Explain double fertilisation in flowering plants with a labelled diagram.

1. 1 · Define. Double fertilisation is the fusion of one male gamete with the egg and the other with the secondary nucleus, inside the embryo sac.
2. 2 · Pollination and the pollen tube.
   • Pollen lands on the stigma and grows a pollen tube.
   • The tube passes through the style and enters the ovule by the micropyle.
   • It releases two male gametes into the embryo sac.
3. 3 · Draw the ovule with the pollen tube. [Diagram: L.S. of ovule with integuments, micropyle, nucellus, embryo sac with egg, synergids, antipodals and secondary nucleus; pollen tube with two male gametes]
4. 4 · The two fusions.
   • Syngamy: male gamete + egg → zygote (2n).
   • Triple fusion: male gamete + secondary nucleus → primary endosperm nucleus (3n).
5. 5 · After fertilisation. Zygote → embryo, endosperm nucleus → endosperm, ovule → seed, ovary → fruit.
6. 6 · Link to daily life. The rice and wheat we eat are mostly endosperm made by triple fusion.
7. 7 · Close. Double fertilisation is found only in flowering plants.

Scoring tip: Write 2n beside the zygote and 3n beside the endosperm. Examiners look for these numbers.