Pollination is the pre-fertilization event in which the transfer of pollen grain takes place from the anther of the flower to the stigma of the same plant or different plant. Pollen is transferred by other agents such as wind, water, gravity, animals, insects, or humans. These agents are called pollinators and the plant that provides pollen is called pollenizer. Some plants are self-fertile or self-compatible and can pollinate themselves whereas some are self-incompatible, so they need other pollenizers and pollinators to fertilize them.
Pollination is the essential step before fertilization, the result of this action will be seed and fruit production, thus ensuring continuity of plant species. Thus, pollination plays an important role in maintaining biodiversity, supporting food production, and sustaining the ecosystem.
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Types of Pollination
Pollination can be of two types-
Self-Pollination
The transfer of pollen grains from the anther of the flower to the stigma of the same flower or genetically similar flower is called self-pollination. It is of two sub-types-
Autogamy– It is the transfer of pollen grain from the anther to the stigma of the same flower. This type of self-pollination is preferred by closely arranged anther and stigma and also due to synchrony in the maturity of anther and stigma.
Examples- Pea, Orchid, etc.
Geitonogamy– It is the transfer of pollen grains from anther to the stigma of different flowers in the same plant. It is also known as false cross-pollination.
Examples- Maize, Banana, etc.
Contrivances for self-pollination
Bisexuality- Presence of both male and female sex organs in the same flower i.e. the plant should be bisexual.
Homogamy– The male and female sex organs of a bisexual flower must be mature at the same time to ensure self-pollination.
Cleistogamy- It is the condition in which the bisexual flower never opens but remains closed. Such flowers are called cleistogamous flowers.
Example- Commelina benghalensis.
Bud pollination– Anthers and stigma of bisexual flowers mature before the opening of the bud and thus self-pollination occurs in the bud stage. Example- Rice.
Merits and demerits of self-pollination
Merits
- This process occurs without the help of any agents.
- This process causes less wastage of pollen grains.
- It maintains the purity of the species.
Demerits
- The number of seeds produced is relatively less.
- The seeds produce weaker progeny due to a lack of recombination of contrasting characters.
- Seeds are less viable.
- There is no chance of formation of new varieties.
- In case of sudden environmental changes, it may lead to extinction.
Cross-pollination
When the pollen grains of one flower are transferred to the stigma of another flower of a different plant then it is called cross-pollination.
Xenogamy– When pollination occurs within the same species, then it is called xenogamy.
Hybridism– When cross-pollination occurs within different species, then it is called hybridism.
Contrivances for cross pollination
The different contrivances of cross-pollination are as follows-
Dicliny– It is the condition where unisexual flowers are formed. It may be monoecious i.e. presence of both unisexual flowers on the same plant e.g. Cucurbita, or dioecious i.e. male and female flowers present on different plants. E.g. Carica.
Dichogamy– When anthers and stigma of bisexual flower mature at different times, it is called dichogamy. It may be-
- Protrandry– In this condition, the anther matures earlier than the stigma. E.g. Sunflower, Cotton, etc.
- Protogyny– It is the condition where stigma matures earlier than anthers. E.g. Michelia.
Herkogamy– It is the condition in which natural or physical barriers are present between androecium and gynoecium to prevent self-pollination. E.g. Calotropis.
Self-sterility– It is the condition where pollen fails to germinate on the stigma of the same flower due to some genetic or physiological regions. Example Solanum tuberosum.
Heterostyly– It is the occurrence of stamens or styles of different lengths. In this condition, there are two types of flowers- Pin i.e. flowers having long style and short stamens and thrum i.e. having short style and long stamens.
Pollen prepotency– The phenomenon of rapid germination of foreign pollen over the stigma of its self-pollen is called pollen prepotency.
Sensitive stigma– In some plants, stigmatic lobes are very sensitive. After cross-pollination, they come closer and prevent self-pollination. Example- Martynia.
Male sterility- Flowers are bisexual but the anthers are non-functional.
Merits and demerits of cross-pollination
Merits
- Healthier offspring are produced and are better adapted to struggle for existence for survival owing to the recombination of contrasting characters.
- More viable seeds are produced.
- Production of new varieties.
- Offsprings are better adapted to their environment and in case of sudden environmental changes, they do not undergo extinction.
Demerits
- This process is uncertain and depends upon agents.
- More wastage of pollen grain.
- It can’t maintain the purity of species.
Agents of pollination
There are different types of agents that help in pollination. Some of them are abiotic such as wind, water, etc. and some are biotic such as birds, insects, etc. They are-
Abiotic pollinators
Anemophily (Wind pollinated)
When pollination is affected with the help of agent wind, then it is called anemophily. Examples are maize, paddy, Bamboo, Cyperus, etc.
Characteristic of wind-pollinated flowers
- Anemophilous flowers are generally smaller, have no color, or are showy, petals are inconspicuous and without nectar.
- Stamens with long filaments remain exposed. The anthers are versatile, and a large quantity of pollens is produced.
- Pollen grains are smaller, light, smooth, and dry so that they can be drifted easily by wind.
- The long style exposes the stigma outside the flower. The stigmas are branched, feathery bristly, or sticky so that they can catch floating pollens easily.
Hydrophily (Water pollinated)
When pollination takes place with the help of water, then it is called hydrophily.
Example- Aquatic plants such as Vallisneria, Hydrilla, etc.
Characteristic of water-pollinated flowers
- Flowers are smaller, inconspicuous, light in weight, colorless, and covered by waxy substances. Calyx and corolla are smaller than essential floral parts i.e. androecium and gynoecium.
- The dehiscence of anther takes place so violently that the pollen grains are scattered to a considerable distance.
- Pollen grains are small, lighter in weight, and coated with wax so that they can float on water and be carried by water current.
- Stigmas are long, sticky, and provided with bristles so they can catch pollen easily.
Biotic pollinators
Chiropterophily (Bat pollinated)
When pollination takes place with the help of bats, then it is called chiropterophily and the flowers are called chiropterophilous flowers.
Examples- Anthocephalous (Kadam), Musa (Banana), Bauhinia, etc.
Characteristics of chiropterophillous flowers
- Flowers are large long stalked, dull having a strong scent, and generally open after dusk.
- They are provided with nectar glands and secrete nectar.
- Pollen grains are produced in huge amounts.
Entomophily (Insect pollinated)
When pollination occurs with the help of insects, then it is called entomophily and the flower is called entomophilous flower.
Example- Sunflower, Marigold, etc.
Characteristics of entomophilous flowers
- The entomophilous flowers are larger in shape, bright colored, and showy so that they can attract insects. If single flowers are smaller in shape, then they remain in a group to form inflorescence.
- The flowers which bloom at night emit a sweet scent to attract insects. Most of the entomophilous flowers have special landing platforms for insects.
- The nectar glands situated at the base of the flowers secrete nectar which is one of the characteristic features of entomophilous flowers.
- Pollen grains are larger with rough, spiny, and sticky surfaces so that they can easily stick to the body of the insect.
- The stigmatic surface is rough and sticky to attach pollen grains firmly.
Ornithophilly (bird-pollinated)
If pollination occurs with the help of birds, then it is called ornithophily and the flowers are called ornithophilous flowers.
Examples- Bombax ceiba, Butea monosperma etc.
Characteristic feature of ornithophilous flowers
- Flowers are generally tubular or urn-shaped or cup-shaped, large and showy, and brightly colored to attract birds.
- Flowers are characterized by the presence of nectar glands.
- Large amounts of pollen grains are present. The stamens and carpels usually extend beyond the perianth lobes so that they can be easily pollinated.
Malacophily, Ophiophily and Myrmecophily
When pollination is affected by snails and slugs, then it is called malacophily. Example-Lemna.
When pollination takes place with the help of snakes, then it is called ophiphily. Example- Santalum.
Myrmecophily– When pollination occurs by ants then it is called myrmecophily. Example- Coffee.
Pollination mechanisms and adaptations
Plants have developed many mechanisms and adaptations to attract pollinators to ensure successful pollination. These are as follows-
Floral structures
Usually, most of the flowers are developed according to different pollinators. For example, entomophilous flowers are usually brightly colored, and showy, with nectar-producing glands and fragrances. Ornithophilous flowers are usually larger, and tubular, to attract birds like hummingbirds. However, flowers having abiotic pollinators are usually unattractive. The hydrophilous flowers usually have a coating of cuticles around pollens to protect them from water.
Food reward
Nectar acts as a food reward for pollinators and fragrances help to attract insects and animals.
Energy
Some thermogenic plants can maintain relatively higher temperatures than the environment to maintain the microclimate of the flower. For example, some plants have larger bracts to minimize heat loss. This floral heat provides energy to the pollinators and in return, the flower gets pollinated. Some thermoregulatory plants include- Philodendron selloum, Nelumbo nucifera etc.
Mimicry
In this type, the flowers do not provide food for pollinators, but they mimic the shape, odor, and other insects to attract the pollinators. For example, Acianthera adamantiensis mimics the odor of dog feces to attract flies. In an orchid species Ophrys, the flower resembles a bee to attract male bees for copulation (pseudocopulation) which thus leads to pollination.
Floral traps
In species like Nymphaea, the flowers containing stigmatic fluid open to attract insects, and the visiting insects eventually fall into the cup and get drowned which leads to washing off all the pollen on the visiting insects to pollinate the plants.
Co-evolution
Both plants and their specific pollinators have co-evolved for specialized relationships. For example, orchids have developed certain structures to attract insects like bees.
Importance of pollination
Pollination is the most important mechanism for maintaining ecological balance and biodiversity.
Genetic diversity and evolution– Cross-pollination enhances genetic diversity in plants by a combination of genetic material from both parents. Pollination drives evolution, helping plants to develop new varieties to survive in a competitive environment.
Biodiversity conservation- Pollination plays a very important role in the preservation of biodiversity as it allows different kinds of plant species to reproduce which provides food, shelter, and habitats for other living organisms.
Maintaining purity- Self-pollination is responsible for maintaining the purity of the species.
Ecosystem Stability- Pollination maintains healthy plant populations that help in maintaining ecosystem stability. The plants produced through pollination can be considered as a base of terrestrial ecosystems as they are the primary producers in food chains.
Economic benefits– Pollination contributes to economic growth worldwide by producing various commercial crops, helps in producing products like honey, and also supports horticulture and forestry industries.
Threats and challenges to pollination and pollinators
Habitat destruction– The most common ways of habitat destruction are overgrazing, land clearing, destruction of food sources, destruction of mating sites for pollinators, and irrigation. Some plants require biotic pollinators and the destruction of habitat leads to a decrease in the population of pollinators thus resulting in less pollination.
Pathogens and predators– Pathogens and predators cause collapses in pollinator populations. Some disease-causing pathogens lead to a significant loss in the population of biotic pollinators such as bees.
Pollution- Pollinators such as bees are considered bioindicators of pollution. Different pollutants may contaminate pollen grains and also may cause harm to pollinators thus leading to a decrease in pollinator population.
Climate change– The changes in climatic conditions such as temperature may lead to changes in the timing of both flowering plants and pollinator activity. The pollinators and plants may adapt differently to changing climates which may cause disturbances in pollination.
Precipitation– High precipitation lowers the pollinator’s activity. However low precipitation may also cause drought periods which leads to a reduction in flower production.
Conservation Efforts and sustainable practices
Minimize pesticides- Organic farming can be promoted and minimizing the use of harmful chemicals should be done for conservation purposes. Diverse varieties of crops can be planted to avoid monoculture.
Other strategies– Important pollinators should be identified and public awareness can be done to conserve them.
Hunting of wild pollinators such as bees for honey must be avoided.
Conservation of and plantation of important specific flowers can be done to conserve pollinators.
Different gardens such as butterflies gardens, etc. can be created to provide resources for pollinators.
Conclusion
Pollination is a vital process in plant reproduction and biodiversity that is important to agriculture and food production worldwide. It forms seeds and fruits, which are essential in sustaining ecosystems and supporting wildlife and humans for sustenance. Pollination also drives genetic diversity and evolution, making plants responsive to environmental changes. Besides its ecological significance, pollination greatly helps the world economy by increasing crop yields and improving crop quality. Maintaining pollinators and, thus, sustainable pollination practices are vital to food security, balance in ecosystems, and healthy environments for the next generations.
References
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