Pollination refers to the transfer of pollen from the anther of a stamen to the stigma of a carpel.
What is a pollen?
Pollen represents the male gametophyte of angiosperms as well as gymnosperms. A diploid microspore mother cell divides by meiosis to form four haploid microspores, which further divide by mitosis to develop into pollen. So, pollen grains are formed in the pollen sacs (microsporangia) of the anther by the process of microsporogenesis. In angiosperms, a pollen grain typically consists of two haploid cells, i.e. a large vegetative cell and a small generative cell, which floats in the cytoplasm of the vegetative cell. This generative cell further divides by mitosis to form two male gametes. So, pollination is, in fact, a process of transfer of male gametes in angiosperms.
Types of pollination
Based on the source of pollen, pollination can be divided into three main types:
- Autogamy
- Geitonogamy
- Xenogamy
1) Autogamy
- Autogamy refers to the transfer of pollen from the anther to the stigma of the same flower.
- Autogamy can only occur if the plant has bisexual flowers.
- Autogamy is true self-pollination, i.e. genetically as well as functionally self-pollination.
- Autogamy is not dependent on the presence of pollinators, as pollination occurs within the same flower.
2) Geitonogamy
- Geitonogamy refers to the transfer of pollen from the anther to the stigma of a different flower of the same plant.
- Geitonogamy can occur in two types of plants:
- Monoecious plants having unisexual flowers, i.e. staminate (male) flowers and pistillate (female) flowers on the same plant. Examples – maize, coconut, castor, cucurbits, etc.
- Monoecious plants having bisexual flowers.
- Geitonogamy is a complex case, as genetically it is self-pollination while functionally it is cross-pollination.
- Geitonogamy is dependent on the presence of pollinators, as pollen grains are transferred from one flower to another.
3) Xenogamy
- Xenogamy refers to the transfer of pollen from the anther to the stigma of a different plant of the same species.
- Xenogamy can occur in all types of flowering plants:
- Dioecious plants, i.e. male and female flowers are located on different plants, and pollen grains are transferred from the male plant to the female plant. Examples – date palm (Phoenix dactylifera), papaya, etc.
- Monoecious plants having unisexual flowers.
- Monoecious plants having bisexual flowers.
- Xenogamy is true cross-pollination, i.e. genetically as well as functionally cross-pollination.
- Xenogamy is also dependent on the presence of pollinators, as pollen grains are transferred from one plant to another.
Agents for pollination
The pollinating agents are broadly divided into two groups:
- Abiotic agents
- Biotic agents
Abiotic agents
Wind pollination / Anemophily
- Wind is the most common among abiotic agents.
- Pollen grains are small, light and non-sticky.
- The carpel has a feathery stigma and style to capture the pollen.
- Flowers lack bright colours, fragrance and nectar.
- A large number of pollen grains are required, as the process is non-directional.
- Flowers are often found grouped into an inflorescence.
Wind-pollinated flowers usually have a single ovule in each ovary and well-exposed stamens. In maize, the “tassels” of the corn cob are the stigma and style, waving in the wind to trap pollen.
Water pollination / Hydrophily
- A rare type of pollination, observed in less than 30 genera of angiosperms, mostly monocots.
- Pollen grains are surrounded by a mucilage layer to make them unwettable.
- Flowers lack bright colours, fragrance and nectar.
- A large number of pollen grains are required, as the process is non-directional.
- Most aquatic angiosperms, like water hyacinth (Eichhornia), water lettuce (Pistia) and lotus (Nelumbo), perform wind or insect pollination.
- Examples – Vallisneria, Hydrilla, Zostera.
In Vallisneria, the female flower reaches the water surface on a long stalk, and the male flowers are released onto the surface and carried to it by water currents. In sea grasses like Zostera, the female flowers stay submerged, and the long, ribbon-like pollen grains are carried inside the water.
Biotic agents
Insect pollination / Entomophily
- A wide range of insects carry out pollination in angiosperms, including bees, butterflies, moths, ants, flies, beetles, etc.
- Bees are the most common pollinating agents and contribute to about 75% of total pollination worldwide.
- Pollen grains are surrounded by a sticky substance to help them attach to the insect’s body.
- Flowers are brightly coloured, fragrant and either large in size or grouped into an inflorescence to attract insects.
- Nectar, pollen and safe egg-laying sites are the rewards offered by the plants to sustain the visits of insects.
- The chances of pollination are high, so fewer pollen grains are required.
- Pollinators like flies and beetles are attracted towards foul smells.
Amorphophallus has one of the tallest flowers (about 6 feet) and offers safe egg-laying sites. The moth and Yucca depend on each other completely: the moth lays its eggs in the ovary, and in return pollinates the flower.
Bird pollination / Ornithophily
- Birds like the hummingbird and sunbird have a long bill and tongue, which allow them to easily obtain nectar from the flower. During this process, pollen gets dusted on the bill of the bird, and when this bird reaches another flower to obtain nectar, pollination can occur.
Human pollination / Anthropophily
Humans carry out pollination in artificial hybridisation, where only the desired pollen grains are used. The anthers are removed from the flower bud (emasculation) and the flower is covered with a bag (bagging) to prevent unwanted pollen.