(a) Explain the development of female gametophyte from a megaspore mother cell in an angiosperm.
(b) Draw a labelled diagram of a fully developed embryo sac.

Explanation

(a) The nucleus of the functional megaspore divide mitotically to form two nuclei which move to the opposite poles, formating the 2-nucleate embryo sac. Two mare more sequential mitotic nuclear divisions result in the formation of the 4-nucleate and later the 8-nucleate stages of the embryo sac. After the 8-nucleate stage, cell walls are laid down leading to the organisation of the typical female gametophyte or embryo sac. Six of the eight nuclei are surrounded by walls and are organised into, the nuclei, called polar nuclei are situated below the egg apparatus in the large central cell. Three cells are grouped at nuclei are situated below the egg apparatus in the large central cell. Three cells are grouped at the micropylar end and constitute the egg apparatus. The egg apparatus, in turn, consists of two synergids and one edd cell. The synergids have special cellular thickenings at the micropylar tip called filiform apparatus, which play an impotant role in guiding the pollen tubes into the synergid. Three cells are at the chalazal end and are called the antipodals. Thus, a typical angiosperm embryo sac, at maturity, though 8-nucleate is 7-celled.