2. OVERVIEW OF A TOPIC
DEFINITION
LOCATION
STRUCTUREOF OVUM
3. DEFINITION
The ovule is a component of the female reproductive system of seed plants.
It is where female reproductive parts are produced and maintained.
Ovules are housed in ovaries at the base of the carpel, which includes an aperture
at the top, stigma, and a neck called a style.
After fertilisation, these cells eventually develop into a seed, which subsequently
ripens to form a full-grown adult plant.
Ovule is also known as the megasporangium in flowering plants.
4. LOCATION
The ovule is found inside the
gynoecium, a part of the flower
present in angiosperms or flowering
plants.
The fruit wall eventually develops
from the ovary of the gynoecium,
which produces more than one ovule.
A funiculus structure, resembling a
stalk, connects the ovules to the
placenta. Plant species exhibit various
ovule attachment or placentation
patterns, including:
5. The ovule is composed of the integuments
forming the outermost layer, the nucellus, and
the female gametophyte (known as an embryo
sac in angiosperms), found in the core.
STRUCTURE OF OVUM
6. Integuments:
A layer of protection surrounds the
ovule called an integument.
Once the ovule has undergone
fertilisation, the integuments
transform into the seed coat.
Gymnosperms usually have one
integument (unitegmic),
whereas angiosperms typically
possess two integuments (bitegmic).
7. The chalaza:
• where the nucellus and
integuments are
connected, is located
across from the
micropyle.
• The integuments do not
entirely encase the
nucellus; a small gap
called the micropyle
remains at the apex.
• A male gametophyte,
pollen, can enter the
ovule through the
micropyle opening and
fertilise it.
8. Nucellus:
The most significant component of the
ovule is the nucellus. It contains nutritive
tissue and the embryo sac, and in some
flowering plants, it is still present after
fertilisation as a source of nutrition for
the embryo.
The nucellus, a component of the ovule’s
inner structure, forms a layer of
sporophytic or diploid cells near the
integuments. Nucellar embryony, an
asexual reproduction process that occurs
in some plants, allows the diploid
nucellus tissue to give birth to the
embryo inside the seed. The perisperm
nourishing the embryo may grow from
the nucellus after fertilisation.
9. What are Antipodals and Synergids?
During fertilization process, a pollen tube grows into one of the synergid cells,
ceases growth, ruptures and releases its two sperm cells into this cell. Generally,
antipodal cells are present in the chalazal end.
Antipodals are nutritive in function and they nourishes the embryo sac.
10. STRUCTURE OF OVUM
WHAT IS RAPHE ?
Raphe is an elongated mass of
tissue, containing vascular
bundle, and lying on the side
of an anatropous ovule,
between the chalaza and the
attachment to the placenta.
Hilum is the scar on the
Testa where it was attached
to the funicle. Chalaza is
the opposite side of the side
bearing micropyle of an
ovule.
WHAT IS HILUM?
These are scars on the seed
coat which marks the place
at which seeds get fastened
to the tissues of the
ovaries. These regulate the
dynamics between the embryo
and the external
surroundings.
They form a potential route
for the invasion of
pathogens in seeds.
The permeability of seeds
is based on various
aspects, such as the cutin.
WHAT IS MICROPYLE?
Small pores are seen at one
terminal of the hilum of
seeds. It is from here that
the pollen tube gets into
the ovaries at the time of
fertilisation.
At the time of germination
of seeds, water passes in
the seed through the
micropyle, and it serves as
the path for invasion of
pathogens in seeds, as seen
in Hilum.
11. FUNCTIONS OF OVUM
The ovule is an essential component in sexual reproduction. A pollen grain
develops a pollen tube through the style after contacting the stigma of a flower of
the same species. This tube subsequently reaches the ovary and travels to the
ovule. After that, fertilisation can occur as the pollen grain’s nucleus travels down
the tube to combine with the nucleus inside the embryo sac.
It should be noted that pollen, which includes the male gametophytes, serves as
the male equivalent of the embryo sac.
