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Pollination
2013-02-14, 8:05 PM

Pollination

                The process of transfer of pollen grains from anther to stigma is known as pollination. The pollination may be:                                                                                                                                               

Ø  Direct pollination

 In gymnosperms, ovules are present outside the ovary hence pollen grains falls directly on the ovules.

Ø  Indirect pollination

 In angiosperms, ovules are enclosed inside the ovary hence the pollen grains falls on the on the stigma but not on the ovules. These are again classified into 2 types.

o   Cleistogamy

     The pollination which occurs in unopened flowers                                            

   Examples: Commenlina bengalensis, Oxalis acetosella, Viola odorata

o   Chasmogamy

 The pollination which occurs in opened flowers. It is the most common pollination in all type of flowers. It is sub divided into:

·         Self pollination/Autogamy:

The transfer of pollen grains from anther to stigma in the same flower.

Examples: Lily, Rose

·         Cross pollination/Allogamy:

The transfer of pollen grains from anther to stigma of another flower but same species. It occurs in 2 types

§  Geitonogamy

The pollination between two flowers present on the same plant and the pollen grains of one flower falls on the stigma of other flower.  

Example: Cucurbita.

§  Xenogamy

The pollination occurs between two flowers present on different plants and the pollen grains of one flower of same type present on another plant.

Example: Pyrus malus



Ø  Advantages of Cross pollination

·         Productivity of seed is more through cross pollination.

·         Seeds are large, healthy, germinate quickly and the yield is more.

·         Plants occupy very less space and the production of flowers is more.

·         Plants show genetic variation.

·         Plants have resistance for disease. 

Ø  Contrivances for Cross pollination

·         The flower of same plants shows different modifications by which self pollination is prevented and cross pollination is carried out.

·         If flowers are bisexual self pollination occurs and unisexual cross pollination takes place (flowers may be staminate/Pistillate)

Examples: Vallisneria, Papaya

·         In some flowers, androecium and gynoecium mature at different times hence cross pollination takes place.

·         In some bisexual flowers, androecium and gynoecium mature at same time but self pollination is prevented by the arrangement of stigma and stamens at different lengths which promotes cross pollination. 

Ø  Agents for cross pollination

The pollen grains are not capable of reaching stigmas spontaneously hence they take the help of external agents like air, water and birds/animals. Based on these agents, they are of 3 types:       

  • Anemophily

This pollination which occurs through the wind. The plants in this may be short (Oryza) or big and long (Borassus). These flowers exhibit some special features like:

 

·         Plants grow in places where the velocity of wind is more.

·         Flowers are not showy and produce more amount of pollen (dry and light in weight.)

·         Flowers are bisexual and protandrous.

·         Perianth lobes may be reduced/absent where as essential organs elaborates.

·         Stamens show versatile fixation(drooping filaments).

·         Inflorescence is normally spike, catkin/spadix.

·         Some flowers with winged pollen grains (Pinus).

·         Stigmas may be sticky which helps in catching pollen.

·         Flowers do not produce scent/nectar.

 

  •        Hydrophily

This pollination occurs through the water majorly in hydrophytes. The plants in this produce long, needle like pollen grains. These are of 2 types:

·         Epihydrogamy

The pollination which occurs on the surface of water.

Example: Vallisneria

§  Pollination in Vallisneria

 Male flowers are detached from the male plant and float on the surface of water. At the same time, female flowers come on to the surface of water by a long pedicel hence, the pollen touches the stigmas and cross pollination occurs.

After pollination, the pedicels of female flower coils like a spring a pull the flowers to the bottom and thus fertilization takes place under the water.

·         Hypohydrophily

The pollination which occurs below the surface of water.

Example: Zostera 

§   Pollination in Zostera

At maturity, anthers produce needle like pollen grains. These will remain suspended in water. The styles are long with sticky stigmas to hold the pollen grains. Hence the pollination occurs below the water.

  •         Zoophily

This  pollination occurs by the animals. Based on the type of animals they are 5 types:

·         Ornithophily:Pollination by birds

Examples: Delonix - Humming birds,Bigonia –crows and mynas.

·         Cheiropteriphily: Pollination by bats.   

                           Examples:Oroxylum,Musa

·         Malocophily: Pollination by slugs and snails.

                   Example: Lemna sulami

·         Ophiophily: Pollination by snakes

                   Example:Arisaema

·         Cantharophily: Pollination by beetles. Example:Cycas.

·         Entomophily: Pollination by insects.

                   Example: Bees.

 

Ø  Characters of Entomophilous flowers

·         Size: Large and showy to attract the insects. If the flowers are inconspicuous and attract the insects.

 Head inflorescence: Sun flower

·         Colour: Flowers become showy with coloured petals and perianth lobes. Insects are attracted by these colors.

Examples: Bouganvillea ,Passiflora

·         Scent: The olfactory sense is more in insects than in human beings. Generally nocturnal flowers are not showy but produce scent to attract insects. 

                                           Examples: Nyctanthus – Night Jasmine

    Jasminum officinale – Jasmine

Some flowers emit smell of rotten meat which attracts the flies

Example: Rafflesia arnoldi

Sometimes scented leaves attracts the insects

Example: Coriandrum Some flowers attracts insects by both (colour, scent)

Example: Rose.

·         Pollen: Flowers produce more amount of pollen to attract insects (sterile and proteinaceous)

Examples: Argemone ,Calotropis

·         Nectar: Flowers produce honey to attract insects.

Example: Ixora 

Generally, nectaries are present at the base of the ovary. In some flowers they are present in different parts of the plant other than flowers (extra-floral nectaries).

Examples: Pass flora – Petioles, Euphorbia – Involucre

    

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