What is double fertilisation? Describe it with a labelled diagram, and state the fate of its products.
How to lay out the answer. Follow the frame, then write it in your own words.
1 · Define in one line. Double fertilisation is the occurrence of two fusions, syngamy and triple fusion, in the same embryo sac of a flowering plant.
2 · Entry of male gametes. The pollen tube enters a synergid through the filiform apparatus and releases two male gametes.
3 · Syngamy. One male gamete (n) fuses with the egg (n) to form the zygote (2n).
4 · Triple fusion. The other male gamete (n) fuses with the two polar nuclei (n + n) to form the primary endosperm nucleus (3n).
5 · Draw the embryo sac at fertilisation. [Diagram: pollen tube, synergid, male gametes, egg, polar nuclei, antipodals, labels for syngamy and triple fusion]
6 · State the fate. Zygote → embryo; primary endosperm cell → endosperm, which feeds the embryo.
7 · Link to daily life. The endosperm of rice, wheat and coconut that we eat comes from triple fusion.
8 · Close in one line. So double fertilisation gives both the embryo and its food supply at the same time.
Scoring tip: Write the ploidy (n, 2n, 3n) beside each fusion. It is the quickest way to show the examiner you understand the process.
