When mature pollen grains, released from anther, are carried to stigma of a flower of the same or different species, it is called pollination. Pollination is transfer of pollen grains from the anther to the stigma of a flower.
Pollination is of two types:
1. Self-pollination: Transfer of pollen grains to stigma of the same or from another flower borne on the same plant as in the pea family e.g. pea and gram.
2. Cross-pollination: Transfer of pollen grains from a flower to stigma of another flower borne on another plant of the same species e.g. in palm and maize.
Pollination results in fertilization and stimulates the ovule to get converted into seed. New varieties of plants are formed through new combination of genes in case of cross pollination. During pollination pollen tube produces growth hormones which stimulate ovary to develop into fruit.
Cross pollination is brought about by various external agencies such as, wind, insects, water, birds and other animals.
Pollination by Wind (Anemophily)
Flowers are small, without colour, nectar and scent. Flowers produce a large number of pollen grains to allow for wastage when pollen-grains are carried by wind to another flower. The pollen grains are small, light and sometimes provided with Wings. The stigmas are comparatively large, protruding and some times hairy, to trap pollen grains from wind for example, grasses and some cacti.
Pollination by insects (Entomophily)
Flowers are usually large, coloured and showy to attract insects. Some of these flowers secrete nectar to attract insects. Salvia flowers show special adaptations for pollination by bees.
Pollination by Water (Hydrophily)
This takes place in aquatic plants. Pollen grains are produced in large numbers. Pollen grains float on surface of water till they land on the stigma of female flowers e.g. Hydrilla, Vallisnaria.
Pollination by Animals (Zoophily)
Flowers of such plants attract animals by their bright colour, size, and scent for example sun bird, pollinates flowers of Canna, and gladioli, and Squirrels pollinate flowers of silk cotton tree.
1. Unisexuality: Flowers may be only male or female, borne on different plants e.g. papaya, palm, or may be borne on the same plant, e.g. maize.
2. Dichogamy: Male and female sex organs mature at different times. In sweet pea, and Salvia, Anther matures earlier than the stigma and in custard apple (sharifa) carpel matures earlier than the anther.
3. Self Sterility: Pollen grains are incapable of affecting fertilization even after being placed on the stigma of the same flower e.g. Petunia, apple.