquatic plant that does not rely on water for pollination but instead depends on insect pollination.

The correct answer is Water hyacinth.