Pollination
The transferral of pollen grains from the opened anthers to the receptive stigma of the carpel is called pollination. It is of two types- SELF POLLINATION and CROSS POLLINATION.
SELF POLLINATION
Self pollination involves the transfer of pollen grains from the anthers of a flower to a stigma of the same flower or genetically similar flower. It occurs by two types s-
a) Autogamy- ( autos=self, gamos=marriage)
It is a kind of pollination in which the pollen from the anthers of a flower are transferred to the stigma of the same flower. It occurs by three methods -
i) Cleistogamy (kleisto=closed,gamos= marriage)
Some plants never open to ensure complete self pollination. This condition is called Cleistogamy, e.g, oxalis, viola etc. These cleistogamous flowers are bisexual small, inconspicuous, colourless and do not secrete nectar.
ii) Homogamy
Anthers and stigma of bisexual flowers of some plants mature at the same time. They are bought close to each other by growth, bending or folding to ensure self pollination. This condition is called Homogamy.
E.g Mirabilis( Four 'o, clock), potato, sunflower plants etc.
iii) Bud pollination
Anthers and stigma of the bisexual flowers of some plants mature before the opening of buds to ensure self pollination, e.g , Wheat, Rice, peas etc.
B) Geitonogamy ( geiton- neighbour, gamos- marriage)
It is a kind of pollination in which pollen from anthers of one flower are transferred to the stigma of another flower borne on same plant. It usually occurs in plants which shows monoecious conditions ( unisexual, male and female flowers are borne on the same plants).
ADVANTAGES of SELF pollination -
1. Chances of pollination are more.
2.self pollination maintains purity of the race and avoids mixing.
3.It needs not to produce a large number of pollen grains.
4. Flowers need not to possess devices such as large and showy petals , presence of scent and nectar,etc to attract pollinators.
DISADVANTAGES of Self pollination -
1 . Progeny continuously gets weaker after every generation.
2. Less chances of the production of new species and varieties.
CROSS POLLINATION
It involves the transfer of pollen grains from the flower of one plant to the flower of another plants. It is called xenogamy or allogamy .
The main floral characteristics which facilitate cross pollination are-
i) Herkogamy - Flowers possess some mechanical barriers on their stigmatic surface to avoid self pollination e.g, presence of gynostegium and pollinia in calotropis.
ii)Dichogamy - pollen and stigma of the flower mature at different times to avoid self pollination. It is of two types - protogyny and protandry.
iii) Self incompatibility - In some plants, the mature pollen fall on the receptive stigma of the same flower but fails to bring about self pollination. It is called self incompatibility.
iv) Male sterility- the pollen grains of some plants are not functional.Such plants set seeds only after cross pollination.
v) Dioecism- Cross pollination always occurs when plants are unisexual and dioecious i.e , male and female flower occurs on separate plants e.g, papaya, cucurbits etc
vi) Heterostyly - the flower of some plants have different lengths of stamens and styles so that self pollination is not possible e.g, primula, Linum etc.
Agents for cross pollination
a) Abiotic agents - such as wind current, gravity, water, etc.
b) Biotic agents - such as animal pollinators , insects, birds.