Fertilisation

1. Germination of the pollen grain. The pollen grain lands on the stigma and absorbs the sugary fluid there. The tube nucleus directs the growth of the pollen tube, which pierces the stigma, grows down the style and reaches the micropyle of the ovule. The tube nucleus degenerates, and the generative nucleus divides mitotically into two male nuclei (N).
2. Double fertilisation (a feature of flowering plants):
   • One male nucleus (N) fuses with the egg nucleus (N) → zygote (2N) → embryo (this is the true fertilisation).
   • The second male nucleus (N) fuses with the two polar nuclei (2N, the central cell) → endosperm nucleus (3N, triploid). This is called triple fusion.
   • The two fusions together are the double fertilisation.
3. The triploid endosperm nucleus divides repeatedly to form the endosperm tissue, which feeds the young embryo. In some seeds the endosperm remains outside the embryo and occupies a part of the seed.

Worked example 7. If the plant is diploid with 2N = 20: pollen nucleus = 10; egg = 10; zygote = 20; endosperm = 30; polar nuclei each = 10; nucellus (maternal) = 20.