Types of embryo sac in plants.

THE EMBRYO SAC - STRUCTURE AND TYPES IN ANGIOSPERMS.

1. THE EMBRYO SAC STRUCTURE AND TYPES IN ANGIOSPERMS. By: Arpit B.S c . Life Sciences

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. The functional mature embryo sac is sac-like, haploid, short-lived and heterotrophic structure. It usually possesses, viz.,1. The E gg Apparatus 2.Antipodals. 3.The Central Cell.

3. 1. EGG APPARATUS : It is a 3-celled structure always present in the central wall towards the micropylar end. b) It possesses a pair of synergids and the egg cell. c) All three cells are intactly present as a triangular structure. d) Their protoplasts are highly polarised differentially.

4. A. 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 F illiform Apparatus . It is a wedge or spheroidal structure made up of the cellulosic microfibrils.. They play a role in the absorption of nutrients , attracting the pollen tube or involved as the target site for the pollen tube entry. They may degenerate soon after the fertilization.

5. B. The Egg Cell: It is a highly specialized and haploid structure with the polarized protoplast. It is sharing the wall with t he 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 in a zygote due to the Syngamy (Fertilization) with the fusion of a male gamete. It is highly specialized and haploid structure with the polarized protoplast. 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-polyploidy). They may be persistent or ephemeral and proliferate in number, also sometimes becomes haustorial. They help in the nourishment of embryo sac initially and the embryo later on.

6. 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 and 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.

7. Types Of Embryo Sacs:- Monosporic Embryo Sac- In this type, only one out of the four megaspores take part in the development of embryo sac. All the nuclei in such an embryo sac are genetically identical because they are derived through mitosis of the single nucleus. Bisporic Embryo Sac- In this type, two megaspore nuclei take part in embryo sac formation. Here, the first meiotic division is followed by wall formation, so that a dyad is formed. Only one of the dyad cell undergoes the second meiotic division whereas the other one degenerates. Tetrasporic Embryo Sac- In this type, both the meiotic divisions are not accompanied by cell plate formation hence all the four haploid nuclei take part in the formation of embryo sac, a tetrasporic embryo sac is more heterogeneous than a bisporic embryo sac because the four products of meiosis involved in its formation are genetically different. Basis for classification- Number of megaspores taking part in the development of embryo sac. Number of divisions occurring in nucleus of functional megaspore. Organization of nuclei in mature embryo sac.

8. 1 .Monosporic Embryo Sac- There are two types of monosporic embryo sacs. Polygonum Type - The embryo sac is formed by the chalazal megaspore of the tetrad and is 8-nucleate. The mature embryo sac comprises a 3-celled egg apparatus ,three antipodals, and a binucleate central cell. This Monosporic 8-nucleate embryo sac, formed by three divisions of the functioning megaspore, occurs in more than 70% of angiosperms. The nucleus divides thrice to form 8 nuclei embryo sac. Of the 8 nuclei, 3 at the micropylar end give rise to the egg and two synergids;3 at the chalazal give rise to antipodals; remaining 2 in the centre as polar nuclei or fused to form secondary nucleus.

9. ii. Oenothera Type- 4 nucleated embryo sac ( Egg apparatus- 3 cells and Polar N ucleus- single cell) Develops from micropylar megaspore. Does not have antipodals. Example- Onagraceae family

10. 2.Bisporic Embryo Sac- Develops from one of the two dyads formed as a result of the first meiotic division (Meiosis l) of Megaspore Mother Cell. Both the nuclei of the functional dyad take part in the formation of embryo sac. Each nucleus undergoes two mitotic divisions and the mature embryo sac is 8-nucleated. The 8 nuclei are organised into antipodals, egg apparatus and polar nuclei as in Polygonum type of embryo sac. The 4 nuclei derived from one megaspore nucleus are genetically different from the other four derived from the second megaspore nucleus. On the basis of the position of functional dyad bisporic embryo sacs are of two types- Allium Type : Develops from the chalazal end. Endymion Type : Develops from the micropylar end.

11. 3.Tetrasporic Embryo Sac - There are seven types of tetrasporic embryo sacs. Adoxa Type- It has 8 nuclei which are formed by the mitotic division of the four haploid nuclei of the coeno-megaspore. The arrangement of the 8 nuclei in the embryo sac is the same as in Polygonum type. Example- Adoxa,Tulipa,Ulmus etc.

12. ii. Plumbago Type- In this, synergids and antipodals are absent. Out of the four haploid coeno-megaspore one migrates to the micropylar end, one at the chalazal end and two at the lateral sides. Each nuclei divides again and formed four groups of two nuclei. One of the nucleus from each group moves to the centre of the cell and form 4 polar nuclei. The remaining nucleus at the micropylar end is cut off by a membrane and form the egg. There are no synergids. The other three nuclei usually disappear but occasionally they too may be cut off by membranes and appear as accessory egg cells. Example- Plumbaginaceae family.

13. iii. Pinaea Type- The four haploid nuclei of the coeno-megaspore undergo two successive mitotic divisions forming 16 nuclei. T hese nuclei arrange themselves in four groups of four each, one at the micropylar end, one at the chalazal end and one each on the two lateral sides. Now one nucleus from each groups migrates to the centre, and these four nuclei in the centre form polar nuclei. The three nuclei at the micropylar end are cut off by membranes and form the egg apparatus. The remaining 3 groups of nuclei( one chalazal and two lateral) degenerate at maturity. Highly polyploidy, 5n,primary endosperm nucleus is formed after double fertilization. Example- Penaeaceae family.

14. iv. Peperomia Type- Egg apparatus characterized by single synergid. The four haploid nuclei of coeno-megaspore undergo two successive mitotic division forming 16 nuclei. Two nuclei at the micropylar end form egg and a synergid, 8 fuse in the centre of the cell to form a polar nucleus and the remaining 6 at the chalazal end formed antipodals. Example- Peperomia family.

15. v. Drusa Type- 16 nucleate embryo sac. Characterized by large number of antipodals. In the mature embryo sac 3 nuclei form egg apparatus. Two act as polar nuclei and the remaining 11 nuclei are cut off by membrane and form antipodal cells. The number and organisation o f nuclei may vary due to irregularity in the divisions. Example- D rusa, Rubia, Chrysanthemum , etc.

16. vi. Fritillaria Type- The four haploid nuclei of coeno-megaspore arrange themselves in two groups- 3 at the chalazal end in the form of a triploid nucleus and one haploid at the micropylar end. The triploid chalazal and the haploid micropylar nucleus undergo two mitotic divisions as a result, four triploid nuclei are formed at the chalazal end and four haploid at the micropylar end. In mature embryo sac 3 haploid nuclei organise into egg apparatus, 3 triploid into antipodals and remaining one haploid and triploid nuclei move to the centre where they fuse to form a tetraploid polar nucleus. Example- Fritillaria, Lilium , etc.

17. vii. Plumbagella Type- Initial development is similar to Fritillaria type and a triploid nucleus is formed at the chalazal end and a haploid at the micropylar end. Each of these nuclei undergoes a single mitotic division and form two groups of two nuclei each. One triploid nuclei from chalazal end and one haploid nucleus from the micropylar end fuse at the centre and form tetraploid polar nucleus. One haploid nucleus at the micropylar end forms the egg and one triploid nucleus at the chalazal end form the single antipodal. There is no synergids.

18. REFERENCES:- https :// images.app.goo.gl/4on7W6zKiUaUVSSs5 www.toppr.com http://eacharya.inflibnet.ac.in/data-server/eacharya-documents/57189ee98ae36c5c22225878_INFIEP_307/179/ET/307-179-ET-V1-S1__ file1.pdf http:// www.mgcub.ac.in/pdf/material/202004300509276c475952aa.pdf https:// www.google.com/url?sa=i&url=https%3A%2F%2Fwww.facebook.com%2FE.K.Botany%2F&psig=AOvVaw23hNndB4mFVaN2sj2QBPOz&ust=1600433798225000&source=images&cd=vfe&ved=0CAkQjhxqFwoTCKjbndme8OsCFQAAAAAdAAAAABAm

19. THANK YOU