Science > Biology > Botany > Reproduction in Plants > Pollination and its Types
In this article, we are going to study important step in sexual reproduction in plants i.e. pollination. We shall also study its types.
Transfer of pollen grains (shed from the anther) to the stigma of a pistil is termed pollination. Depending on the source of pollen, pollination can be divided into two types.
a) Self Pollination i) Autonomy ii) Geitonogamy
b) Cross Pollination i) Xenogamy ii) Hybridization
Self Pollination:
Transfer of pollen grains from the anther to the stigma of the same flower or different flower produced on the same plant is called self-pollination.
In autogamy, pollination is achieved within the same flower. Transfer of pollen grains from the anther to the stigma of the same flower.
In geitonogamy, there is a transfer of pollen grains from the anther of one flower to the stigma of the different flowers of the same plant.
Advantages of Self Pollination:
- It is a sure method of fertilization.
- No external agencies or medium required for the pollination.
- This is the most economical method of pollination.
- For this method, attractive flowers, fragrance, Vivid colours and nectar production is not required.
- There is less wastage of pollen grains.
- Genetic stability can be maintained in the progeny.
Disadvantages of Self Pollination:
- Progeny shows less vigour due to continued self-pollination.
- No possibility of introduction of new desirable characters
- Undesirable characters cannot be eliminated.
- It does not help in evolution.
- Disease resistant capacity becomes less.
Cross Pollination:
Transfer of pollen grains from the anther of a flower to the stigma of another flower produced on the different plant having dissimilar genetic makeup is called cross-pollination. It is also called allogamy.
In xenogamy, pollination is achieved by transfer of pollen grains from the anther of a flower to the stigma of another flower produced on the different plant but of same species.
In hybridization, pollination is achieved by the transfer of pollen grains from the anther of a flower to the stigma of another flower produced on the different plants but of different species.
Advantages of Cross Pollination:
- Progeny shows enhanced vigour.
- Offspring are more viable and resistant.
- There is a possibility to get new desirable characters.
- It involves genetic recombination and hence brings variations.
- The yield of the crop can be maintained.
- It helps in evolution.
- Undesirable characters of the plant can be eliminated.
Disadvantages of Cross Pollination:
- Pollination may fail due to distance barrier.
- Flowers have to totally depend on the external agencies for pollination.
- More wastage of pollen.
- It may introduce some undesirable characters.
- It is not economical because a lot of energy of the plant is wasted in attracting carriers.
- Genetic Purity is not maintained.
Agencies of Pollination:
As pollen grain are non-motile, they require an external agency to transfer themselves from anther to the stigma. There are two categories of agencies of pollination
- Abiotic agents: Non-living physical factors like water, wind
- Biotic agents: Living organisms like insects, birds, bats, and animals.
Anemophily:
When pollen is transported by wind, this mode of pollination is called anemophily. It is the most primitive type of pollination. Many of the world’s most important crop plants are wind-pollinated. These include wheat, rice, corn, rye, barley, and oats.
Many economically important trees are also wind-pollinated. These include pines, spruces, firs, and many hardwood trees, including several species cultivated for nut production.
Wind-pollinated plants do not invest in resources that attract pollinating organisms, such as showy flowers, nectar, and scent. Instead, they produce larger quantities of light, dry pollen from small, plain flowers that can be carried on the wind. Female structures on wind-pollinated plants are adapted to capture the passing pollen from the air, but the majority of the pollen goes to waste.
Hydrophily:
When pollen is transported by water, this mode of pollination is called hydrophily. and plants are called hydrophilous. It is a very rare type of pollination, even in aquatic plants. water is a regular mode of transport for the male gametes among the lower plant groups such as algae, bryophytes, and pteridophytes.
Some examples of water pollinated plants are Vallisneria and Hydrilla which grow in fresh water and several marine sea-grasses such as Zostera.
Water-pollinated plants do not invest in resources that attract pollinating organisms, such as showy flowers, nectar, and scent. Instead, they produce unwettable pollen. Stigma is long and sticky. Generally, flowers are unisexual. There are two types of hydrophilic pollination Hypohydrophily and b) Epihydrophily
Hypohydrophily:
Hypohydrophily is a true hydrophily that occurs beneath the surface of the water. It occurs in completely submerged plants and their pollen grains are waterborne. Example: Zostera marina, Ceratophyllum, etc.
Ceratophyllum desnersum:
In Ceratophyllum desnersum (that is, a submerged freshwater plant), the male flower bears 30 to 45 stamens. The mature anthers break at the base, mount to the surface of the water and dehisce there. The liberated pollen germinates and sinks in water. While sinking, they come in contact with the stigma of female flowers to produce pollination.
Zostera marina:
In Zostera marina, the pollen grains are elongated (up to 2,500 mm), like a needle and without exine. They have similar specific gravity as that of water; thus float beneath the surface of the water. Whenever they reach the stigma, they coil about it and germinate.
Epihydrophily:
Hypohydrophily is a pseudo hydrophily that occurs on the surface of the water. In Vallisneria male and female plants are separate. At maturity, male flowers are detached from male inflorescence and begin to float on the water surface. The coiled female plant undergoes uncoiling at maturity and reaches the water surface. The male flowers surround the female flower and undergo anthesis i.e. formation of a mature pollen grain.
Entomophily:
When pollen is transported by insects, this mode of pollination is called entomophily and plants are called entomophilous. Insect-pollinated plants have large and attractive flowers with vivid and bright colours. They have fragrance and nectar. If flowers are small they are grouped and called an inflorescence. e.g. sunflower. If flowers blossom at night they are white and have a very pleasant fragrance.
To sustain animal visits, the flowers have to provide rewards to the animals. Nectar and pollen grains are the usual floral rewards. For harvesting the reward(s) from the flower the animal visitor comes in contact with the anthers and the stigma. The body of the animal gets a coating of pollen grains, which are generally sticky in animal-pollinated flowers.
When the animal carrying pollen on its body comes in contact with the stigma, it brings about pollination. In some species, floral rewards are in providing safe places to lay eggs.
There is a relationship between a species of moth and the plant Yucca where both species – moth and the plant cannot complete their life cycles without each other. The moth deposits its eggs in the locule of the ovary and the flower, in turn, gets pollinated by the moth. The larvae of the moth come out of the eggs as the seeds start developing.
Not all insects are causing pollination. They visit flowers for nectar only. Such insects are called pollen robbers or nectar robbers.
In salvia, the stamen is bifurcated into two connective branches. The upper branch of connective bears fertile anther lobe while the lower one is sterile. When insects enter flower for nectar, it pushes the lower sterile anther backward, which results in bending of the upper fertile anther. Now, upper fertile anther comes in contact with the insect body and pollens are dusted on the insect’s body. When such a dusted insect visits another flower with matured gynoecium, the pollens are received by stigma. This mechanism is called the lever mechanism.
Ornithophily:
When pollen is transported by birds, this mode of pollination is called ornithophily and plants are called ornithophilous. Bird-pollinated plants have large and attractive flowers with vivid and bright colours. They have thick and fleshy flower parts. Corolla is tubular and funnel-shaped. They don’t have fragrance. They produce a large amount of sugary nectar. Pollen grains are sticky. The common pollinating birds are sunbirds, hummingbirds, crow, Bulbul.
E.g. Callistemon (bottlebrush), Bignonia, Butea, Bombax ( Silk Cotton).
Chiropterophily:
When pollen is transported by bats, this mode of pollination is called chiropterophily and plants are called chiropterophilous. Bat-pollinated plants have large and stout flowers to hold the weight of a bat. These flowers are open at night only and produce the fermented fragrance of rotten fruit to which bats are attracted. Flowers have a large number of stamens and produce pollens in large numbers.
E.g. Anthocephallus (kadamb), Kigelia pinata, Adansonia (Baobab tree), Bauhinia.