(a ) Trace the development ofmegaspore mother cell up to the formation of a mature embryo-sac in a flower ing plant.
(b) Draw a labelled diagram of the structure of mature dicot embryo.

Explanation

The process of formation of megaspores from megaspore mother cell is called megasporogenesis. Ovules generally differentiate a single megaspore mother cell [MMCI] In the micropylar region of the nucellus. The megaspore mother cell undergoes meiosis resulting in formation of four megaspores: In majority of flowering plants. one of the megaspore is functional while the other three degenerate

Only the functional megaspore develops in female gametophyte (embryo sac). This method of embryo sac formation from a single megaspore is termed monosporic development. The nucleus of the functional megaspore divides mitotically to form two nuclei which move to the opposite poles forming two nucleate embryo sac. Two 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 organisation of the typical female gametophyte or embryo sac. Six of the eight nuclei are surrounded by cell walls and organised into cells: the remaining two nuclei, called polar nuclei are nuclei situated below the egg apparatus in Antipodas the large central cell. Three cells are grouped together at the micropylar end and constitute egg apparatus, IL consists of two synergids and one egg cell. Three cells at the chalazal end are called antipodals. Thus, a typical angiosperm embryo sac at maturity. though eight nucleate is seven celled.