Pollination

Before understanding the types of pollination, it is important to understand what pollination is. So, pollination is the transfer of pollen grain from the anther (male part) to the stigma (female part) of a plant. This is followed by fertilisation and the production of seed and fruit. Thus, pollination is the pre-fertilization stage in plants.

Thus pollination is a type of sexual reproduction seen in plants. Pollination takes place in angiosperms—the pollen transfers from the male cone to the female cone in gymnosperms.

Types of pollination

The process of pollination is broadly classified into two types: 1. Self-pollination 2. Cross-pollination.

Let’s discuss this one by one

1) Self-pollination

It is further divided into two types:-  1. Autogamy and 2. Geitonogamy

1)Autogamy

In this process, pollen is transferred from the anther to the stigma of the same flower. For this coordinated opening of anther and stigma, maturation is a prerequisite.

Conditions:-

1.   Synchronization of anther and stigma:- when pollen is released from anther, stigma should be ready to receive the pollen simultaneously.
2.   The distance and position:- anther and stigma should be close enough for pollination.

It generally occurs in cleistogamous flowers because, in such flowers, anther and stigma are not exposed and lie close to each other. This makes it more feasible for self-pollination.

In Autogamy, pollinators are rarely required. So these flowers do not have nectar, smell, or colours to attract the pollinators.

It is the closest form of inbreeding and leads to homozygosity.

Eg:- Sunflower, Orchid, Peas, Commelina, Groundnuts, Tridax

2) Geitonogamy:

In this process, the transfer of pollen grain takes flare from the anther to stigma between different flowers of the same plant. Though it is a type of self-pollination, it requires pollinators. Thus geitonogamy is the type of self-pollination; it has very less extent of change in genetic composition but requires an external carrier for pollination just as in cross-pollination.

It does not take place in cleistogamous flowers but chasmogamous flowers.

Eg:- Grains and cereals.

Advantages of self-pollination:

1.   If a given genotype is suited to an environment, self-pollination helps to keep the same traits stable for generations.
2.   Can take place without pollinating agents
3.   Less wastage of pollens
4.   Self-pollinating plants do not need external carriers.
5.   Species can be maintained with purity
6.   Flowers do not need to produce nectar, smell, or coloured petals to attract pollinators.
7. Pollination can take place even in the absence of pollinators and can grow in areas like the Arctic or high elevations

Disadvantages of self-pollination:

1.   No adaptation to the environment during the change or pathogenic attack
2.   Inbreeding depression due to exposure to a recessive mutation
3.   Genetic defects cannot be eliminated by genetic recombination

Most of the self-pollinating flowers are small, and they may pollinate before the bud opens.

2)Cross-pollination

Scientifically this is known as xenogamy. In this, the pollen grain transfers from the anther of one flower to the stigma of another flower of another plant. It can also be said that it transfers pollen from one plant to another. Takes place in cleistogamous flowers.

This is the only method by which unisexual plants can reproduce—also known as heterogamy.

The movement of pollen occurs by wind or by animals.

Based on the type of pollinating agents, cross-pollination can be of various types.

1. Hydrophilous: Pollination takes place with the help of water. Flowers are small. They do not have an odour or colourful petals. Pollens are adapted to float on water:- Hydrilla, Zostera, Vallisneria, Ceratophyllus

2.   Zoophilous – Here, the pollinating agents are animals such as bats, birds, etc. The pollen is sticky to be carried from one flower to another flower very easily. Eg:- Amorphophallus, Dieffenbachia, Monstera, Philodendron,
3.   Anemophilous – In this type, pollination occurs with the help of wind. Here also the flowers are small and inconspicuous. The pollen is light weighted and non-sticky. They are sometimes winged. Eg:- Poplar,  Beech,  Alder,  Oak, Chestnut, Willow, maize, rice, etc
4.   Entomophilous – The pollinating agents here are insects. Insects are the major pollinating agents. The flowers are often attractive with big and bright petals. They also produce nectar and fragrance. The nectar-producing flowers attract bees, butterflies, ants, etc. The pollens are spiny and sticky so that insects can easily carry them. Pollination by ants is known as myrmecophily. Pollination by bees, butterflies, moths, etc. is known as hymenoterophily. At least 30% of crop plant species depend on bees and other insect pollination. Eg:- Jasmine, Rafflesia, Euphorbia, Bougainville, Salvia, Fig etc
5.   Ornithophilous- Here, the pollinating agents are birds. Birds pollinate very few plants. The flowers are bright yellow and red colours. Flowers that birds pollinate are tubular. Eg:- Bottlebrush, red Coral tree, silk cotton

Advantage of cross-pollination:-

1.   It is beneficial as it introduces new genes due to fertilisation between two genetically different gametes
2.   It improves the offspring’s resistance to diseases and changes in the environment. Thus adaptive
3.   Recessive characters are eliminated due to genetic recombination.

Disadvantages of cross-pollination:-

1.   High production of pollen grain to ensure pollination. Thus high wastage.
2.   There is a high probability that the good qualities may be eliminated due to the recombination of genes. Also, unwanted characters may be added due to the same.
3.   External carriers are a must.

As cross-pollination allows a higher rate of genetic diversity, there are many ways developed by plants to avoid self-pollination, also known as self-incompatibility. This prevents inbreeding and promotes the production of new genotypes in plants. Self-incompatibility is considered one of the main reasons for the spread and success of angiosperms.

Eg:- Maturation of pollen and stigma at different times, while some other flowers develop physical features to prevent self-pollination like primrose i.e. they develop different lengths of stigma and anther.

Some other plants have male and female flowers at different parts of the plants, so self-pollination is difficult.

And some other plants have male and female flowers in different plants, so self-pollination is impossible.

There is another mechanical mechanism called hand pollination. It is done when self and cross-pollination are not desirable. A cotton swab or brush are tools used for pollen transfer.

Conclusion- 

Thus pollination is a type of sexual reproduction seen in plants. Pollination takes place in angiosperms—the pollen transfers from the male cone to the female cone in gymnosperms.As cross-pollination allows a higher rate of genetic diversity, there are many ways developed by plants to avoid self-pollination, also known as self-incompatibility. In this process, pollen is transferred from the anther to the stigma of the same flower.