AP BIOLOGY

Detailed explanation-1: -The megaspore mother cell (2n) undergoes meiosis during megasporogenesis and gives rise to four haploid nuclei. Out of these four haploid nuclei, three degenerate and one functional megaspore undergoes mitotic divisions to give rise to embryo sac (mature female gametophyte).

Detailed explanation-2: -In embryo sac, n, 2n, 3n conditions are found respectively in antipodal, zygote, and endosperm. In a majority of flowering plants, one of the megaspores remains functional while the other three degenerate. Only the functional megaspore then develops into the female gametophyte or ‘embryo sac’.

Detailed explanation-3: -An embryo sac is said to form when the haploid megaspore nucleus divides. It possesses two haploid nuclei and six haploid cells which do not have cell walls. In some cases, the haploid nuclei combine forming a single cell (endosperm motor cell).

Detailed explanation-4: -Ploidy level of endosperm is generally triploid or 3n.

Detailed explanation-5: -Sequence of development during the formation of embryo sac is: Archesporium