Pollination: Definition
Pollen grains are transmitted from the Anther to the Stigma during pollination. When pollen grains land on ovule, it is known as Direct pollination e,g Gymnosperms, and some primitive Angiosperms. When pollen grains land on Stigma and form a pollen tube it is known as Indirect pollination e,g Angiosperms. Pollination’s function in fruit and seed production has been recognised since ancient times. The Arab and Assyrian kings used to perform a special religious ceremony in which the female inflorescence of the date palm was touched by the male inflorescence to ensure good fruiting. However, Thomas Millington, towards the end of the 17th century, provided the scientific underpinning for this method.
Types of Pollination
Pollination is of the following types
Self Pollination
The transfer of pollen grains from an Anther to Stigma of the same flower or a flower on the same plant is known as Self-Pollination. It is of the following types
Autogamy (Auto = Self, Gamy = marriage)
Autogamy means the pollination of a flower by its pollens.
Geitonogamy: (Geitonos = Neighbour, Gamy = marriage)
When the pollens of a flower pollinate any other flower present on the same plant it is called Geitonogamy. Thus this type of self-pollination occurs between two different flowers present on the same plant.
Contrivances or Adaptations for Self-Pollination
Homogamy: When male and female parts of a flower mature at the same time it is known as Homogamy. It favours self-pollination because in a bisexual flower pollen are released at the time when stigma is receptive e,g Mirabilis Jalapa
Cleistogamy: The production of flowers that never open is known as Cleistogamy and the flowers are known as Cleistogamous flowers. In such flowers self-pollination is carried out within closed buds. Thus it is a remarkable way of avoiding cross-pollination e,g Viola, Oxalis, Drosera etc
Bud pollination: In some plants like rice, pea, petunia, pollination occurs within bud conditions as male and female parts mature before anthesis.
Merits
1. It is a sure and economic method of pollination
2. No need to develop extra devices as pollinators are not required.
3. Self-pollination is used to maintain the purity of some useful and desirable varieties.
4. It is used to develop pure lines
5. It needs not to produce a large number of pollen grains.
Demerits
1. It does not favour evolution
2. No new characters/traits are developed.
3. Progeny continuously gets weaker after every generation.
Cross-pollination
The transfer of pollen grains from the Anther of the flower on one plant to the Stigma of the flower on another plant is called Cross-pollination. It is also called Xenogamy or Allogamy. Cross-pollination occurs in both dioecious and monoecious species but dioecious species are necessarily cross-pollinated. Cross-pollination leads to cross-fertilization (Fusion of male and female gametes from two different flowers), which has the advantage of genetic recombination.
Contrivances or adaptations for cross-pollination
The flowers show several adaptations that induce cross-pollination.
Herkogamy: In some bisexual flowers the structure of anthers and styles is such that autogamy is mechanically impossible. This condition is known as Herkogamy.e,g Caryophyllaceae, Orchidaceae, Asclepiadaceae etc.
Heterostyly: It is a condition that refers to dimorphic flowers that have styles of different lengths. It is a unique adaptation that allows members of the Oxalidaceae, Polygonaceae, Rubiaceae, and other families to prevent self-pollination.
Dicliny or Unisexuality
Monoecious plants: When male and female flowers are born on the same plant e,g Maize, Cucurbits, Castor etc
Dioecious plant: Male and female flowers are born on separate plants in a dioecious plant, e.g. Carica papaya, Cannabis etc
Agents of pollination
The biotic agents include living organisms such as insects, birds, bats, snails, slugs etc and the abiotic category includes nonliving agents like air, water etc
Types of cross-pollination
Anemophily (Wind pollination)
Anemophily is a mode of pollination or transfer of pollen grains from the anther to stigma through the agency of wind. The wind-pollinated flowers are called Anemophilous Flowers. The adaptations that distinguish Anemophilous flowers are as follows
1. Flowers are large in number, especially male flowers.
2. Corolla is either absent or reduced.
3. Pollen grains are small, light, dry, dusty, and occasionally winged, allowing them to be blown long distances with ease (up to 1300km)
4. Pollen grain production is huge as 0.5 million in cannabis, 25 million in maize.
5. In pinus pollens are double-winged and wings represent an extension of the exine.
6. Yellow cloud of pollens of pinus during the spring sea is known as a sulphur shower.
7. It is non-directional pollination and thus involves maximum wastage of pollens.
8. In Urtica pollens are released forcefully due to the sudden bursting of anthers to favour wind pollination a process known as Gun powder mechanism.
9. Anemophilous pollens cause Asthma, Bronchitis and other respiratory disorders.
Hydrophily (pollination by water)
Hydrophily is a method of pollination in which pollen grains are transferred from the anther to the stigma with the use of water. For pollination, aquatic plants make excellent use of water.
Hydrophily is of two types
1. Hypo-hydrophily
2. Epi-hydrophily
1. Hypo-hydrophily: Submerged plants such as Najas, Ceratophyllum, and Zostera exhibit hypo-hydrophily, which is the pollination of flowers below the water surface. Zoostera marina is an aquatic marine submerged angiosperm having the largest ribbon-shaped pollen grains with the same density as that of water. They float in water and on coming in contact with stigma coil themselves around it to bring about pollination inside water known as Hypo-hydrophily.
2. Epi-hydrophily: It is the pollination of flowers at the surface of the water. It is more frequent than hypo-hydrophily. E,g Vallisneria spiralis a dioecious plant is the most common example of epi-hydrophily. Vallisneria’s male and female flowers are both submerged. The whole male flower breaks away and floats on the surface of the water. A lengthy spiral stalk raises the female blooms to the surface open anthers to come into touch with the stigmas when free-floating male flowers get caught in the cap-shaped female flower. The female flower is withdrawn inside the water after pollination by coiling the stalk, where fruit development takes place.
Entomophily(Pollination by insects)
Flowers that are pollinated by insects acquire special adaptations to attract them.
1. Large-sized flowers, If flowers are inconspicuous, present in the group called inflorescence to make them conspicuous to attract the pollinator.
2. Pollen grains sticky or with spinous exine
3. Bright coloured petals and tepals
4. Scent and odours
5. Nectar in special glands
6. Edible pollen
7. Special mechanism