Sexual reproduction in flowering plants involves two separate events: pollination, the transfer of pollen, and fertilisation, the fusion of gametes. They must not be confused — pollination comes first, and fertilisation follows only if a pollen tube grows successfully.
Pollination
Pollination is the transfer of pollen grains from an anther to a stigma.
- Self-pollination — pollen is transferred to the stigma of the same flower or another flower on the same plant.
- Cross-pollination — pollen is transferred to the stigma of a flower on a different plant of the same species. This mixes genes and increases variation.
Pollen is carried by agents such as insects (insect pollination) or wind (wind pollination), matched to the flower's structure.
Fertilisation
After landing on a compatible stigma, a pollen grain absorbs sugary fluid and grows a pollen tube down through the style towards the ovule, its growth controlled by the tube nucleus. The generative nucleus divides into two male gametes. The tube enters the ovule through a tiny pore, the micropyle, and releases the two male gametes into the embryo sac. This is where double fertilisation, unique to flowering plants, takes place:
- one male gamete fuses with the egg cell to form a diploid zygote, which grows into the embryo;
- the other male gamete fuses with the two polar nuclei to form a triploid cell that develops into the endosperm, the food store.
Key idea
Pollination = transfer of pollen (physical). Fertilisation = fusion of gametes (nuclear). Double fertilisation forms a diploid zygote AND a triploid endosperm.
Example
A bee carries pollen from one hibiscus to the stigma of another (cross-pollination). A pollen tube grows down the style; its two male gametes bring about double fertilisation, forming the embryo and the endosperm.
Remember
Egg + male gamete → zygote (embryo). Two polar nuclei + male gamete → endosperm (food). Cross-pollination increases variation.