[Biology Class Notes] on What is Pollination? Pdf for Exam

Pollination means the transfer of pollen grains from the stamen of a 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 fertilizes the egg cell of the ovule which starts the development of the embryo. There is also another sperm cell in the pollen tube that combines with two subsidiary sexual nuclei of the ovule. This initiates the formation of reserve food tissue. The growing ovule eventually develops into a seed.

Agents of Pollination

In general terms, pollination is the process of reproduction in plants. It is the transfer of pollen to an ovule that 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 Pollan grains transfer.

There are three types of agents than can be observed:

1. Anemophily: Anemophily refers to pollen 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 the same 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.

2. Zoophily: Zoophily is the method in which animals help in pollination. There are three categories of zoophily

• Entomophily: Pollinations carried out by insects.

• Ornithophily: Pollination carried out by birds.

• Chiropterophily: Pollination carried out by bats.

There are various factors that play a part in zoom filly such as the centre of the flower, nectar position, edible SAP and pollen which attracts insects. Whilst in ornithophily, tiny birds like the hummingbird and honey crushes feed on the nectar of flowers like begonia and thereby pollinate them. In chiropterophily, trees like java and bauhinia are pollinated by bats.

3. Hydophily: Hydrophily or 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.

Pollination in Plants

There are two modes of pollination in plants:

1. Self-Pollination: This type of pollination occurs when the pollen grains of a flower to the stigma of the same flower. 

The pollination process is very fast and 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.

Self-pollination is of two types:

• Autogamy- It is a type of self-pollination method where the pollen grains are transferred to the stigma of the same flower. 

• Geitonogamy- This is a type of self-pollination method where pollen grains are transferred from the anther of one plant to the stigma of another flower of the same plant.

2. Cross-Pollination: The 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 that 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 carpel may mature at different times.

Advantages and Disadvantages of Self-Pollination 

Self-pollination has both advantages and disadvantages. In the section, we will discuss both.

The Advantages of Self-Pollination Include:

• It ensures that all recessive characters are eliminated.

• There is less wastage of pollen grains compared to cross-pollination.

• The purity of the species is maintained as there is diversity in genes.

• External agents such as wind, water and pollinating agents are not involved.

• The quantity of pollen grains produced from Poland 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.

• Pollination does not yield new varieties.

• Since there is no mixing of genes, self-pollination reduces the strength and vitality of the race.

Advantages and Disadvantages of Cross-Pollination

Cross-pollination has both advantages and disadvantages. In the section, we will discuss both.

The Advantages of Cross-Pollination are:

• 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. Cross-pollination is beneficial in the plant race as it introduces new genes to the gene pool due to fertilization between different genes.

• Spring plants show better immunity against diseases.

• Pollination introduces gene variety to the species. 

• Cross-pollination improves fertility resistance.

The Disadvantages of Cross-Pollination Include:

• Excessive wastage in Pollen grains. There is a high loss of pollen grains that need to be produced in large quantities to ensure fertilization.

• Due to genetic recombination, good characteristics of a species may get lost and unwonted characteristics may get introduced.

• There is a high probability that good traits can be eliminated and unwanted traits added because of genetic reunification.