The Embryo sac Structure of Angiosperms

This is a part of the online class for the Botany undergraduate students, Government First Grade College, Yelahanka, Bangalore

1. THE EMBRYO SAC STRUCTURE IN ANGIOSPERMS By P B MALLIKHARJUNA GFGC YB 17.04.2020

2. A typical mature embryo sac or the female Gametophyte is usually a 7-celled, 8-nucleated sac like structure present in the nucellus of the ovule. It is derived from a megaspore due to megagametogenesis (multiple mitotic divisions ) The functional mature Embryo sac may be a sac- like, haploid, short-lived and heterotrophic structure . It usually possesses, viz., 1) The Egg Apparatus 2) Antipodals , and 3) The Central cell.

3. An Embryo sac structure

4. Egg Apparatus : It is a 3-celled structure always present in the central cell towards the micropylar end. It possesses a pair of synergids and the egg cell. All three cells are intactly present as a triangular structure. Their protoplasts are highly polarized differentially.

5. Synergids : These are elongated cells with a pointed or hooked end towards the micropylar end. The walls of synergids are incomplete. They possess a mass of finger-like projections of the wall into the cytoplasm called Filiform Apparatus . It is a wedge or spheroidal structure made up of the cellulosic microfibrills . They play a role in the absorption of nutrients, attracting the pollen tube due to the chemotropic nature or maybe involved as the target site for the pollen tube entry. They may degenerate soon after fertilization . The ultrastructure of a synergid cell

6. B) The Egg cell : It is a highly specialized and haploid structure with the polarized protoplast. It is sharing the wall with the antipodals and the central cell towards the micropylar end. While, the distal portion has a thin wall. It contains the vacuolated cytoplasm with more plastids towards the micropylar end and with a large nucleus towards the distal end. Its nucleus or the female gamete results a Zygote due to the Syngamy (Fertilization) with the fusion of a male gamete

7. 2. Antipodals : Antipodals are usually present in the central cell towards the chalazal end. There are three antipodal cells with a variable nuclear condition (haploid – polyploid ). They may be persistent or ephemeral and proliferate in number, and also sometimes become haustorial . They help in the nourishment of Embryo sac intially and the embryo later on.

8. 3. The Central Cell : The central cell is the largest cell of the Embryo sac. It is the mother cell of the endosperm after fertilization. It encompasses the egg apparatus towards the micropylar end and the antipodals towards the chalazal end. It is also a polarized cell with a large central vacuole with the scanty but organelle-nutrient rich peripheral cytoplasm. A paired or fused polar nuclei are present just below the egg apparatus. It is usually diploid but often exists with a polyploidy condition. The polar nuclei may be fused and resulting the Primary Endosperm Nucleus (PEN) which later on fuses with the nucleus of another male gamete ( Triple fusion ) and develops into endosperm .

9. Conclusion : The mature embryo sac metamorphoses into the embryonic and the endospermic structure in the ovule soon after double fertilization . --------------THANK YOU-----------------