What is double fertilization? Describe fertilized embryo sac with a neat labelled diagram.

Explanation

Drawing with any six labellings

Three cells are grouped together at the micropylar end and constitute the egg apparatus.
(ii) The egg apparatus consists of 2 syner gids and one egg cell.
(iii) The synergids have special cellular thickenings at the micropylar tip called filiform apparatus. It helps to guide the pollen tubes into the synergid.
(iv) Three cells are at the chalazal end are called the antipodals. The large central cell has two polar nuclei. Thus, a typical mature angiosperm embryo sac is 8- nucleate and 7-celled.
Detailed Answer:
Female gametophyte (Embryo sac)
1. In most of the flowering plants, only one megaspore is functional while the other three degenerate.
2. The functional megaspore develops into the female gametophyte or embryo sac.
3. This method of embryo sac formation from a single megaspore is termed as monosporic development.
Development of Female gametophyte
The nucleus of the functional megaspore divides mitotically to form two nuclei which move towards the opposite poles, forming two-nucleated embryo sac.
Two more sequential mitotic nuclear divisions result in the formation of the fournucleated and later the eight nucleated stages of the embryo sac.
These divisions are strictly free nuclear, i.e. nuclear divisions are not followed immediately by cell wall formation.
After eight-nucleate stage, the organization of the typical female gametophyte or embryo sac takes place.
Generally six of the eight nuclei are surrounded by cell walls and organized into cells.
The remaining two nuclei called the polarnuclei are found below the egg apparatus in the large central cell. Distribution of the cells within the embryo sac
The three cells consisting of two synergids and one egg cell which are grouped together at the micropylar end constitute the egg apparatus.
The synergids have special cellular thickenings at the micropylar tip called filiform apparatus.
The filiform apparatus helps to guide the pollen tubes into the synergid.
Three cells at the chalazal end organize as the antipodals.
Thus, a typical mature angiosperm embryo sac at maturity is eight-nucleate and seven-celled.