Sexual reproduction in flowering plants

Two parents mix their DNA

In sexual reproduction two parents each give a gamete, a special sex cell. A male gamete joins a female gamete. This joining is fertilisation, and it makes one cell, the zygote, that grows into the new individual.

Because the young get DNA from two parents, sexual reproduction gives more variation than asexual reproduction. That makes it a great help to a species.

The parts of a flower

The flower is the reproductive part of a flowering plant.

• Sepals. The green leaf-like parts at the base, which protect the bud.
• Petals. Often brightly coloured, they attract insects.
• Stamens. The male parts. Each has a stalk, the filament, and an anther at the top, which makes pollen grains. The pollen carries the male gamete.
• Carpel, or pistil. The female part in the centre. At its base is the swollen ovary, with one or more ovules. Each ovule holds a female gamete, the egg cell. Above it are the long style and the sticky stigma.

A flower with both stamens and carpel is bisexual, as in hibiscus and mustard. A flower with only stamens or only a carpel is unisexual, as in papaya and watermelon.

Pollination

Pollen must reach the stigma. The transfer of pollen from an anther to a stigma is pollination.

• Self-pollination. The pollen lands on the stigma of the same flower, or of another flower on the same plant.
• Cross-pollination. The pollen goes to a flower on a different plant of the same kind. Insects, birds, wind and water carry it.

Fertilisation

After landing on the stigma, a pollen grain grows a pollen tube. The tube grows down through the style and reaches the ovule in the ovary. The male gamete travels down the tube and joins the egg cell inside the ovule. This is fertilisation, and it makes the zygote.

Seeds, fruits and germination

After fertilisation:

1. The zygote divides many times and forms an embryo inside the ovule.
2. The ovule develops a tough coat and becomes a seed. The seed holds the future plant.
3. The ovary grows and ripens into a fruit.
4. The petals, sepals, stamens, style and stigma usually shrivel and fall off.

When conditions are right, with water, air and warmth, the seed germinates. The embryo grows a root, the radicle, and a shoot, the plumule. It uses food stored in the seed, often in the cotyledons, until it can make its own.

Only fertilised ovules become seeds. See how the numbers add up below.

Try it in Python

The program counts the pods and seeds on one pea plant. Each pollinated ovary grows into a pod, and each fertilised ovule becomes a seed.

Practise with new numbers

Common mistakes

• "Pollination and fertilisation are the same." Pollination moves pollen to the stigma. Fertilisation is the joining of the gametes inside the ovule, which comes later.
• "The ovary becomes the seed." The ovule becomes the seed. The ovary becomes the fruit.
• "Every ovule becomes a seed." Only a fertilised ovule becomes a seed.
• "Pollen is the male gamete." The pollen grain carries the male gamete, which travels down the pollen tube.
• "Petals make the seeds." Petals attract insects. They usually fall off after fertilisation.

Remember these

• Sexual reproduction joins a male and a female gamete. It gives more variation than asexual reproduction.
• Stamens (anther and filament) are male. The carpel (stigma, style and ovary with ovules) is female.
• Pollination brings pollen to the stigma. A pollen tube carries the male gamete to the ovule, where fertilisation makes a zygote.
• The ovule becomes the seed, and the ovary becomes the fruit. The seed germinates into a new plant.

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 two parts of a stamen and say what the anther makes.

2. A bee carries pollen from a sunflower in one field to a sunflower in the next field. What kind of pollination is this?

3. How does the male gamete reach the egg cell in the ovule?

4. After fertilisation, which part of the flower becomes the seed, and which becomes the fruit?

5. Why do the young from sexual reproduction show more variation than those from asexual reproduction?