Introduction
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:
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.
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.
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:
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.
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.
Interesting Facts
Wheat, maize, rice, and soybeans, are self-pollinating or wind-pollinated.
In the United States, bees are brought to crops like cucumbers, melons, and strawberries that are grown for profit.
The loss of pollinators has been demonstrated mostly by the disappearance of bees.
Key Features
Pollination adaptation traits by plants attract insects like bees and moths.
Flowers pollinated by wind have many stigmas.
Both the flower and the moth alter their shapes to facilitate pollination.
Important Questions
1) What is Allogamy?
Ans: Allogamy, or cross-pollination, is when pollen from one flower's stamen travels to the stigma of another flower on the same plant. Cross-pollinating plants have numerous methods to prevent self-pollination, such as organised reproductive organs or stamens and carpels maturing at different periods. Pollen from one flower pollinates the identical flowers. It developed when pollinators weren't reliable pollen transporters and is particularly common in annual crops and plants that expand in new areas.
2) What are the various flowers that are pollinated by insects?
Ans: There are various flowers pollinated by insects. Flies are attracted to rotting-smelling flowers. These nectar-producing flowers are brown or purple. They can be found on the carrion flower Stapelia Rafflesia, dragon arum Dracunculus, and corpse flower Amorphophallus. Pollen is a good source of protein, and nectar is a good source of energy. Wasps are also significant insect pollinators, fertilising several fig varieties. The yucca moth, which pollinates yucca plants, is one of the best-studied moths in relation to these plants.
FAQs on Pollination
1. What is pollination and why is it essential for plants?
Pollination is a fundamental biological process involving the transfer of pollen grains from the male part of a flower (anther) to the female part (stigma). This process is essential because it is the prerequisite for fertilisation in flowering plants, which leads to the production of seeds and fruits. This ensures the continuation and propagation of the plant species.
2. What are the two primary modes of pollination found in plants?
The two primary modes of pollination are based on the source of the pollen:
- Self-Pollination: This occurs when pollen from the anther of a flower lands on the stigma of the same flower or another flower on the very same plant.
- Cross-Pollination: This involves the transfer of pollen from the anther of a flower on one plant to the stigma of a flower on a different plant of the same species.
3. How does cross-pollination provide a genetic advantage over self-pollination?
Cross-pollination provides a significant genetic advantage by promoting genetic diversity. When pollen from two different parent plants combines, it results in new combinations of genes in the offspring. This variation can lead to increased vitality, better resistance to diseases, and greater adaptability to changing environmental conditions. Self-pollination, in contrast, limits the gene pool, which can lead to inbreeding depression over generations.
4. What are the different agents that facilitate pollination?
Pollinating agents, or vectors, are the means by which pollen is transferred. They are broadly categorised into two types:
- Abiotic Agents (non-living): This includes pollination by wind (anemophily) and water (hydrophily).
- Biotic Agents (living): This includes pollination by insects (entomophily), birds (ornithophily), bats (chiropterophily), and other animals.
5. How are flowers adapted for pollination by wind versus pollination by insects?
Flowers show specific adaptations based on their pollinating agent. Wind-pollinated flowers are typically small, inconspicuous, and lack bright colours, fragrance, or nectar. They produce large quantities of light, dry pollen and have large, feathery stigmas to easily trap airborne pollen. In contrast, insect-pollinated flowers are usually large, brightly coloured, and produce nectar and a scent to attract insects. Their pollen grains are often sticky or spiky to adhere to the insect's body.
6. What are the key events that occur after a pollen grain successfully lands on a compatible stigma?
Once a pollen grain lands on a compatible stigma, it absorbs moisture and nutrients, leading to the germination of a pollen tube. This tube grows down through the style and into the ovary, where it enters an ovule. The male gametes are then released from the pollen tube to fertilise the female egg cell, initiating the formation of an embryo and the development of the ovule into a seed.
7. What is the functional difference between autogamy and geitonogamy?
Both are types of self-pollination. Autogamy is the transfer of pollen to the stigma of the exact same flower, ensuring fertilisation even in isolation. Geitonogamy is the transfer of pollen to a different flower on the same plant. Ecologically, geitonogamy functions like cross-pollination as it often requires a pollinating agent (like an insect) to move pollen between flowers. However, genetically it is equivalent to autogamy because the pollen and ovule come from the same parent plant.
8. Why would a plant evolve for cross-pollination even though it is less certain than self-pollination?
Although self-pollination is a more reliable method, plants evolve for cross-pollination to avoid the long-term disadvantages of inbreeding. The primary benefit of cross-pollination is the creation of genetic variation, which is crucial for the long-term survival of a species. This diversity allows a population to adapt to new environmental challenges, such as pests, diseases, and climate change, providing a significant evolutionary advantage that outweighs the uncertainty of relying on external pollinating agents.
