Pollination is of two types viz., Self Pollination and Cross Pollination.
Self-pollination includes pollination of the stigma of the same (intersexual) flower (autogamy) or stigma of another flower belonging to the same plant (geitogamy). Self-pollination is possible in only those plants, which bear bisexual flowers, and only such bisexual flowers which achieve anther dehiscence and receptivity of stigma simultaneously.
Self-pollination has two major adaptations, which are known as:
a) Homogamy, when the anthers and stigmas of a flower or of different flowers of the same plant matures at the same time
b) Cleistogamy, when the flower does not open and pollination occurs within the flower itself.
Cleistogamy may be constitutional (inherent) or ecological (induced by severe environmental condition).
Commelina benghalensis (kankauoa) bears two types of flowers: (a) aerial flowers, which are brightly coloured (blue or violet), chasmogamous, and insect pollinated; and (b) underground, dull and cleistogamous flowers, borne on the subterranean branches of the rhizome.
Cross-pollination is defined as the transfer of pollen grain to the stigma of another flower of another plant. It is also known as allogamy or xenogamy. It brings about genetic recombination and variation.
Agencies for Cross Pollination:
There are several agents for cross-pollination. They are as follows:
1. Anemophily- Wind pollinated flowers are generally unisexual. The anthers produce large number of tiny, smooth and dry pollen grain since they have to travel long distances and there is a huge wastage. The flowers are inconspicuous and not showy, nor they emit any scent or produce nectar for attracting insects. Stigmas are long and often feathery. Anemophily is common in cereals, grasses, palms etc.
2. Hydrophily- Aquatic plants particularly the submerged ones have this kind of pollination. In Naias and Ceratophyllum pollination occurs completely under water. In Hydrilla, Elodea, Vallisneria, etc. pollination between the floating pollen grains and stigmas occurs at the surface of the water.
3. Zoophily- A vast number of flowers are pollinated by different kinds of animals. They are usually highly specialized types. Some types are as follows:
a) Entomophily (insect pollinated)- The entomophilous flowers are large and brightly colored to attract the insects. They are often aggregated into compact inflorescences. They emit scent and secrete nectar. The pollen grains are sticky and have rough surfaces. Bees, Butterflies, moths etc are some of the common insects involved in cross-pollination.
b) Ornithophily (bird pollinated)- Some flowers produce large amount of nectar for which very small birds like the humming birds, etc. visit them and pollinate them. Plants like red silk cotton, bottle brush, coral tree are visited by various birds which bring pollen grains with them.
c) Chiropterophily (bat pollinated)- Bat pollinated plants bear large sized flowers that also produce large amount of nectar and emit scent. The sausage tree (Kigelia pinnata), Bauhinia megalandara, Eperua falcate is bat pollinated.
d) Malacophily (pollination by molluscus)- Many aroids are believe to be pollinated by snails.
e) Myrmecophily (ant pollinated)- Certain plants are believed to be visited by ants in search of food.