Pollination means or it can be defined as the transfer of pollen grains from the stamen of flower to its ovule bearing organisms. In cycads and conifers in which the ovules are exposed the pollen is caught in a drop of fluid which is secreted by the ovule. In flowering plants, the ovules are in a hollow organ called the pistil and the pollen is deposited on the pistil’s surface called the stigma. Here, the pollen germinates and gives rise to the pollen tube. The pollen tube grows towards the ovule base and through double fertilisation, one sperm cell fertilises the egg cell of the ovule which starts the development of the embryo. There is also another sperm cell in the pollen tube which combines with two subsidiary sexual nuclei of the ovule. This initiates the formation of reserve food tissue. The growing ovule eventually develops to seed.
In general terms, pollination is the process of reproduction in plants. It is the transfer of pollen to an ovule which enables fertilisation and the production of seeds and this process requires at least one pollination agent. In this article, we will discuss what are the most important agents for pollen grain transfer.
There are three types of agents than can be observed:
Anemophily: Anemophily refers to pollens grains transferred with the help of air. This type of pollination is observed in large forests where the air carries pollen from one tree to different trees in another area. It can be on same the tree as well. It is most effective for anemophilous flowers which have versatile, free-swinging, light anthers and they can be carried easily by the wind. Examples of anemophilous plants are coconut, palm, maize etc.
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Zoophily: Zoophily is the method in which animals help in pollination. There are three categories of zoophily
Entomophily: Pollination carried out by insects.
Ornithophily: Pollinations carried out by birds
Chiropterophily: Pollination carried out by bats
There are various factors which play a part in zoophily. Such as the scent of the flower, nectar position, edible sap and pollen which attracts insects. Whilst in ornithophily tiny birds like the hummingbird and honey thrushes feed on the nectar of flowers like Begonia and thereby pollinate them. In chiropterophily, trees like java and bauhinia are pollinated by bats.
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Hydrophily: Hydrophily or water pollination is very common in plant groups such as algae, pteridophytes, and bryophytes. For many aquatic plants, such as water hyacinth and lily, flowers emerge above the water level and are pollinated by insects or wind. The pollen grains are protected by a mucilaginous covering in most water-pollinated species.
There are Two Modes of Pollination in Plants
Self-pollination: In this method of pollination, it occurs when the pollen grain of a flower falls directly from the anther to the stigma of the same flower. The pollination process is very fast effective, however, it reduces genetic diversity in the species of the plant as the sperm and the egg cells of the flower share some of the same genetic information.
Advantages and disadvantages of self-pollination
Self-pollination has both advantages and disadvantages. In this section, we will discuss both.
The Advantages of Self-Pollination include:
It ensures that all recessive characters are eliminated.
There is less wastage pollen grains compared to cross-pollination
The purity of the species is maintained as there is no diversity in genes.
External agents such as wind, water and pollinating agents are not involved.
The quantity of produced pollen grains from pollen has a good success rate in the rate of pollination.
The Disadvantages of Self-Pollination Include:
The offspring have low immunity against diseases.
The vitality and the hybrid vigour or heterosis of the plant species are reduced.
Self-pollination does not yield new varieties.
Cross-Pollination: It is a method of pollination that takes place when pollen is delivered from the stamen of one flower to the stigma of a flower on another plant of the same species.
Plants which have adapted for cross-pollination have several mechanisms to prevent self-pollination. Their reproductive organs may be arranged in a way that self-pollination is unlikely, or the stamens and carpels may mature at different times.
Advantages and Disadvantages of Cross-Pollination
The Advantages of Cross-Pollination are:
High-quality seeds are produced and the plant species show a high rate of heterosis and vigour.
All unisexual plants reproduce by this method of pollination.
Genetic recombination with the species results in the elimination of all recessive traits.
Offspring plants show better immunity against diseases.
Cross pollinations introduce gene variety with the species.
The Disadvantage of Cross-Pollination include:
Excessive wastage in pollen grains
Due to genetic recombination, good characteristics of a species may get lost and unwanted characteristics may get introduced.
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1. What is the significance of pollination?
A: Pollination in plants is important due to many reasons. Some of them are mentioned below.
It is a pre-requisite for fertilisation in all flowering plants.
Pollination is an important process that brings the two types of gametes closer to fertilisation which ultimately results in zygote formation.
It helps in the production of seeds and fruits, which ensure continuity of the plant species.
Cross-pollination produces offspring with a combination of characters from both parent varieties. Seeds from cross-pollinated plants are more viable and tolerant and adaptable to various environmental factors.
Pollination helps in genetic recombination between plants. Due to this, new characters can be observed in the offspring.
Premature abscission of the ovary is prevented.