Transfer of pollen grains from anther to stigma is called as pollination. It is of two types, self-pollination and cross pollination.
Type # 1. Self-Pollination:
It is the transfer of pollen grains from the anther to the stigma of same flower or to the different flower on the same plant.
Self-pollination can be of two types:
a. Autogamy:
When pollen grains of a flower fall on the stigma of the same flower. There are three methods by which this process occurs.
i. Bud Pollination:
Here pollination takes place at the bud stage only. Such pollination is found in bisexual homogamous flower e.g. Pea, Rice, etc.
ii. Cleistogamy:
Flowers which never open and are called cleistogamous flowers. Cleistogamous flowers are colourless, bisexual, nectarless and small in size e.g. Oxalis, Viola, etc.
iii. Homogamy:
In homogamy both anther and stigma mature at the same time. Mature anther and stigma come near each other and pollination takes place e.g. Mirabilis jalapa, Argemone mexicana.
b. Geitonogamy:
In geitonogamy, pollens of one flower are transferred to the stigma of another flower of the same plant.
Advantages of Self-Pollination:
i. Flowers may not be showy, large and may not produce nectar.
ii. Large number of pollen grains are not required.
iii. Chances of pollination are more.
iv. Pure lines are generated by self-pollination.
v. Parental characters can be preserved.
Disadvantages of Self-Pollination:
i. New varieties cannot be formed.
ii. Defective genes cannot be eliminated.
iii. New characters cannot be introduced.
iv. After a few generations the offspring produced is weak.
Type # 2. Cross-Pollination/Xenogamy/Allogamy:
When pollen grains of a flower fall on the stigma of flower of another plant but of the same species, it is called cross-pollination.
Nature has provided different mechanisms to favour cross-pollination:
i. Herkogamy:
Morphological barriers develop between anther and stigma which do not permit self-pollination e.g. Calotropis.
ii. Self-Incompatibility:
This situation is observed when fertilisation does not take place even when viable pollen grains fall on receptive stigma of the same flower.
iii. Heterostyly:
Under this condition plant bears two or more types of morphologically different flowers. In such flowers, stigma and style may lie at different levels (Fig. 15). Due to this self-pollination does not occur, e.g. Oxalis.
iv. Unisexuality/Dicliny:
Plants having flowers which are either pistillate or staminate. In such flowers only cross- pollination is possible e.g. Papaya, Cucumber, etc.
v. Dichogamy:
These are bisexual flowers but anther and stigma mature at different times. When androecium matures first it is called protandry e.g. Rose, Citrus, Salvia, etc. When gynoecium matures first it is called protogyny e.g. Apple, Palm, etc.
In cross-pollination, pollen grains from one flower are carried to the stigma of another flower. For this certain biotic and abiotic agents are required. Biotic agents can be insects, man, birds, bats. The abiotic agents can be wind and water.
Depending on the pollinating agents, pollination can be of the following types:
a. Entomophily,
b. Anemophily,
c. Ornithophily,
d. Chiropterophily,
e. Hydrophily and
f. Malacophily.
a. Entomophily:
When pollination takes place by insects it is called entomophily. Common insects participating in pollination are butterfly, moth, wasp, honey bee, etc.
An entomophilus flower should have any of these characters:
i. Flower is showy, brightly coloured and big.
ii. It should have nectar or fragrance.
iii. Flower should produce large number of pollen grains.
iv. In some flowers instead of petals other parts become brightly coloured, e.g. calyx in Brassica, bracts in Bougainvillea.
v. Certain plants have edible pollen grains.
b. Anemophily:
When pollination takes place by wind it is called Anemophily (Fig. 16a). For this the flower may not be showy or brightly coloured. Nectar or fragrance may also not be there. Instead pollen grains are light, small and dry. They may even develop wings. Pollen grains are produced in large numbers.
c. Ornithophily:
Pollination taking place with the help of birds is called Ornithophily. Flowers are bright and showy to attract the birds. They have nectar for which the birds visit the flowers and in turn pollinate them. A few birds which pollinate the flowers are Sunbird, Crow, Parrot, Humming bird, etc.
d. Chiropterophily:
Pollination brought about by bats is Chiropterophily and the flowers are chiropterophilus. Such flowers have very strong smell. They are dull but produce large number of pollens. Bats being nocturnal get attracted towards the flowers because of odour e.g. Adansonia (Baobab tree), Anthocephalus (Kadam tree).
e. Hydrophily:
Pollination brought about by water is hydrophily and the flowers are called hydrophilus (Fig. 16b). Hydrophilus flowers are colourless, odourless and nectarless. Pollen grains have a protective coating so that they do not decay in water. Pollen grains are long and needle like so that they can float easily on water surface e.g. Vallisenaria.
Advantages of Cross-Pollination are:
a. New varieties are formed.
b. Variations can be induced.
c. Diseases found in parents can be eliminated in offspring.
d. Best qualities of both the parent plants can be incorporated in offspring.
Disadvantages of Cross-Pollination are:
a. It may happen that desired qualities are lost and undesired qualities appear.
b. This process is less economical.
c. Pollination may not always occur.
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