Pollination is defined as the process of transfer of pollen grains from anther to the stigma of the same flower or of different flower of the same species.
- Pollen grains are immobile so when they are shed from the anthers, they may reach stigma by a number of means.
- Flowering plants have evolved many adaptation to achieve pollination.
- Pollination is of two types: Self-pollination and cross-pollination.
It refers to transfer of pollen from anther to the stigma of same flower or to the stigma of another flower of same plant. There are two types of self-pollination: autogamy and geitonogamy.
- Auto: self, gamos: marriage.
- It refers to transfer of pollen from anther to the stigma of same flower.
- It is only possible when the flower is bisexual and male and female part mature at the same time.
- No need of external agencies for pollination.
- Example: wheat, rice and pea.
- Geiton: neighbour, gamos: marriage.
- It refers to transfer of pollen from anther to the stigma of another flower of same plant.
- Flower may be unisexual or bisexual.
- From the genetical point of view geitonogamy is self-pollination because all flowers of the same plant are genetically identical.
- From the ecologically, it is considered as cross pollination.
- It may need external agencies for pollination.
Note: Although geitonogamy is functionally cross-pollination involving a pollinating agent, genetically it is similar to autogamy since the pollen grains come from the same plant.
It is the transfer of pollen from anther to stigma of another plant. Sometimes it is also known as true or real cross-pollination. It is genetically as well as ecologically cross pollination.
- It is also known as xenogamy.
- Xenos: strange, gamos: marriage.
- It requires external agencies like water, air or insects etc.
- Xenogamy is the only type of pollination which brings genetically different types of pollen grains to the stigma.
- External agencies may be biotic or abiotic.
Majority of flowering plants produce hermaphrodite flowers and pollen grains are likely to come in contact with the stigma of the same flower. Continued self-pollination result in inbreeding depression. Flowering plants have developed different way to discourage self-pollination and promote cross-pollination, these are known as outbreeding devices.
- Separate sexes: The most obvious is to separate the sexes in different flowers either on the same plant (monoecious and unisexual like castor and maize) or on separate plant (dioecious like papaya). Monoecious and unisexual condition prevent autogamy but not geitonogamy. Dioecious condition prevent both autogamy and geitonogamy.
Dichogamy: Androecium and gynoecium of a bisexual flower mature at different time. It is of two types:
(a) Protandry: It is the process of maturation of androecium earlier than gynoecium.
(b) Protogyny: It is the process of maturation of gynoecium earlier than androecium.
- Self incompatibility: It is a genetic mechanism to prevent self-pollen (from the same flower or other flowers of the same plant) from fertilisation by inhibiting pollen germination or pollen tube growth in the pistil. The pollen is rejected by the stigma of the same flower and is only capable of pollinating a flower with different "incompatibility alleles".
- Herkogamy: Male and female sex organs in a bisexual flower arranged at different levels. So that the pollen cannot come in contact with the stigma of the same flower. It prevents autogamy.
- Pollen release and stigma receptivity not coordinated. Either the pollen is released before the stigma becomes receptive or stigma becomes receptive much before the release of pollen. It prevents autogamy.
Agents of Pollination
Plants requires different types of agent to complete pollination, especially cross-pollinating plant. There are two main group of agents:
(i) Abiotic agents like wind and water
(ii) Biotic-agents which include animals of different types such as insects, birds, bats, snails, etc. Abiotic agents
In anemophily pollen grains reach the stigma through winds. Plant have certain features to perform anemophily or we can say plant has certain adaptation for air pollination.
- Lots of pollen grains are formed as compared to the number of ovules available for pollination.
- Pollen are small, light, smooth and dry. Female reproductive organ i.e. stigma is large and feathery for trapping the pollen grain.
- Anemophilous flowers are neither attractive nor have fragrance.
- They don't have nectar glands.
- Pollination by water is known as hydrophily.
- There are two types of hydrophily: Epihydrophily and Hypohydrophily.
- In Epihydrophily pollination take place on the surface of water. Example: Vallisneria.
- In Hypohydrophily pollination take place inside the water. Example: Zostera.
Pollination by animals is called Zoophily. Generally in zoophilous plants, flowers are very large, attractive and maximum number of nectar glands are present. Pollen have sticky surface. To sustain animal visits, the flowers have to provide rewards to the animal. Pollen, nectar, providing safe places to lay eggs, colour, fragrance are the usual floral rewards. For obtaining rewards from the flower the animal comes in contact with the anthers and the stigma. Body of the animal gets a coating of pollen grains. When the animal carrying pollen on its body comes in contact with the stigma, pollination takes place.
- Majority of flowering plants use a range of animals as pollinating agents. Insect (Bees, butterflies, beetles, ants, moths), birds (sunbirds and humming birds) and bats are common agents.
- Among animals, insects particularly bees are the dominant biotic pollinating agents.
- Even larger animals such as some primates (lemurs), arboreal rodents, or reptiles (gecko lizard and garden lizard) also work as agents.
Types of Zoophily
Question: Write the name of platn which have tallest flower.
Answer: Flower of Amorphophallus (Height is around 6 feet).
Question: Explain the relationship between Yucca plant and species of moth for pollination.
Answer: Yucca plant and species of moth, both cannot completer their life cycles without each other. Flower provide safe places to lay eggs. The moth deposits its eggs in the locule of ovary. In turn, flower gets pollinated by moth.
Question: What are pollen/nectar robbers?
Answer: Many insects may consume pollen or the nectar without bringing about pollination. Such floral visitors are known as pollen/nectar robbers.
Question: Explain pollination in Salvia.
Answer: In salvia pollination take place by the insects, it means by entomophily. Salvia have lever mechanism or turn pipe mechanism. The flowers of salvia have two stamen. Stamen has two lobes. The uppper lobe is fertile and the lower one sterile. When the insect lands on the lower lip, the fertile lobe automatically brings down to touch the back of insect and thus depositing the pollen grains on the back of insect. The flower is protandrous, and on the maturity of the stigma it bends down and touches the back of the insect and receives the pollen grains from it.
Question: Explain pollination in Vallisneria
Answer: Vallisneria pollination take place by water (Hydrophily). In Vallisneria, the female flower reach the surface of water by the long stalk and the male flowers or pollen grains are released on to the surface of water. Pollen grain reach to the female flowers passively by water currents. As soon the pollination is over, the stalk of the female flower becomes spirally coiled and pulls the female flower down into the water. The further process take place under water.