POLLINATION

Pollination:

·         Transfer of pollen grains from the anther to the stigma of a pistil is termed as pollination.

·         Both male and female gametes are non-motile.

 

Kinds of pollination:

Autogamy:

·         Pollination within same flower.

·         In open and exposed anthers and stigma autogamy is rare.

·         Viola, Oxalis and Commelina produce two types of flowers:

o    Chasmogamous: exposed anther and stigma

o    Cleistogamous: closed anther and stigma.

·         Cleistogamous flower is invariably autogamous and assured seed set even in the absence of the pollinator.

Geitonogamy:

·         Pollination between two flowers of the same plant.

·         Pollination by pollinating agent.

·         Genetically similar to the autogamy.

Xenogamy:

·         Transfer of pollen grains from the anther to the stigma of different plant.

·         It is commonly called as cross-pollination.

·         It brings genetically different types of pollen grains to the stigma.

 Agents of pollination:

·         Plant use two abiotic agent i.e. wind and water for pollination.

·         One biotic agent for pollination such as animals.

·         Majority of plant use biotic agent for pollination.

·         Few plant use abiotic pollinating agent.

Anemophily:

·         Pollinating agent is wind.

·         Plants produces enormous amount of pollen when compared to the number of ovules available for pollination to compensate the uncertainties of pollination.

·         Flowers with well exposed stamens.

·         Large feathery stigma to trap air-borne pollen grains.

·         Most wind pollinated flower contains single ovule in one ovary and numerous flower packed into an inflorescence e.g. corn cob.

·         Pollen grains are light and non-sticky.

Hydrophily:

·         Pollination by abiotic agent like water.

·         This type of pollination is very rare, about 30 genera, mostly monocot.

·         Vallisneria, Hydrilla and Zostera are the common example for Hydrophily.

·         All aquatic plants are not Hydrophily.

·         Pollen grains released into the surface of water and carried to the stigma by air current as in Vallisneria.

·         In sea grass the flowers remained submerged.

·         Pollen grains are long, ribbon like and carried passively inside the water

·         Pollen grains are protected from wetting by mucilaginous covering.

Pollination by biotic agent:

·         Majority of flowering plants use a range of animals as pollinating agents.

·         Among the animal, insect particularly bees are the dominant biotic agents for pollination.

·         Insect pollinating flowers are very large, colorful, fragrant and rich in nectar.

·         Small flowers present in cluster to make them conspicuous.

·         Flower pollinated by flies and beetles secrete foul odours.

·         Nectar and pollen grains are the usual floral rewards for insects.

·         In some species floral rewards are in providing safe places to lay eggs: e.g. Amorphophallus.

·         A species of moth and Yucca plant cannot complete their life cycle without each other. The moth deposits its eggs in the locule of the ovary and the flower in turn get pollinated by the moth.

·         Many insects may consume pollen or nectar without bring about pollination. Such floral visitors are referred as pollen/nectar robbers.

 

Outbreeding Devices:

·         Majority of the flowering plants produce hermaphrodite flower and undergo autogamy.

·         Continuous autogamy or self-pollination results in inbreeding depression.

·         Flowering plants have developed many devices to avoid self pollination and to encourage cross-pollination. Such devices are called Outbreeding devices.

o    Pollen released and stigma receptivity is not synchronized.

o    Spatial separation of anthers and stigmas

o    Anther and stigma are placed at different positions.

o    Self incompatibility.

o    Production of unisexual flowers.

Pollen pistil Interaction:

·         All the events – from pollen deposition on the stigma until pollen tubes enter the ovule – are together referred as pollen-pistil interaction.

·         Pollination does not guarantee the transfer of the right type of pollen grain to the right type of stigma.

·         The pistil has the ability to recognize the pollen whether it is compatible or incompatible.

·         If it is right type the stigma allow the pollen to germinate.

·         If it is wrong type the stigma rejects the pollen, preventing germination.

·         The ability of the pistil to recognize the pollen by continuous dialogue mediated by chemical like Boron, Inositol and sucrose level.

·         Following compatible pollination, the pollen grain produce pollen tube through one of the germ pore.

·         Content 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 grain is in 2-celled stage the generative cell divides and forms two male gametes inside the pollen tube.

·         If the pollen grain is in 3- cell stage the pollen tube carry two male gametes from the beginning.

·         Pollen tube enters into the ovule through micropyle and then into the embryo sac through synergids guided by filiform apparatus.

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