Why sexual reproduction?
In sexual reproduction two parents each give a special cell called a gamete. Each gamete carries half the usual DNA, so when they fuse the offspring gets the normal full amount. Because DNA from two different individuals is mixed, the offspring are more varied than in asexual reproduction — and variation helps the species survive changes.
Gametes are made by a special kind of division that halves the DNA; this keeps the DNA amount the same from one generation to the next.
Parts of a flower
The flower is the reproductive part of a plant. Sepals (green) protect the bud; petals (coloured) attract insects. The stamen is the male part: a stalk called the filament and the anther, which makes pollen grains. The carpel (pistil) is the female part: the sticky stigma, the long style, and the swollen ovary holding ovules; each ovule has an egg cell.
Flowers with only stamens or only carpels are unisexual (papaya, watermelon). Flowers with both are bisexual (hibiscus, mustard).
Pollination: self and cross
Pollination is the transfer of pollen from the anther to the stigma. If pollen reaches the stigma of the same flower (or same plant), it is self-pollination. If it goes to a flower on a different plant, it is cross-pollination, carried by wind, water, insects or birds. Cross-pollination mixes DNA of two plants, so it gives more variation.
Fertilisation: the pollen tube
After landing on a suitable stigma, the pollen grain germinates and grows a pollen tube through the style to the ovule. The male gamete moves down the tube and fuses with the female gamete (egg) inside the ovule. This fusion is fertilisation, and the fused cell is the zygote.
After fertilisation: seed and fruit
The zygote divides many times to form an embryo inside the ovule. The ovule develops a tough coat and becomes a seed. The ovary grows fast and ripens into a fruit. Petals, sepals, stamens, style and stigma usually shrivel and fall.
The seed holds the future plant (embryo) with stored food. In the right conditions of water, air and warmth it germinates into a seedling. The embryo has a plumule (future shoot), radicle (future root) and cotyledons (seed leaves with food).
Key formulas and definitions
- Stamen (male) = filament + anther (makes pollen)
- Carpel/pistil (female) = stigma + style + ovary (holds ovules)
- Pollination: anther → stigma (self or cross)
- Fertilisation: male gamete + egg → zygote
- Zygote → embryo · Ovule → seed · Ovary → fruit
- Embryo parts: plumule (shoot), radicle (root), cotyledon (food)
Worked examples
1. How is pollination different from fertilisation?
Pollination only moves pollen from anther to stigma. Fertilisation happens later, inside the ovule, when the male gamete fuses with the egg. Pollination comes first; fertilisation needs a pollen tube.
2. A papaya plant has only male flowers. Can it make fruit alone? Explain.
No. Its flowers are unisexual and have only stamens; there is no ovary to become a fruit. Pollen must reach a female papaya plant's flowers, whose ovaries then form fruits.
3. Why do gametes carry only half the DNA?
If each gamete had full DNA, the zygote would get double DNA and the amount would keep doubling each generation. Halving it in gametes keeps the DNA amount constant in every generation.
4. Trace what happens to each part of a mango flower after fertilisation.
Zygote → embryo; ovule → the seed (stone); ovary wall → the fleshy fruit; petals, stamens, style and stigma dry and fall; sepals may stay as a small crown or fall.
Common mistakes
- Writing that pollination and fertilisation are the same thing.
- Saying the ovary becomes the seed. Ovary → fruit; ovule → seed.
- Calling the whole stamen the anther. The anther is only the top part that makes pollen.
- Thinking the pollen grain itself enters the ovule. Only the pollen tube grows down and carries the male gamete.