MEGASPOROGENESIS AND DEVELOPMENT OF FEMALE GAMETOPHYTE:

Formation of megaspore from megaspore mother cell is called megasporogenesis.

Here we are going to study the monosporic type of development of embryo common example is Polygonam. In this only one megaspore situated near chalazal end remain functional other degenerate.

Other are bisporic & tetrasporic.

STEPS IN DEVELOPMENT ARE:

1.Occur in nucellus much before it is surrounded by integuments.

2. Single archesporium cell divide periclinally to form outer parietal and an inner sporogenous cell.

3.The sporogenous cell directly behave as megaspore mother cell.

4.The diploid megaspore mother cell enlarge in cell & divide by meiosis to form four haploid cell or tetrad cell. Out of these four three degenerate.

5.The functional megaspore now divide mitotically  to form two nuclei out of which one move towards micropylar end and other towards chalazal end.

6.Both the nuclei divide further to form 8 nucleated stage 4 at chalazal end & 4 at micropylar end.

6.The embryo sac enlarges in size form vacuole in the centre.

7. Now out of  the 4 nuclei at micropylar end 2 differentiated into 2 synergid cells and one form egg and the fourth one move towards centre called polar nuclei. 2 Synergid and 1 egg collectively called egg apparatus.

9. Out of the 4 nuclei at the chalazal end 3 form antipodal cell and one move towards centre called polar nuclei.

Hence the embryo sac is a 7 celled 8 nuclei structure with 2 synergid and egg at micropylar end 3 antipodal cells at chalazal end and two polar nuclei in single cell in the Centre

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FUNCTIONS OF EMBRYO SAC CELLS

Egg : It fuse with one male gamete to form zygote.

Synergids :They direct the growth of pollen tube by secreting certain chemicals ,function as shock absorber during the penetration of pollen tube and their filiform apparatus absorb nutrient into the embryo sac,they degenerate after fertilization.

Polar nuclei : They fuse with second male gamete to form triploid cell which form endosperm this is known as triple fusion feature of angiosperms. Endosperm provide nourishment to the developing embryo.

Antipodal cell: Most of the cases they degenerate before or after fertilization but in some cases they provide nourishment to the embryo sac.