Structure and evolution of female gametophyteof gymnosperms

1. STRUCTURE AND EVOLUTION
OF FEMALE GAMETOPHYTE
OF GYMNOSPERMS
PRESENTED BY:-
• GOSWAMI KHUSHBOOBEN.B.
• M.SC.BOTANY SEM-1
• PAPER - CBO - 402
• DEPARTMENT OF LIFE
SCIENCE H.N.G.U. ,PATAN.

2. CONTENTS
• Introduction
• Structure of female gametophyte
• Evolution of female
gametophyte
• References

3. INTRODUCTION
• Gymnosperms (Gymnos = nacked and
Sperma = seed) are naked seeded
plants.
• These group of plants have ovules
which are not encosed and thus the
seeds remain nacked.

4. • Most of the gymnosperms are evergreen,
woody perennials with shrubby or tree
like habit.
• They show xerophytic characters.
• Many of them are very large in size.

5. Fig:- Female cone

6. STRUCTURE OF FEMALE GAMETOPHYTE
• Female gametophyte develop in female
cone.
• The female cone also has a central axis
on which bracts known as
megasporephylls are present.
• In the female cone , megaspore mother
cells are present in the megasporangium.

7. • The megaspore mother cell divides by
meiosis to produce foue haploid
megaspores.
• One of the megaspores divides to from the
multicellular female gametophyte, while
the others divide to from the rest of the
structure.
• The female gametophyte is contained
within a stucture called the archegonium.

9. OVULE
• The ovules are naked and are borne on
megasporophylls that are usually
spirally arranged around a central axis.
• The ovules are sessile and those of cycas
are the largest in the plant kingdom.
The ovules consists of a parenchymatous
mass of cells called the nucellus or the
megasporangium.

10. • The necellus encloses a single diploid
megaspore mother cell (MMC) that
undergoes meiosis to form a linear tetrad
of 4 haploid megaspores, out of which only
(usually the lower) remains functional,
whereas the rest degenerate.
• The functional megaspore enlarges and
undergoes free nuclear division to form
large number of the nuclei.

12. • This is the young female gametophyte .
• By now the nucellus becomes enveloped by
a single massive integument that grows
around leaving a small pore or the
micropyle at one end.
• This is the micropylar end of the ovule.
• This integumented megasporangium is
called the ovule.

14. • Now centripetal wall formation starts around
the free nuclei of the female gametophyte.
• It continues till the whole female
gametophyte becomes cellular.
• The single integument in gymnospermous
ovule consists of three layers : (1) Sarcotesta
or the outer fleshy layer, (2) sclerotesta or
the middle stony layer, and (3) the inner
sarcotesta or the inner fleshy layer.

16. • In pinus , Ephedra and gnetum the integument is
partly fused and partly free from the nucellus.
• In cycas the integument is fused with the
nucellus except in the apical region.
• In podocarpus gracilior the entire integument
grows into a micropylar tube in mature ovule.
• The apical region of the nucellus degenerates to
form a pollen chamber in which the semi –
germineted pollen grains.

17. • Towards the micropylar end the female
prothallus developes two or more archegonia
or the female sex organs.
• The archegonia have a short or long neck made
of 2 (cycas, Zamia, Ginkgo), 4 (Taxus) , 8 (Biota),
and 8 tiers of 4 cell each (Ephedra).
• The ventral canal cell is present in Ginkgo and
Pinus where as it is absent in cycas and
Ephedra, which have only ventral canal nucleus.
The venter has a large egg or the oosphere.

18. • In Biota orientalis, Cryptomeria and some
other conifers the archegonia occur in groups
and form and archegonial complex.
• Such groups may also develope at the
chalazal end of the female gametophyte,
e.g., Biota orientalis and Cryptomeria
japonica.

19. EVOLUTION OF FEMALE GAMETOPHYTES
• The fundamental characteristic of all
the spermetophytes “sensulato” to
produce seeds and that the seed or
more exactly the ovule that will develop
into a seed on maturation , is a
tegumented megasporangium that the
plants disperse togather with their
contents.

20. • The nucellus of an ovule contains a megaspore
that develops into the female gametophyte
(prothallus, archegonia) and is hence
comparable to megasporangium.
• It is surrounded by one or two protective
structure called the integuments .
• In oder to elucidate the origin and evolution of
ovule the botanists have to analyse
ontogenetically and phylogenetically the
various elements of this body .

21. • The research in this direction has almost
been exclusively resrticted to the integument
, and it is still a subject of dispute.
Regarding ovules morphological nature,
three main theories have been advanced:-
1). The axial theory, which held that nucellus is
of the nature of a bud bearing integuments
that are regared as lateral foliar
appendages.

22. 2). The sui-generis theory that regarded the
ovule to be an independent structure borne
on either foliar or axial organs.
3). The foliar theory regarded the ovule to be
hpmplogous to, with three lobed leaflet of
the female sporophyll, the two lateral lobes
of the leaflet are fused to form the outer
integument, the nucellus is an emergence
borne on the cup shaped terminal lobe.

23. • Benson (1940) produced that a pteridosperm
ovule is a synangium in which the central
sporangium has retained a single megaspore,
where as the surrounding ring of sporangia
became sterile and formed the integument.
• Oliver and Scott (1940) regarded the
integument to be a multiple structure as
evidenced by its free lobing. They regarded that
the present integument was once a
comparatively un specialized cupul like
indusium.

24. De Haan (1920) concluded:-
• The integuments of pteridospermous,
gymnospermous and angiospermous ovules
are homologus.
• The integuments are not homologus with the
indusial of ferns and lycopods.
• The integuments are originally composed of
units that are now regarded as teloms.

25. Halle (1933, 1937) was of the view that the cupules
and the synangi of pteridosperm ovules were
borne on the leaves and were formed by the
lateral concrescence of groups of cyclically
arrenged teloms.
• Nucellar modification concept :- this theory was
put forth and later elaborated by Andrews in 1961.
According to this theory the lagenostomalean
integument is derivable from the nucellus or
sporangiun wall by its vascularisation and growth
and lobing of the apex. it dose not account for the
origine of cupule.

26.  The telomes concept:- it is the synthesis of
of the views of smith (1961), Long (1960) and
Andrews (1961). They all consider the
derivation of pteridosperm ovule from a
terminal fern megasporangium surrounded
by sterile and fertile telomes as those of
hedesia corymbosa.

27. CONCLUSION
• The preridospermous ovule has arisen
independently a number of times , but the
process of evolution has followed a similar
pettern , which is explained by the telome
concept.
• This group has been regarded by Beck (1960,
1962, 1970) as ancestral to both pteridosperms
and cordaitales. This is suggestive of a
monophyletic origin for the gymnosperms.

28. REFERENCES
• Botany for a degree students – Gymnosperms
- Vasistha P C , S. Chand & company
• www. Google.com.

29. THANK YOU