(a) Explain how some plants are adapted for achieving pollination through insects.
(b). Explain how pollination takes place in vallisneria and submerged aquatic plants.

Explanation

(a) Characters of Entomophilous Flowers : In majority of the plants, cross pollination takes place through agents like insects and such flowers are called entomophilous flowers. These show the following characters.
Colour : Normally petals of these flowers are brightly coloured to attract insects for the purpose to carry out cross pollination. In some plants, other parts of the flower are also modified into petaloid structure. e.g. In bougainvillea, bracts are brightly coloured, in canna stamens are petaloids.
Nectar : It is the sugary substance secreted by gamopetalous corolla of entomophilous flowers. Insects while collecting nectar, unknowingly brings about cross pollination. In some plants, nectary glands are developed on involucre of cyathium inflorescence. e.g. poinsettia.
Scent: Some entomophilous flowers emit a strong scent during night, to attract the insects and take their service in cross pollination. E.g. nightqueen, jasmine. 
(b) Characters of Hydrophilous flowers : Generally, the pollen grains of submerged plants like hydrilla have the same specific gravity as that of water, so that pollen grains suspend and float in water at different depths. 

• In vallisneria special types of characters are developed for cross pollination. It is dioecious. Male plants bear small sessile male flowers in submerged spadix. These flowers detach and float on the surface of the water. Female plants bear solitary female flowers on the spirally coiled stalks. When flowers mature the stalks uncoil and bring the flowers to the surface of the water. When male flowers come in contact with a female flower, the antlers burst open and sticky pollen grains deposit on the stigma. After the pollination, the stalk of the female flower coils and thus flower is brought down into water, where fruit formation takes place.