Pollination and its Types Class 12 Botony Notes - Sexual Reproduction in Plants

Pollination and its types CBSE NCERT Notes Class 12 Biology 

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Pollination - 

The transference of pollen grains from the anther to the stigma is called pollination. It occurs in angiosperms.

Pollination

Types of pollination :

1. Self-pollination

2. Cross-pollination

Self Pollination – 

It is the transference of the pollen grains from the anther of a flower to the stigma of either same flower or genetically same flower. In autogamy, the transfer of pollen grains from the anther of a bisexual flower takes place on the stigma of the same flower. It occurs in several cereals, some peas, and flowers which do not open. In Geitonogamy, pollination takes place between flower of the same plant. Self-pollinated flowers are generally same, inconspicuous, colourless, odourless and nectarless.

Following conditions are necessary for self-pollination:

1. Homogamy – In this condition anthers and stigmas of a flower mature simultaneously. The pollen grains reach the mature stigma either by contact, wind, gravity, raindrops or even insects. The filaments bend down and bring the ripe anthers in contact with stigma. In potato curling of style takes place. Thus the pollen grains fall on the stigma.
2. Cleistogamy – It occurs in bisexual flowers which never open. Thus only self-pollination can occur. In groundnut, fruits are formed underground. 

Advantages of Self Pollination – 

• Parental characters can be preserved by self-pollination. 
• Chances of pollination are more.
• Self- pollination maintains the purity of the race and avoids mixing. 
• It need not to produce a large number of pollen grains. 
• 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 – 

• Weaker characters or defects cannot be eliminated.
• No useful characters can be introduced in the race.
• The immunity of race towards disease falls.
• The seeds become smaller and fewer. 
• Continued self-pollination leads to the death of species.
• The plants become less adapted to changes in the environment.

Cross-Pollination – 

It is the transference of pollen grains from the anthers of one flower to the stigma of genetically different flower. It is also known as Xenogamy. Cross-pollination is performed with the help of external agency like wind, water, animals etc. It is also carried out by plant breeders for maintaining races of cultivated plants and producing varieties. 

Following are the contrivances or devices to bring cross-pollination. 

1. Unisexuality or Dicliny – In unisexual or dicliny flowers, stamens and carpels occur in different flowers i.e. male and female. Two types of flowers may be born on the same type of plants or in different plants. The former types of plants are monoecious and later types are dioecious.
2. Dichogamy – This is the condition in which anthers or stigma of flower mature at different times. Therefore self-pollination is not possible. When anthers mature earlier than stigma, it is called protandry and when stigma matures earlier than anthers it is called protogyny. In nature, protandry is more common. 
3. Prepotency – In many cases, the pollen grains of anthers on other follower germinate more rapidly than the pollen grains of the same flower. Therefore, cross-pollination is favoured.
4. Self-sterility – Here, the pollen grains of flower are incapable of completing growth on the stigma of same flower. This is because of non-availability of growth-promoting hormones, presence of incompatibility genes.
5. Heterostyly – This is the condition in which two or more types of flowers are produced having different length of styles and stamens. When style is longer, flower is called pin-eyed and when stamens are longer, flower is called thrum-eyed. Thus, cross-pollination can take place.
6. Herkogamy – In this condition, some part of the flower acts a barrier to self-pollination and favour cross-pollination. The anther and stigma cannot come in contact because of barriers.

Different modes of Cross-pollination – 

Cross-pollination is of following types depending on the agent which participate in pollination. The various types are – 

1. Anemophily – It is the pollination by wind. The wind picks up pollen grains from the dehisced anther of a flower and drops to the stigma of another flower. Such plants grow in large groups, the flowers are produced above the foliage and is some flowers are produced before foliage. Since many pollen grains are wasted, so large quantities of pollen are produced. The pollen grains are dry and unwettable. Male flowers generally hang and so shaken by the wind to shed the pollen grains. Both the stigma and anthers are exposed. The stigmas are either sticky, hairy, feathery or branched to increase the surface area for catching pollen grains. The pollen grains are small, light and dusty. Some have wing-like structures to be carried by wind to far away. The flowers are generally small, inconspicuous and without colour, nectar or smell.
2. Hydrophily – It generally occurs in aquatic submerged plants. The perianth (fused sepals and petals) are unwettable because of the waxy coating. The pollen grains are light and covered with wax. Stigma is sticky but unwettable. There is no scent, colour and nectar. Common examples are Vallisneria, Lemna etc. 
3. Entomophily (Insect pollination) – It is the most common and most specialized mode of pollination and also most interesting type. Some flowers are modified to be pollinated by certain insects only. The insect visit flowers to get nectar, pollen grains, shelter and depositing their eggs and pollination is brought by chance. The entomophilous flowers are large, showy, variously coloured to attract insects. Some moths visit flowers at night (Rat-ki-Rani) because of smell emitted at night. Most of the flowers have smell, colour, and nectar. In some flowers pollen grains are edible. Bees are the most common visitors to the flowers to collect honey (nectar) and pollens. The pollen grains are spiny and sticky so as to stick to the bodies of insects. The stigma is also sticky to catch the pollen grains. Some flowers provide shelter to insects from heat.
4. Ornithophily – It is the pollination by birds. Small birds like sunbirds and humming birds bring about pollination. Plants like Begonia, Bombax, Agave, Indian coral tree are pollinated by Crow, Menach, Bulbul, and Parrot.
5. Chiropterophily – It is the pollination by bats. The bats pollinated flowers are large, stout with strong odour, abundant nectar, and pollen grains. 
6. Malacophily – It is the pollination by snails. 

Advantages of Cross-Pollination –

• Cross-pollination brings about genetic recombination and production of new varieties.
• It results in healthy and strong offsprings due to the phenomenon of hybrid vigour.
• Variations caused due to cross-pollination may result in the production of diseases resistant plants.
• Cross-pollination results in the production of seeds in self – sterile plants.

Disadvantages of Cross-Pollination –

• Cross-pollination is not economical. The plants waste a lot of energy and food materials in unnecessary adaptation and devices to bring about pollination. 
• It is uncertain because a factor of chance is always involved.
• It involves the addition of some undesirable character or loss of some important characters. 

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