Describe the formation of the female gametophyte in a flowering plant. Draw a labelled diagram of a mature embryo sac.

1. 1 · Define in one line. The embryo sac is the female gametophyte; it forms inside the nucellus of the ovule from a functional megaspore.
2. 2 · Megasporogenesis. One megaspore mother cell (2n) in the micropylar nucellus divides by meiosis into four haploid megaspores; three degenerate and one is functional (monosporic development).
3. 3 · Free-nuclear divisions. The nucleus of the functional megaspore divides by mitosis three times, giving 2, 4 and then 8 free nuclei, four at each pole.
4. 4 · Cell organisation.
   • Micropylar end: egg apparatus of one egg and two synergids with the filiform apparatus.
   • Chalazal end: three antipodals.
   • Centre: one central cell with two polar nuclei.
5. 5 · Draw the mature embryo sac. [Diagram: chalazal end, antipodals, polar nuclei, central cell, egg, synergids, filiform apparatus, micropylar end]
6. 6 · Close in one line. So the mature embryo sac is 7-celled and 8-nucleate.

Scoring tip: Use the words 'free-nuclear' and 'monosporic'; both are examiner keywords. Count cells and nuclei separately.