Agents of Pollination
- Xenogamy or cross pollination is performed with the help of an external agency which may be abiotic (wind, water) or biotic (animals).
- Cross pollination is named after the agency that assists it, e.g., anemophily (wind pollination), hydrophily (water pollination),
entomophily (insect pollination), ornithophily (bird pollination), chiropterophily (bat pollination) and malacophily (snail pollination).
Cross pollinating flowers with different agencies have different characteristic features.
Characteristics of cross pollinating flowers
Wind pollinated flowers- Flowers are small and inconspicuous.
- Pollen grains are dry, light and non sticky.
- Well exposed stamens
- Large often feathery stigma to trap air-borne
pollen grains. - Single ovule in each ovary and numerous
flowers packed into an inflorescence. - Common in grasses
Insect pollinated flowers- Flowers are large, colourful, fragrant and rich in
nectar - The flowers produce an odour which may be
pleasant or foul. - When the flowers are small, a number of flowers
are clustered together into an inflorescence to
make them conspicuous. - Observed in jasmine, Rosa, Magnolia, etc.
Water pollinated flowers- Flowers are small and inconspicuous.
- Pollen grains are long, ribbon like and
protected from wetting by a muci-
laginous covering. - Stigma is long, sticky but unwettable
- Observed in Vallisneria, Zostera,
Ceratophyllum.
Outbreeding Devices or Contrivances to Ensure Cross Pollination
- Continuous self-pollination results in inbreeding depression. Therefore, angiosperms have developed many devices to
discourage self-pollination and encourage cross-pollination. - Dichogamy - Pollen release and stigma receptivity are not synchronised in some species. Either the pollen is released before the
stigma becomes receptive or stigma becomes receptive much before the release of pollen. In protandry, anthers mature earlier
than stigma of the same flower, e.g., Sunflower; in protogyny stigma mature earlier, e.g., Mirabilis. - Heterostyly - In some species the anther and stigma are placed at different positions so that the pollen cannot come in contact
with the stigma of the same flower. E.g., Primula, Lythrum. - Self incompatibility - This is a genetic mechanism that prevents self-pollen (from the same flower or other flowers of the
same plant) from fertilising the ovules by inhibiting pollen germination or pollen tube growth in the pistil. E.g., Tobacco, potato. - Dicliny - Flowers are unisexual so that self-pollination is not possible. The plants may be monoecious (bearing both male and
female flowers, e.g., Maize) or dioecious (bearing male and female flowers on different plants, e.g., Mulberry, papaya). - Prepotency - Pollen grains of another flower germinate more rapidly than the pollen grains of the same flower over the stigma,
e.g., apple, grape. - Herkogamy - The different mechanical devices to prevent self-pollination and promote cross pollination, e.g., Stigma lies inside a
flap in pansy, anthers occurs inside corolla pocket in Kalmia.
Pollen - Pistil Interaction
- It is a dynamic process that occurs from the time of pollen deposition over the stigma to the time of pollen tube entry into ovule.
- It is a safety measure to ensure that illegitimate crossing do not occur.
- Pollen grains of a number of plants may settle over a stigma.
- Pollen - pistil interaction ensures that only the right pollen belonging to same species would germinate while others fail to do so.
- Knowledge of pollen-pistil interaction is helpful to plant breeders in manipulating pollen growth even in incompatible cases.
Artificial Hybridisation
- Artificial hybridisation has been used by plant breeders for crop improvement programme.
- In artificial hybridisation, it is important to make sure that only the desired pollen grains are used for pollination and the
stigma is protected from contamination. - This is achieved by emasculation and bagging technique.
- Emasculation is removal of stamens from the floral buds of female parent (if the female parent bears bisexual flowers)
so that chances of self-pollination are eliminated. - Bagging is covering of emasculated flowers with a bag made of butter paper to prevent contamination of its stigma with
unwanted pollens. - When the stigma of bagged flower attains receptivity, mature pollen grains collected from the anthers of the male parent
flower are dusted on the stigma and the flowers are rebagged and the fruits are allowed to develop.