Sexual reproduction in flowering plants involves the fusion of a male gamete nucleus with a female gamete nucleus to form a zygote. The flower is the reproductive organ, and its structure is adapted to bring these gametes together.
Parts of an insect-pollinated flower
The stamen is the male part, made of the anther (produces pollen) and the filament. The carpel is the female part, made of the stigma, style and ovary (containing ovules). Brightly coloured petals, scent and nectar attract insects.
Key idea
Pollination is the transfer of pollen from an anther to a stigma. Self-pollination is within the same flower or plant; cross-pollination is between different plants of the same species.
Pollination and fertilisation
Insect-pollinated flowers have large petals, sticky and spiky pollen, and sticky stigmas. Wind-pollinated flowers have small dull petals, large feathery stigmas hanging outside, and smooth light pollen produced in huge amounts. After pollination the pollen grain grows a pollen tube down the style to the ovule. The male gamete nucleus travels down this tube and fuses with the female gamete nucleus in the ovule — this is fertilisation.
Worked example
A gardener wants to breed two flower varieties. To cross-pollinate, pollen from the anther of plant A is placed on the stigma of plant B. The pollen tube grows to the ovule, fertilisation occurs, and the resulting seed carries genes from both parents, giving variation.
Seed and fruit formation
After fertilisation the ovule becomes the seed and the ovary wall becomes the fruit. The zygote develops into an embryo (plumule and radicle) with a food store (cotyledons).
Remember
- Pollination = transfer of pollen; fertilisation = fusion of nuclei.
- Ovule → seed; ovary → fruit.
- Cross-pollination increases variation.