Explain about Embryo sac.

The nucleus of the functional megaspore divides mitotically to form two nuclei which move to the opposite poles forming the two nucleate embryo sac.

Two more sequential mitotic nuclear divisions result in the formation of the 4 -nucleate and later 8 nucleates stage of the embryo sac. These mitotic divisions are strictly free nuclear that is nuclear divisions are not followed immediately by cell wall formation.

After the 8 nucleate stages, 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 cell walls and organised into cells; the remaining two nuclei, called polar nuclei are situated below the egg apparatus in the large central cell. There is a characteristic distribution of the cells within the embryo sac.

Three cells are grouped together at the micropylar end and constitute the egg apparatus. The egg apparatus, in turn consists of two synergids and one egg cell.

The syneregids have special cellular thickenings at the micropylar tip called filiform apparatus which play an important role in guiding the pollen tubes into the synergid.

Three cells are at the chalazal end and are called the antipodals.

The large central cell as mentioned has two polar nuclei.

Thus a typical angiosperm embryo sac, at maturity though 8 -nucleate is 7 celled.

Panchanan Maheshwari has classified female gametophyte on the basis of how many megaspore, take part in the development of embryo sac, into monosporic, biosporic and tetrasporic embryo sac.