Pollination: kinds and agents

The bridge between the anther and the stigma

Pollen is made in the anther. The egg is in the ovule. They are far apart, and pollen cannot walk or swim by itself. Pollination is the transfer of pollen grains from the anther to the stigma of a pistil. Every seed starts with this step.

Three kinds of pollination

The kind depends on where the pollen comes from.

Autogamy. Complete autogamy needs a bisexual flower in which the anther and the stigma lie close together and are ready at the same time. Some plants, such as Viola (common pansy), Oxalis and Commelina, make two kinds of flowers.

• Chasmogamous flowers open and expose the anthers and the stigma, like the flowers of other species.
• Cleistogamous flowers never open. The anthers and the stigma lie close, and the pollen falls on the stigma inside the bud. These flowers are always autogamous. They set seed even if no pollinator is present.

Geitonogamy. The pollen goes from one flower to another flower of the same plant. It needs a pollinating agent, so it is a cross-pollination in how it happens. But the pollen is genetically the same as the plant, so the result is like autogamy.

Xenogamy. The pollen goes to a flower of another plant. It is the only kind that brings pollen with different genes to the stigma.

Agents of pollination

Plants cannot move, so most of them hire a carrier. The carrier may be a non-living agent (wind or water) or a living agent (an animal).

Wind

Wind pollination is common in grasses. The flowers have these features.

• Light, non-sticky pollen that the wind can carry.
• Well-exposed stamens, so the pollen is shed into the air.
• A large, often feathery stigma, which traps pollen from the air.
• A single ovule in each ovary, and many flowers packed in an inflorescence.

A maize cob is an example. The silky tassels at the top of the cob are the styles and the stigmas, and they wave in the wind to catch pollen.

Water

Pollination by water is rare. It is limited to about 30 genera, mostly monocots. Examples are Vallisneria and Hydrilla, which grow in fresh water, and sea-grasses such as Zostera.

• In Vallisneria, the female flower reaches the surface on a long stalk. The male flowers or the pollen are released on to the surface and drift on the water until some reach the female flowers.
• In sea-grasses, the female flowers stay under water, and the pollen is released inside the water. The pollen grains are long and ribbon-like, and a coat of mucilage protects them from wetting.

Many water plants do not use water for pollination. In the water hyacinth and the water lily, the flowers rise above the water and are pollinated by insects or wind. Wind and water flowers are not colourful, and they make no nectar.

Animals

Most flowering plants are pollinated by animals. Insects, mostly bees, do the most work. Butterflies, flies, beetles, wasps, ants and moths also visit flowers, and so do birds and bats. Even some lemurs, tree-dwelling rodents and lizards (geckos and garden lizards) pollinate some plants.

• Animal-pollinated flowers are large, colourful, fragrant and rich in nectar.
• Small flowers are often clustered in an inflorescence, which makes them look bigger.
• Their pollen grains are sticky, so they cling to the visitor.
• The visitor gets a reward: nectar or pollen. In some plants the reward is a safe place to lay eggs.

A close partnership is the one of Yucca and a moth. The moth lays its eggs in the locule of the flower, and in doing so pollinates it. The larvae hatch only when the seeds begin to grow. Neither can survive without the other.

The three agents side by side:

• Wind: light, dry pollen. Small flowers that are not colourful. No nectar. A large, feathery stigma.
• Water: long, ribbon-like pollen in mucilage (sea-grasses). Flowers that are not colourful. No nectar. An exposed stigma.
• Animal: sticky pollen. Large, colourful, scented flowers. Nectar. A small, sticky stigma.

You should find that the grains needed rise steeply when the chance falls. A plant that leaves pollination to the wind, with a very small chance for each grain, must make a huge number of grains.

Try it in Python

The program uses the same model to compare wind and animal pollination.

Practise with new numbers

Common mistakes

• Saying that geitonogamy is a kind of autogamy. It needs a pollinator and two flowers, but genetically it is like autogamy.
• Saying that cleistogamous flowers are cross-pollinated. They never open, so they are always autogamous.
• Saying that all water plants are pollinated by water. Water lily and water hyacinth flowers are pollinated by insects or wind.
• Giving wind-pollinated flowers bright petals and nectar. They have none, because no animal is needed.
• Saying that xenogamy needs a different species. It needs a different plant of the same species.

Remember these

• Autogamy: same flower. Geitonogamy: another flower of the same plant. Xenogamy: another plant.
• Only xenogamy brings genetically different pollen. Geitonogamy is genetically like autogamy.
• Cleistogamous flowers (Viola, Oxalis, Commelina) never open, and they set seed without any pollinator.
• Wind: light pollen, feathery stigma, one ovule. Water: about 30 genera, for example Vallisneria and Zostera. Animals: colourful, scented flowers, sticky pollen, nectar.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Name the three kinds of pollination, and say where the pollen comes from in each.

2. What is a cleistogamous flower, and why is it sure to set seed?

3. Why is geitonogamy genetically like autogamy?

4. List three features of a wind-pollinated flower.

5. Give two examples of plants that are pollinated by water.

References

• NCERT (2023). Biology, Textbook for Class XII, chapter 1, Sexual Reproduction in Flowering Plants.
• Clark M. A. et al. (2018). Biology 2e, OpenStax, chapter 32, Plant Reproduction.