52 MTBIOLOGY TODAY | JULY ‘15development of male gametophyte
• The development of the male gametophyte is more or less
uniform in all flowering plants.
• Development of male gametophyte is precocious that is
it begins inside the microsporangium or pollen sac.
• The size of the nucleus increases and it divides mitotically
to produce a bigger vegetative or tube cell and a
smaller generative cell.
• The tube cell has a vacuolate cytoplasm which is rich in
food reserve and cell organelles. Its nucleus becomes large
and irregular.
• The generative cell undergoes a lot of change in shape
during pollen development. Its cytoplasm is highly reduced
but contains the usual cell organelles along with a
prominent nucleus.
• Male gametes are formed by a mitotic division in the
generative cell. This may take place after the release of
pollen from the anther or while the pollen are still confined
to the anther. Thus, in the former condition the pollen are
shed at the 2-celled stage and in the latter at the 3-celled
stage.
• After reaching the stigma, the intine grows out through a
germ pore into slender pollen tube.
piStil, ovule and embryo Sac
• Gynoecium is female reproductive whorl of the flower. It is
composed of one or more carpels or pistils. The carpels
are modified leaves which bear the ovules, and also called
the megasporophylls.
• Each carpel consists of three parts, i.e., stigma, style
and ovary.
• The stigma is the terminal end of the style
upon which the pollen grains fall, and
is generally knob-like and sticky.
• The style is the slender projection
of the ovary and bears stigma at
its terminal end. The surface of the
style may be smooth or covered
with hairs. In many cases, these
hairs collect the pollen grains.
• The swollen basal part of the carpel
which is single or many chambered, is termed ovary.
• The ovary contains one or more round or oval, egg-like
bodies, known as the ovules which are attached to the
placenta. Each ovule contains a large oval cell known
as the embryo sac. The ovary gives rise to the fruit and
ovules give rise to the seeds after fertilisation.
Structure of ovule
• An ovule or integumented megasporangium develops
from the base or the inner surface of the ovary. It is
small, generally an oval structure and consists chiefly of
a central body of tissue, the nucellus and one or two
integuments.
• Each ovule is attached to the placenta by a small stalk
called the funiculus. The ovule lacking the funicle is called
sessile.
• The place of attachment of the funiculus with the main
body of the ovule is called the hilum.
• In an inverted ovule, the funicle fuses with the main body
of the ovule, forming a sort of ridge, known as the raphe.
The upper end of the raphe which is the junction of the
integuments and the nucellus is called the chalaza.
• The nucellus, a mass of parenchymatous cells, is equivalent
to megasporangium.
• On the basis of quantity of nucellus, ovules are of two types:
tenuinucellate (nucellus is very thin) and crassinucellate
(nucellus is massive).
• Ovules having single integument are called unitegmic
ovule. It is common in Gamopetalae. Ovules having 2
integuments are called bitegmic ovule, e.g., members of
Polypetalae and monocots. When nucellus is not surrounded
by integuments, the ovule is called ategmic, e.g., Santalum
album, Loranthus.
Supernumerary integuments, i.e., more than two
integuments have also been reported in some taxa. In
Asphodelus and Trianthema, a third integument, called aril,
arises from the base of the ovule and it covers completely
the other two integuments. In many Euphorbiaceae, an
outgrowth arises from the tip of the outer integument
which turns backward and partially envelops the ovules.
This outgrowth is called caruncle.
• A small opening is left at the apex of the
integuments; this is called the micropyle.
When there are two integuments then
the inner integument is formed first
followed by the formation of the
outer integument.
• In the basal part of the
nucellus, in between the embryo
sac and vascular bundles, there
is a group of cells with lignified
and suberized walls, which is called