Pollination is process of transfer of pollen grains from the anther to the stigma of a pistil.
- Some external agents help the plants for pollination. Depending on the source of pollen, pollination is 3 types.
a. Autogamy: In this, pollen grains transfer from theanther to the stigma of the same flower.
Complete autogamy is rare in flowers with exposed anthers and stigma. Autogamy in such flowers requires
synchrony in pollen release and stigma receptivity. Also, the anthers and stigma should lie close to each
other to enable self-pollination.Plants like Viola (common pansy), Oxalis & Commelina
produce 2 types of flowers:
Chasmogamous flowers: They are similar to flowers of other species with exposed anthers and stigma.
Cleistogamous flowers: They do not open at all. Anthers & stigma lie close to each other. They are
autogamous as there is no chance of cross-pollination.
When anthers dehisce in the flower buds, pollen grains come in contact with the stigma for pollination.
Cleistogamous flowers produce assured seed-set even in the absence of pollinators.
b. Geitonogamy: In this, pollen grains transfer from the anther to the stigma of another flower of the same plant. It is functionally cross-pollination involving a pollinating agent. But it is genetically similar to autogamy since the pollen grains come from the same plant.
c. Xenogamy: In this, pollen grains transfer from anther to the stigma of a different plant. This brings genetically different types of pollen grains to the stigma.
Agents of Pollination
1. Abiotic agents (wind & water) of pollination
Pollination by wind (anemophily):
- More common abiotic agent.
- Ways for effective pollination:
o The flowers produce enormous amount of pollen.
o The pollen grains are light and non-sticky so that they can be transported in wind currents.
o They often possess well-exposed stamens (for easy dispersion of pollens into wind currents).
o Large, feathery stigma to trap air-borne pollen grains.
- Wind pollinated flowers often have a single ovule in each ovary and numerous flowers packed into an inflorescence.
- E.g. Corncob – the tassels are the stigma and style which wave in the wind to trap pollen grains. Wind-pollination is quite common in grasses.
Pollination by water (hydrophily):
- It is quite rare. It is limited to about 30 genera, mostly monocotyledons. E.g. Vallisneria & Hydrilla (fresh
water), Zostera (marine sea-grasses) etc.
- As against this, water is a regular mode of transport for the male gametes among the lower plants. It is believed, particularly for some bryophytes & pteridophytes, that their distribution is limited because of the need for water for the transport of male gametes and fertilisation.
- In Vallisneria, the female flower reaches the surface of water by the long stalk and the male flowers or pollen grains are released on to the surface of water. They are carried by water currents and reach the female flowers.
- In sea grasses, female flowers remain submerged in water. Pollen grains are long and ribbon like. They are
carried inside the water and reach the stigma.
- The pollen grains of most of the water-pollinated species have a mucilaginous covering to protect from wetting.
- Not all aquatic plants use hydrophily. In most of aquatic plants (water hyacinth, water lily etc), the flowers emerge above the level of water for entomophily or anemophily.
- Wind and water pollinated flowers are not very colourful and do not produce nectar.
2. Biotic agents (animals) of pollination
- Majority of flowering plants use animals as pollinating agents. E.g. Bees, butterflies, flies, beetles, wasps, ants, moths, birds (sunbirds and humming birds) bats, some primates (lemurs), arboreal (tree-dwelling) rodents, reptiles (gecko lizard & garden lizard) etc.
3 - Pollination by insects (Entomophily), particularly bees is more common.
- Often flowers of animal pollinated plants are specifically adapted for a particular species of animal.
- Features of insect-pollinated flowers:
o Large, colourful, fragrant and rich in nectar. Nectar & pollen grains are the floral rewards for pollination.
o When the flowers are small, they form inflorescence to make them visible.
o The flowers pollinated by flies and beetles secrete foul odours to attract these animals.
o The pollen grains are generally sticky.
- When the animal comes in contact with the anthers and the stigma, its body gets a coating of pollen grains. When it comes in contact with the stigma, it results in pollination.
- Some plants provide safe places as floral reward to lay eggs.
E.g. Amorphophallus (it has the tallest flower of about 6 feet). A species of moth and the plant Yucca cannot
complete their life cycles without each other. The moth deposits its eggs in the locule of the ovary and the flower gets pollinated by the moth. The larvae of the moth come out of the eggs as the seeds start developing.
- Many insects consume pollen or nectar without bringing about pollination. They are called pollen/nectar robbers. Outbreeding Devices:
Majority of flowering plants produces hermaphrodite flowers can undergo self-pollination. Continued selfpollination results in inbreeding depression.
To avoid self pollination and encourage cross-pollination, there are some devices in plants:
a. Avoiding synchronization: In some species, pollen release and stigma receptivity are not synchronized.
Either the pollen is released before the stigma becomes receptive or stigma becomes receptive before the release of pollen. It prevents autogamy.
b. Arrangement of anther & stigma at different positions:
This prevents autogamy.
c. Self-incompatibility: It is a genetic mechanism to prevent self-pollen (from the same flower or other
flowers of the same plant) from fertilization by inhibiting pollen germination or pollen tube growth in the pistil.
d. Production of unisexual flowers: If male & female flowers are present on the same plant (i.e., monoecious,
e.g. castor & maize), it prevents autogamy but not geitonogamy. In dioecious plants (e.g. papaya), male and
female flowers are present on different plants (dioecy). This prevents both autogamy and geitonogamy.
Pollen-pistil Interaction:
- It is a dynamic process involving pollen recognition followed by promotion or inhibition of the pollen.
- This interaction takes place through the chemical components produced by them.
- If the pollen is compatible (right type), the pistil accepts it and promotes post-pollination events. The pollen grain germinates on the stigma to produce a pollen tube through one of the germ pores. The contents of the pollen grain move into the pollen tube. Pollen tube grows through the tissues of the stigma and style and reaches the ovary.
- If the pollen is incompatible (wrong type), the pistil rejects the pollen by preventing pollen germination on
the stigma or the pollen tube growth in the style.
- In some plants, pollen grains are shed at 2-celled condition (a vegetative cell & a generative cell). In such
plants, the generative cell divides and forms the two male gametes during the growth of pollen tube in the stigma.
- In plants which shed pollen in the 3-celled condition, pollen tubes carry 2 male gametes from the beginning.
Pollen tube, after reaching the ovary, enters the ovule through the micropyle and then enters one of the
synergids through the filiform apparatus. The filiform apparatus present at the micropylar part of the synergids
guides the entry of pollen tube.
- A plant breeder can manipulate pollen-pistil interaction, even in incompatible pollinations, to get desired hybrids.
Artificial hybridisation:
- It is one of the major approaches of crop improvement programme.
- In this, desired pollen grains are used for pollination. This is achieved by emasculation & bagging techniques.
- Emasculation is the removal of anthers (using forceps) from the bisexual flower bud of female parent before the anther dehisces.
- Emasculated flowers are covered with a suitable bag (made up of butter paper) to prevent contamination of its stigma with unwanted pollen. This is called bagging.
- When the stigma attains receptivity, mature pollen grains collected from anthers of the male parent are dusted on the stigma. Then the flowers are rebagged and allowed to develop the fruits.
- If the female parent produces unisexual flowers, there is no need for emasculation. The female flower buds are bagged before the flowers open. When the stigma becomes receptive, pollination is carried out using the
desired pollen and the flower rebagged.
- Some external agents help the plants for pollination. Depending on the source of pollen, pollination is 3 types.
a. Autogamy: In this, pollen grains transfer from theanther to the stigma of the same flower.
Complete autogamy is rare in flowers with exposed anthers and stigma. Autogamy in such flowers requires
synchrony in pollen release and stigma receptivity. Also, the anthers and stigma should lie close to each
other to enable self-pollination.Plants like Viola (common pansy), Oxalis & Commelina
produce 2 types of flowers:
Chasmogamous flowers: They are similar to flowers of other species with exposed anthers and stigma.
Cleistogamous flowers: They do not open at all. Anthers & stigma lie close to each other. They are
autogamous as there is no chance of cross-pollination.
When anthers dehisce in the flower buds, pollen grains come in contact with the stigma for pollination.
Cleistogamous flowers produce assured seed-set even in the absence of pollinators.
b. Geitonogamy: In this, pollen grains transfer from the anther to the stigma of another flower of the same plant. It is functionally cross-pollination involving a pollinating agent. But it is genetically similar to autogamy since the pollen grains come from the same plant.
c. Xenogamy: In this, pollen grains transfer from anther to the stigma of a different plant. This brings genetically different types of pollen grains to the stigma.
Agents of Pollination
1. Abiotic agents (wind & water) of pollination
Pollination by wind (anemophily):
- More common abiotic agent.
- Ways for effective pollination:
o The flowers produce enormous amount of pollen.
o The pollen grains are light and non-sticky so that they can be transported in wind currents.
o They often possess well-exposed stamens (for easy dispersion of pollens into wind currents).
o Large, feathery stigma to trap air-borne pollen grains.
- Wind pollinated flowers often have a single ovule in each ovary and numerous flowers packed into an inflorescence.
- E.g. Corncob – the tassels are the stigma and style which wave in the wind to trap pollen grains. Wind-pollination is quite common in grasses.
Pollination by water (hydrophily):
- It is quite rare. It is limited to about 30 genera, mostly monocotyledons. E.g. Vallisneria & Hydrilla (fresh
water), Zostera (marine sea-grasses) etc.
- As against this, water is a regular mode of transport for the male gametes among the lower plants. It is believed, particularly for some bryophytes & pteridophytes, that their distribution is limited because of the need for water for the transport of male gametes and fertilisation.
- In Vallisneria, the female flower reaches the surface of water by the long stalk and the male flowers or pollen grains are released on to the surface of water. They are carried by water currents and reach the female flowers.
- In sea grasses, female flowers remain submerged in water. Pollen grains are long and ribbon like. They are
carried inside the water and reach the stigma.
- The pollen grains of most of the water-pollinated species have a mucilaginous covering to protect from wetting.
- Not all aquatic plants use hydrophily. In most of aquatic plants (water hyacinth, water lily etc), the flowers emerge above the level of water for entomophily or anemophily.
- Wind and water pollinated flowers are not very colourful and do not produce nectar.
2. Biotic agents (animals) of pollination
- Majority of flowering plants use animals as pollinating agents. E.g. Bees, butterflies, flies, beetles, wasps, ants, moths, birds (sunbirds and humming birds) bats, some primates (lemurs), arboreal (tree-dwelling) rodents, reptiles (gecko lizard & garden lizard) etc.
3 - Pollination by insects (Entomophily), particularly bees is more common.
- Often flowers of animal pollinated plants are specifically adapted for a particular species of animal.
- Features of insect-pollinated flowers:
o Large, colourful, fragrant and rich in nectar. Nectar & pollen grains are the floral rewards for pollination.
o When the flowers are small, they form inflorescence to make them visible.
o The flowers pollinated by flies and beetles secrete foul odours to attract these animals.
o The pollen grains are generally sticky.
- When the animal comes in contact with the anthers and the stigma, its body gets a coating of pollen grains. When it comes in contact with the stigma, it results in pollination.
- Some plants provide safe places as floral reward to lay eggs.
E.g. Amorphophallus (it has the tallest flower of about 6 feet). A species of moth and the plant Yucca cannot
complete their life cycles without each other. The moth deposits its eggs in the locule of the ovary and the flower gets pollinated by the moth. The larvae of the moth come out of the eggs as the seeds start developing.
- Many insects consume pollen or nectar without bringing about pollination. They are called pollen/nectar robbers. Outbreeding Devices:
Majority of flowering plants produces hermaphrodite flowers can undergo self-pollination. Continued selfpollination results in inbreeding depression.
To avoid self pollination and encourage cross-pollination, there are some devices in plants:
a. Avoiding synchronization: In some species, pollen release and stigma receptivity are not synchronized.
Either the pollen is released before the stigma becomes receptive or stigma becomes receptive before the release of pollen. It prevents autogamy.
b. Arrangement of anther & stigma at different positions:
This prevents autogamy.
c. Self-incompatibility: It is a genetic mechanism to prevent self-pollen (from the same flower or other
flowers of the same plant) from fertilization by inhibiting pollen germination or pollen tube growth in the pistil.
d. Production of unisexual flowers: If male & female flowers are present on the same plant (i.e., monoecious,
e.g. castor & maize), it prevents autogamy but not geitonogamy. In dioecious plants (e.g. papaya), male and
female flowers are present on different plants (dioecy). This prevents both autogamy and geitonogamy.
Pollen-pistil Interaction:
- It is a dynamic process involving pollen recognition followed by promotion or inhibition of the pollen.
- This interaction takes place through the chemical components produced by them.
- If the pollen is compatible (right type), the pistil accepts it and promotes post-pollination events. The pollen grain germinates on the stigma to produce a pollen tube through one of the germ pores. The contents of the pollen grain move into the pollen tube. Pollen tube grows through the tissues of the stigma and style and reaches the ovary.
- If the pollen is incompatible (wrong type), the pistil rejects the pollen by preventing pollen germination on
the stigma or the pollen tube growth in the style.
- In some plants, pollen grains are shed at 2-celled condition (a vegetative cell & a generative cell). In such
plants, the generative cell divides and forms the two male gametes during the growth of pollen tube in the stigma.
- In plants which shed pollen in the 3-celled condition, pollen tubes carry 2 male gametes from the beginning.
Pollen tube, after reaching the ovary, enters the ovule through the micropyle and then enters one of the
synergids through the filiform apparatus. The filiform apparatus present at the micropylar part of the synergids
guides the entry of pollen tube.
- A plant breeder can manipulate pollen-pistil interaction, even in incompatible pollinations, to get desired hybrids.
Artificial hybridisation:
- It is one of the major approaches of crop improvement programme.
- In this, desired pollen grains are used for pollination. This is achieved by emasculation & bagging techniques.
- Emasculation is the removal of anthers (using forceps) from the bisexual flower bud of female parent before the anther dehisces.
- Emasculated flowers are covered with a suitable bag (made up of butter paper) to prevent contamination of its stigma with unwanted pollen. This is called bagging.
- When the stigma attains receptivity, mature pollen grains collected from anthers of the male parent are dusted on the stigma. Then the flowers are rebagged and allowed to develop the fruits.
- If the female parent produces unisexual flowers, there is no need for emasculation. The female flower buds are bagged before the flowers open. When the stigma becomes receptive, pollination is carried out using the
desired pollen and the flower rebagged.
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