Describe the structure of typical embryo sac and the functions performed by its various constituents?
In the mature ovule, within the nucellus towards the micropylar end is a large cell called embryo sac. It is also called a female gametophyte. It contains cytoplasm, two free nuclei and six small cells. Two free nuclei fuse to form a secondary nucleus. Out of these, three cells constitute egg apparatus (two synergids and one oosphere or egg cell). The other three cells lie at the chalazal end of embryo sac and are called antipodal cells. They are small and enclosed in thin cell walls. In egg apparatus, egg fuses with male gamete to form zygote or oospore which later gives rise to the embryo. Synergids help in fertilisation. Secondary nucleus fuses with the second male gamete to form primary endosperm nucleus which later on forms endosperm. The three antipodal cells degenerate: A typical embryo sac has three cells that are grouped together at the micropylar end and constitute the egg apparatus. The egg apparatus, in turn, consists of two synergids and one egg cell. Three ceLLs are at the chaLazaL end and are cast the antipodals. The Large centraL ceLL has two poLar nucLei. Thus, a typical angiosperm embryo sac at maturity is 7-celLed and 8-nucLeated.
