Pollination

Pollination

The process of transfer of pollen grains from the anther to the stigma of the flower is called pollination. Pollination occurs at the two-celled stage of pollen grains. Pollination is of two types- Self-pollination and cross-pollination.

Types of pollination

Self-Pollination

The pollination in which pollen grains are transferred to the stigma of the same flower or different flower of the same plant is called self-pollination. However, true self-pollination (autogamy) involves some flower. Thus, it is of two different types- Autogamy and Geitonogamy.

Autogamy is the self-pollination in which pollen grains are transferred to the stigma of the same flower.

Geitonogamy is the pollination in which pollen grains are transferred to the stigma of a different flower of the same plant. It is, therefore, ecologically cross pollination but genetically self-pollination.

True self-pollination (autogamy) is of two types- direct and indirect. Direct autogamy is the type of autogamy in which pollination occurs due to the direct touch between the anthers and stigma by growth or bending (eg-Catharanthus, Mirabilis). Indirect autogamy occurs due to factors like dew, gravity e.t.c.

• Viola, Oxalis, and Commelinaproduce two types of flowers:

⇒Chasmagamous :Flower with exposed anther and stigma.

⇒Cleistogamous : Flower with closed anther and stigma.

Contrivances of self-pollination:

The factors which favour self-pollination are as follows-

1) Bisexuality - The flower having both male and female reproductive organs is a bisexual flower.

2) Homogamy - The process of maturity of male and female organs of bisexual flower at the same time is called homogamy.

3) Cleistogamy - The self-pollination that occurs in a closed flower is called cleistogamy. Eg- Balsam, Oxalis.

4) Chasmogamy - The self-pollination that occurs in an open flower is called chasmogamy. Eg- Mirabilis,Sunflower.

5) Bud pollination - The self-pollination that occurs in bud condition is called bud pollination. Eg- Pea, Wheat, Rice e.t.c.

Advantages of self-pollination:

1) It maintains pure lines.

2) It is more economical as there is less number of pollen grains required.

3) It is independent of other factors.

4) Flower do not require to make attractive parts.

5) It ensures seed production.

6) It eliminates some bad recessive characters.

Disadvantages of self-pollination:

1. New characters are not introduced.

2. Adaptability of race decreases.

3. There is no variation in the new generations.

4. Continuous self-pollination may lead to the extinction of the race.

5. It may carry unwanted characters to new individuals.

6. It has no role in evolution.

Cross-pollination

It is also called allogamy (but allogamy represents xenogamy and geitonogamy). Based on plants, cross-pollination is of two types- Xenogamy and hybridism.

Xenogamyis the cross pollination which takes place between two different flowers of two different plants of same species.

Hybridismis the cross pollination which takes place between two genetically different plants.

Cross pollination is always carried out by some agents. Based on the agents, it is of different types- Anemophily, Hydrophily, and Zoophily.

• Anemophily

.• Pollination by air is called anemophily.

• Plants produce an enormous amount of pollen when compared to the number of ovules available for pollination to compensate the uncertainties of pollination.

• Flowers with well-exposed stamens are usually pollinated by anemophily.

• Large feathery stigma to trap airborne pollen grains.

• Most winds pollinated flower contains single ovule in one ovary and numerous flower packed into an inflorescence.

• Pollen grains are light and non-sticky.

• Hydrophily

• Pollination by an abiotic agent like water is called hydrophily.

• This type of pollination is very rare, about 30 genera, mostly monocot.

•Vallisneria, Hydrilla, and Zosteraare the common example for hydrophily.

• All aquatic plants are not Hydrophily.

• Pollen grains released into the surface of the water and carried to the stigma by the air current as inVallisneria.

• In sea grains, the flowers are submerged.

• Pollen grains are protected from wetting by mucilaginous covering.

• Pollination by biotic agent

• Majority of flowering plants use a range if animals as pollinating agents.

• Among the animal, insects particularly bees, are the dominant biotic agents for pollination.

• Insect pollinating flowers are very large, colorful, fragrant, and rich in nectar.

• Small flowers present in a cluster to make them conspicuous.

• Flower pollinated by flies and beetles secrete foul odours.

• Nectar and pollen grains are the usual floral rewards for insects.

• In some species, floral rewards are in providing safe places to lay eggs.

• A species of moth andYuccaplant cannot complete their life cycle without each other. The moth deposits its eggs in the locule of the ovary and the flower in turn gets pollinated by the moth.

• Many insects may consume pollen or nectar without bringing about pollination. Such floral visitors are referred as pollen/nectar robbers,

Based on the biotic agents involved, pollination is of different types:

1. Entomophily- pollination by insects

2. Ornithophily - pollination by birds

3. Chiropterophily - pollination by bats

4. Malacophily - pollination by snails

5. Ophiophagy - pollination by snakes

6. Anthophily - pollination by human beings in plant breeding technique.

Contrivances to ensure cross-pollination:

1) Unisexuality or dicliny- The flowers having single sex organ is called the unisexual flower.

2) Dichogamy- The maturity of male and female reproductive organ at the different time is called dichogamy. It may be protandry ( anthers mature earlier than pistil), eg- Sunflower, salvia e.t.c. or protogyny (pistil matures earlier than anthers), eg- Mirabilis, Gloriosa e.t.c.

3) Prepotency- The phenomenon in which pollen grains of the same flower germinates more rapidly on the stigma of another flower is called prepotency. Eg- Apple, grape e.t.c.

4) Self-sterility- The phenomenon in which pollen grains of the same flower fail to germinate on the stigma of that flower is called self-sterility. Eg- Tobacco, Potato e.t.c.

5) Heterostyly- The phenomenon having different heights of male and female reproductive organs in a flower is called heterostyly. Eg- Primula, Jasmine e.t.c.

6) Herkogamy- The physical barrier present between male and female organs of a flower to ensure cross-pollination is called herkogamy.

Advantages of cross-pollination:

1) The self-sterile plants can continue their race.

2) It induces variations in new generation.

3) It helps in evolution.

4) It creates better adaptability in the progeny.

5) New and more useful varieties can be produced by this method.

6) Productivity is high.

7) The defective characters are eliminated.

Disadvantages of cross-pollination:

1. It is highly expensive as a large number of pollen grains are required.

2. It is dependent on other factors.

3. Some undesirable characters may remain in the race.

4. The useful characters may be spoiled.

5. The plant has to create a large number of characters to attract biotic agents.

6. Seed production is less ensured.

Reference

Keshari, Arvind K. and Kamal K. Adhikari. A Text Book of Higher Secondary Biology(Class XII). 1st. Kathmandu: Vidyarthi Pustak Bhandar, 2015.

Mehta, Krishna Ram.Principleof biology.2nd edition.Kathmandu: Asmita, 2068,2069.

Jorden, S.L.principle of biology.2nd edition . Kathmandu: Asmita book Publication, 2068.2069.