Question

Study and identify various stages of female gametophyte ovary of a flower.

Answer 1

The functional megaspore forms female gametophyte or embryo sac.

The nucleus of megaspore divides into two, four and finally eight daughter nuclei. Four of which are located at each pole.

One nucleus from each pole migrates to the centre to form two polar nuclei which further fuse to form a diploid fusion or secondary nucleus. Thus, central cell bears two polar nuclei. Three nuclei at the base of embryo sac form antipodal cells. The remaining three nuclei at the micropylar end get surrounded by cytoplasm to form pyriform cells.

These three cells together constitute egg apparatus, which consists of two cells known as synergids or help cells and an egg or oosphere which hangs between them. The synergids bear special cellular thickenings at the micropylar tip called filiform apparatus, which play an important role in guiding pollen tubes into synergies.

The egg cell on fertilization gives rise to zygote, while synergids get disorganized soon after fertilization. The antipodal cells sooner or later also get disorganized. They may, however, be nutritive m function. Thus, a typical angiospermic embryo-sac, at maturity though eight nucleate is seven-celled.