Question

Write an essay on the development of female gametophyte. Illustrate the answer with suitable diagrams.

Answer 1

In the common monosporic (Polygonum-type) pathway:

1. A diploid megaspore mother cell forms in the nucellus of the ovule and undergoes meiosis, producing four haploid megaspores. Usually three degenerate; the remaining functional megaspore develops further.
2. Its nucleus divides mitotically three times without immediate cytokinesis, producing the two-, four- and then eight-nucleate stages. Four nuclei lie at each pole.
3. One nucleus from each pole moves to the centre and becomes a polar nucleus. Three nuclei at the micropylar end form the egg apparatus: an egg cell and two synergids. Three at the chalazal end form antipodal cells.
4. Cell walls form around the egg apparatus and antipodals; the polar nuclei remain in the central cell. The mature embryo sac is therefore seven-celled and eight-nucleate.