The Female gametophyte or embryo sac in angiosperms
is eight nucleated and seven celled. It is called Polygonum type, since it was
first studied in Polygonum divaricatum by Strasburger.
After the last nuclear division in the female
gametophyte, the cleavages are such that all the cells of the embryo sac are
formed within the wall of the parent megaspore.
The embryo sac has three main parts:
1. Egg
apparatus 2. Antipodals 3. Central Cell
Egg
Apparatus:
Three nuclei of the embryo sac towards micropyle
develop into an egg apparatus. As these nuclei are covered by thin walls, they
are considered as ‘cells’.
The egg apparatus consist of one egg cell and two
synergids. The three cells of the egg apparatus an egg cell and two synergids
are arranged in triangular fusion.
EGG
CELL:
The middle cell of the egg apparatus is big and acts
as an egg cell. The egg shows common wall with the two synergids the central
cell.
The wall is
thicker in the micropylar region but becomes thinner towards the
chalazalside.It is absent at the chalazal end in cotton maize. At the micropylar end the lateral walls of
the egg cell appear to join the central cell wall.
The egg cell
becomes highly polarised early in its development. The polarity is expressed by
the aggregation of cytoplasmic elements at the chalazal end of the cell. The
micropylar end of the cell is occupied by a large vacuoles.
SYNGERGIDS:
They are elongated cells present at the micropylar
end of the embryosac. When two synergies are present they lie in contact with
each other and partly embrace the egg. They are pointed or hooked toward the
micropyle..
The wall
around the synergies is incomplete. There is a distinct wall around the
micropyle one third of the cell which thins towards the chalazal end and
finally disappears. As a result the chalazal one third of the cell lacks a
wall.
A prominent
structure called filiform apparatus(FA)is present at the micropylar end of each
synergid. Recent electron microscopic works have revealed that the filiform
apparatus is a mass of finger-like projections of the cell into the cytoplasm..
The
cytoplasm of the synergid is strongly polarised. They are ephemeral structures.
In embryosac with two synergid one degenerates before the entry of the pollen
tube into the embryo sac, whereas the
other one often called the persistent synergid degenerates shortly after the
embryo sac has received the pollen tube discharge.
FUCTIONS:
Looking at the structure and concentration of cell
organelles the synergids appear highly active metabolically three function have
been ascribed to synergids..
1) They play
an important role in directing the pollen tube growth by secreting some
chemotropically active substances..
2) The degenerating synergids forms the seat for
pollen tube discharge in the embryosac..
3) Jenser suggested that the filiform apparatus may
be aiding the synergid in the absorption andtransportation of materials into
the embryosac form the nucellus.
ANTIPODAL
CELLS:
The three nuclei arranged at the posterior side of
the embryo sac are called the antipodal cells. The antipodal cells exhibit the
greatest variation amongst all the cells of the embryosac. Usually they
degenerate before or soon after fertilisation without any appreciable
enlargement.
In many
plants the antipodals are persistent and show some structural cytological
features suggesting their possible role in the nutrition of the embryosac. In
the Caltha palustin they persist upto the octant stage of pro embryo. In
grasses they undergo a series of mitosis divisions leading to the formation of
a large number of antipodal cells. This highest number of antipodal cells known
is 300 recorded in Sasa paniculata.
In Zea mays, during additional divisions in
antipodals, the walls of many cells remain incomplete leaving protoplasmic
continuities between adjacent cells. This results in the formation of
multinucleate protoplasm or syncytium.
Haustorial behaviour of antipodal cells is known in
many plants. In Argemone mexicana the antipodal cells are much larger than the
either the egg or the synergids. After fertilisation they continue to enlarge and
persistupto heart shaped stage of the embryo.
Three main
functions have been attributed to the antipodal cells. Often nutritive role had
been proposed for the antipodal cells especially where they are persistent.
Formation of wall projections in antipodal cells of Maize, Rice, Poppy gives
them the appearance of the so called transfer cells and support the suggestion
that these cells any be associated with the nutrition of embryosac.
The
antipodal cells may also store large quantities of starch, lipids, and proteins
which are utilised by the developing endosperm embryo. Another role ascribed to
antipodals is to produce and secrete substances that control the growth
development of endosperm..
CENTRAL
CELL:
It is the largest cell of the embryosac and the
mother cell of the endosperm. The enlargement
of the embryosac after the last nuclear division is largely due to the inflation
of the large central vacuole of the central cell.
The nuclei
of the central cell called polar nuclei are very large and each possesses a conspicuous
nucleolus.They are present either in the centre of the cell suspended by
cytoplasmic strands, or in the cytoplasm close to the egg apparatus.
Unlike the
egg cell the cytoplasm if the central cell is rich in all cell organelles
appears to be the centre of intense synthetic activity. There are plasmids
containing starch and sometimes proteins and phytoferrtin. In Capsella the
central cell possess numerous sphaerosomes associated with glyoxysomes that
probably convert fat into sugar. The central cells contains sufficient food
reserves that are available for use during fertilisation and early stages of
endosperm development..
The
presence of cell wall projections in the micropylar or chalazal region shows
that central cell draws nutrition from the surrounding nucellus or integuments
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