Question

With the help of a neat and labelled diagram, describe the development of the female gametophyte of angiosperms.

Answer 1

A diploid hypodermal cell at the micropylar end of the nucellus gets differentiated to form an archesporium. Mostly this single-celled archesporium directly functions as a megaspore mother cell (MMC). This diploid MMC (2n) undergoes meiosis to form a tetrad of haploid megaspores (n).

This process is known as megasporogenesis Megaspores are generally arranged in the linear tetrad.

Generally, the chalazal megaspore remains functional while three degenerate gradually. A functional (fertile) megaspore is the first cell of the female gametophyte. It undergoes enlargement and develops into a female gametophyte. The haploid nucleus of a functional megaspore undergoes three successive free-nuclear mitotic divisions.

First mitotic division results in the formation of two nuclei. Both nuclei undergo two successive divisions. this results in the formation of four nuclei at each pole and an 8-nucleated structure is formed. One nucleus from each pole comes to the centre and they function as polar nuclei.

This is followed by the cellular organization to form a 3-celled egg apparatus at the micropylar end, three antipodals at the chalazal end and two polar nuclei remain in the centre. Thus, an 8-nucleated, 7-celled female gametophyte is formed within the functional megaspore; therefore the development is called endospores. Only one megaspore takes part in the formation of the embryo sac; therefore, it is called monosporic. (In some Angiosperms, embryo sac may by bisporic or tetrasporic).