THE EMBRYO SAC - STRUCTURE AND TYPES IN ANGIOSPERMS.

1. THE EMBRYO SAC
STRUCTURE AND TYPES
IN ANGIOSPERMS.
BY:
ARPIT
B.Sc. 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 Egg Apparatus
2.Antipodals.
3.The Central Cell.

4. 1. EGG APPARATUS :
a) 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.

5. 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 Filliform 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.

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 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.

7. 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.

8. 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.

9. 1.Monosporic Embryo Sac- There are two types of monosporic embryo sacs.
i. 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.

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

11. 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.

12. 3.Tetrasporic Embryo Sac-There are seven
types of tetrasporic embryo sacs.
i. 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.

13. 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.

14. iii. Pinaea Type-
• The four haploid nuclei of the coeno-
megaspore undergo two successive mitotic
divisions forming 16 nuclei.
• These 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.

15. 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.

16. 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 of
nuclei may vary due to
irregularity in the divisions.
• Example- Drusa, Rubia,
Chrysanthemum, etc.

17. 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.

18. 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.

20. 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
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