The essentials
After a compatible pollen grain lands on the stigma, it germinates and produces a pollen tube through a germ pore. The tube grows through the stigma and style, enters the ovary and usually enters the ovule through the micropyle. Inside the embryo sac, it enters one of the synergids, guided by the filiform apparatus, and releases two male gametes.
One male gamete fuses with the egg cell; this is syngamy and produces a diploid zygote. The other male gamete fuses with the two polar nuclei of the central cell to form a triploid primary endosperm nucleus (PEN); this is triple fusion. Because two fusions take place in one embryo sac, the event is called double fertilisation. It is a characteristic feature of flowering plants. The central cell becomes the primary endosperm cell, which develops into the endosperm, and the zygote develops into the embryo.
Key points
- Number of nuclei involved in triple fusion: three (one male gamete + two polar nuclei).
- Zygote is 2n; PEN is 3n.
- The pollen tube enters through the micropyle in most plants (porogamy).
- Synergids and antipodals usually degenerate after fertilisation.
Common mistakes
- Do not confuse "triple fusion" with "double fertilisation": triple fusion is one of the two fusions.
- The endosperm is triploid, not diploid, in typical angiosperms.
Exam tips
Ploidy-based numericals are common: if n is given for a pollen grain, endosperm = 3n, embryo = 2n.