Question

Discuss the development of an ovule till the formation of an embryo-sac.

Answer 1

1. An ovule develops from the placenta inside the ovary. It is attached by a funicle and consists of nucellus surrounded by one or two integuments, leaving a micropyle. A megaspore mother cell differentiates in the nucellus.
2. The diploid megaspore mother cell undergoes meiosis to produce four haploid megaspores. Usually three degenerate and one remains functional.
3. The functional megaspore undergoes three successive mitotic divisions without immediate cell-wall formation, producing eight haploid nuclei.
4. One nucleus from each pole moves to the centre to form the two polar nuclei. At the micropylar end, three nuclei form the egg apparatus—one egg and two synergids. At the chalazal end, three nuclei form antipodal cells.

The mature, typical embryo sac is seven-celled and eight-nucleate.