Pollination and Fertilisation

1Pollination

Pollination is the movement of pollen from an anther to a stigma of the same kind of plant. Without pollination there is no fertilisation, and without fertilisation the plant cannot make seeds or fruit.

• Self-pollination: pollen lands on the stigma of the same flower, or of another flower on the same plant.
• Cross-pollination: pollen is carried to a flower on a different plant of the same kind.

2Agents of pollination

An agent of pollination is whatever carries the pollen. Animals that do this are called pollinators: bees, butterflies, moths, beetles, flies, and birds such as sunbirds, which you can see in Rwandan gardens. They visit flowers to drink sweet nectar. As they feed, pollen sticks to their bodies, and some of it rubs off on the next flower they visit.

Flowers pollinated by animals usually have big, brightly coloured petals, a scent, and nectar as a reward. Their pollen is often sticky or spiky so it clings to the animal.

3Pollination by wind

Maize, sorghum, wheat, rice and grasses are pollinated by the wind. Look at a maize plant: the tassel at the top holds the male flowers, and the silky threads on the young cob are the female stigmas.

Wind-pollinated plants make so much pollen because most of it blows away and is lost. A few plants that live in water are pollinated by pollen floating on the water.

4Fertilisation

After a pollen grain lands on the stigma, it grows a thin pollen tube down through the style to an ovule inside the ovary. The male sex cell travels down the tube and joins the female sex cell (the egg) in the ovule. This joining is called fertilisation. Remember: pollination is the journey of the pollen; fertilisation is the joining of the cells.

5After fertilisation

• Each fertilised ovule becomes a seed.
• The ovary swells and ripens into a fruit that protects the seeds.
• The petals, stamens and style wither and fall off.

A fruit with one seed (an avocado or a mango) grew from an ovary with one fertilised ovule. A passion fruit or a tomato has many seeds because many ovules were fertilised.

6Seed dispersal and germination

Seeds are spread away from the parent plant so the young plants do not compete with it for light, water and minerals. Seeds are carried by the wind (light seeds with hairs or wings), by animals (birds eat fruit and drop the seeds elsewhere; hooked seeds stick to fur and socks), by water, or by explosion (dry bean pods twist open and shoot out their seeds). When a seed has water, warmth and air, it germinates: it starts to grow into a new plant.

Try it yourself

1. On which part of the flower must pollen land for pollination to happen?
2. Pollen from a bean flower lands on the stigma of the same flower. Is this self- or cross-pollination?
3. Give two features of a wind-pollinated flower.
4. What grows down the style after pollination?
5. An avocado has one seed, but a passion fruit has many. Why?

Answers

1. The stigma.
2. Self-pollination.
3. Any two of: small dull petals or none; no scent or nectar; lots of light, smooth pollen; anthers hanging out on long filaments; large feathery stigmas.
4. A pollen tube.
5. Each seed comes from one fertilised ovule. The avocado's ovary had one ovule fertilised; the passion fruit's had many.