The process by which pollen grains are transferred from anthers to stigma is referred as pollination. Without pollination there won’t be fruit set and seeds. Hence it is considered as a very important process in plant growth stage.
Pollination is of two types: viz. 1) Autogamy or self pollination and 2) Allogamy or cross pollination.
Transfer of pollen grains from the anther to the stigma of same flower is known as autogamy or self pollination.
Mechanisms promoting self-pollination:
1. Bisexuality. Presence of male and female organs in the same flower is known as bisexuality. The presence of bisexual flowers is a must for self pollination. All the self pollinated plants have hermaphrodite flowers.
2. Homogamy. Maturation of anthers and stigma of a flower at the same time is called homogamy. As a rule, homogamy is essential for self-pollination.
3. Cleistogamy. When pollination and fertilization occur in unopened flower bud, it is known as cleistogamy. It ensures self pollination and prevents cross pollination. Cleistogamy has been reported inpeas, wheat, barley, oats and several other grass species.
4. Chasmogamy. Opening of flowers only after the completion of pollination is known as chasmogamy. This also promotes self pollination and is found in crops like peas and beans.
5. Position of Anthers. In some species, stigmas are surrounded by anthers in such a way that self pollination is ensured. Such situation is found in tomato and brinjal. In some legumes, the stamens and stigma are enclosed by the petals in such a way that self pollination is ensured. Examples are greengram, blackgram, soybean, chickpea and pea.
Transfer of pollen grains from the anther of one plant to the stigma of another plant is called allogamy or cross pollination.
Mechanism promoting cross-pollination
1. Dicliny. It refers to unisexual flowers. This is of two types: viz. i) monoecy and ii) dioecy. When male and female flowers are separate but present in the same plants, it is known as monoecy. Examples are cucurbits, grapes, strawberry, cassava and rubber. When staminate and pistillate flowers are present on different plants, it is called dioecy. It includes papaya, date palm, spinach, hemp and asparagus.
2. Dichogamy. It refers to maturation of anthers and stigma of the same flowers at different times. Dichogamy is of two types: viz. i) protogyny and ii) protandry. When pistil matures before anthers, it is called protogyny such as in sapota. When anthers mature before pistil, it is known as protandry. It is found in aonla, passion fruit
3. Heterostyly. When styles and filaments in a flower are of different lengths, it is called heterostyly
4. Herkogamy. Hinderance to self-pollination due to some physical barriers such as presence of hyline membrane around the anther is known as herkogamy.
5. Self incompatibility: The inability of fertile pollens to fertilize the same flower is referred to as self incompatibility.
6. Male sterility: In some species, the pollen grains are non functional. Such condition is known as male sterility.