Pollination: Types and Mechanisms

By now you know that pollen grains carry the male gametes and the embryo sac houses the female gamete. But here is the catch: neither gamete can move on its own. Both are non-motile (unable to travel independently). So how do they come together for fertilisation? The answer is pollination, one of the cleverest strategies flowering plants have evolved.

What Exactly Is Pollination?

Pollination is the transfer of pollen grains from the anther (where pollen is produced and shed) to the stigma of a pistil. It is not fertilisation itself; it is the essential step that positions the male gametes close enough to the female gamete for fertilisation to happen later.

Because neither gamete can swim, crawl, or fly, flowering plants recruit outside help. They use external agents (wind, water, insects, birds, bats, and other animals) to carry pollen from one place to another. Over millions of years, angiosperms have evolved a remarkable variety of adaptations to make this transfer efficient.

Three Types of Pollination

Depending on where the pollen comes from and where it lands, pollination falls into three categories.

Autogamy: Pollination Within the Same Flower

Autogamy (self-pollination in the strictest sense) occurs when pollen moves from the anther to the stigma of the same flower (Figure 1.9a).

Fig 1.9 (a): Self-pollinated flower

In a typical chasmogamous flower (a flower that opens normally and exposes its reproductive parts), complete autogamy is actually quite rare. Two conditions must both be met for it to work:

• Synchrony — Pollen release from the anther and receptivity of the stigma must happen at the same time. If pollen is shed before the stigma is ready (or vice versa), self-pollination fails.
• Physical proximity — The anther and stigma must be close enough to each other that pollen can land on the stigma without needing to travel far.

Chasmogamous Versus Cleistogamous Flowers

Some plants have found an elegant way to guarantee self-pollination. Certain species produce two kinds of flowers on the same plant:

• Chasmogamous flowers — These are ordinary open flowers, similar to those of most other species. Their anthers and stigma are exposed, so they can be pollinated by external agents.
• Cleistogamous flowers — These flowers never open. The petals stay tightly closed throughout the flower’s life, keeping the anthers and stigma sealed inside.

Fig 1.9 (c): A plant bearing both chasmogamous (open) and cleistogamous (closed) flowers

Inside a cleistogamous flower, the anthers and stigma lie pressed together. When the anthers dehisce (split open to release pollen) inside the closed bud, pollen grains land directly onto the stigma. There is no opening for outside pollen to enter, which means cross-pollination is impossible. This makes cleistogamous flowers invariably autogamous: self-pollination is the only possible outcome, every single time.

The big advantage? Assured seed production. Even if pollinators are completely absent (bad weather, scarce insects, remote habitat), cleistogamous flowers still set seed. It is a reliable insurance policy for reproduction.

Plants that use this dual strategy include Viola (common pansy), Oxalis, and Commelina. Each of these species bears both chasmogamous and cleistogamous flowers on the same individual, giving them the best of both worlds: the genetic mixing that comes with open-flower pollination, and the guaranteed seed set that comes with closed-flower self-pollination.

Geitonogamy: Between Different Flowers on the Same Plant

Geitonogamy occurs when pollen travels from the anther of one flower to the stigma of another flower on the same plant.

From the outside, geitonogamy looks like cross-pollination because a pollinating agent (an insect, the wind, etc.) physically moves pollen between two separate flowers. However, the genetic picture is completely different. Both flowers belong to the same individual plant, so they carry the same set of genes. No new genetic material enters the mix. For this reason, geitonogamy is functionally cross-pollination (it needs an external agent) but genetically equivalent to autogamy (the offspring are no more diverse than if the flower had pollinated itself).

Xenogamy: True Cross-pollination Between Different Plants

Xenogamy is the transfer of pollen from the anther of a flower on one plant to the stigma of a flower on an entirely different plant (Figure 1.9b).

Fig 1.9 (b): Cross-pollinated flower

This is the only type of pollination that brings genetically different pollen to the stigma. Because the two parent plants are separate individuals with their own unique genetic variations, the offspring produced through xenogamy carry new combinations of genes. This genetic diversity is one of the greatest advantages of sexual reproduction, as it gives populations the raw material to adapt to changing environments.

Comparing All Three Types at a Glance

Feature	Autogamy	Geitonogamy	Xenogamy
Pollen source and destination	Same flower	Different flowers, same plant	Flowers on different plants
External agent needed?	Not necessarily (especially in cleistogamous flowers)	Yes	Yes
Genetic outcome	Self-pollination	Genetically same as self-pollination	True cross-pollination (new genetic combinations)
Genetic diversity introduced?	No	No	Yes