Explain the monisporic development of female gametophyte

In monosporic development as in (Polygonum) the development of embryo sac, only one megaspore situated towards chalazal end remains functional. The remaining three megaspores gradually degenerate and finally disappear. The functional haploid megaspore enlarges in size and, by means of three successive mitotic divisions, gives rise to an eight-nucleate embryo sac. Since only one megaspore is involved thus it is called monosporic development.