Structure of Embryo Sac (Female Gametophyte):

In angiosperms, the female ga- metophyte is called embryo sac. Embryo sac is an oval multicellular haploid structure which is embedded in the nucellus towards micropylar half of the ovule. It is covered over by a thin membrane derived from the parent megaspore wall. The typical and most common type of embryo sac, found in 80% flowering plants is Polygonum type

It contains 8 nuclei but 7 cells— 3 micropylar, 3 chalazal and one central. It is formed by one meiosis (formation of 4 megaspores from one MMC) and three mitosis (inside functional megaspore). The three micro­pylar cells are collectively known as egg apparatus (equivalent to one archegonium).

They are pyriform in outline and are arranged in a triangular fashion. The three cells of egg appa­ratus have conspicuous common walls towards micropylar half. They separate and become thin towards the central cell.

One middle cell is larger and is called egg or oosphere. It has a central or micropylar vacuole and a nucleus towards the chalazal end. A filiform apparatus may or may not be present. The remaining two cells are called synergids, cooperative cells or help cells. Each of them bears a filiform apparatus in the micropylar region, a lateral hook, chalazal vacuole and a central nucleus.

A filiform apparatus is a mass of finger like projections of the wall into the cytoplasm. In embryo sac, one synergid degenerates at the time of entry of pollen tube into the embryo sac, whereas, the second one degenerates shortly after the embryo sac has received the pollen tube discharge.

All the three cells of the egg apparatus communicate with one another and to the central cell by plasmodesmata. The egg or oosphere represents the single female gamete of the embryo sac. The synergids help in obtaining nourishment from the outer nucellar cells, guide the path of pollen tube by their secretion and function as shock absorbers during the penetration of pollen tube into the embryo sac.

The three chalazal cells of the embryo sac are called antipodal cells. They are the vegetative cells of the embryo sac which may degenerate soon or take part in absorbing nourishment from the surrounding nucellar cells. Internally they are connected with the central cell by means of plasmodesmata.

The central cell is the largest cell of the embryo sac. It has a highly vacuolated cyto­plasm which is rich in reserve food and Golgi bodies. In the middle, the cell contains two polar nuclei which have large nucleoli. The polar nuclei often fuse to form a single diploid secondary or fusion or definitive nucleus. Thus all the cells of the embryo sac are haploid except the central cell which is first bi-nucleate and then becomes diploid due to fusion of polar nuclei.