Ovule Structure, Types & Embryo Notes for Botany Semester Students
1.
An ovule isthe female reproductive structure of seed plants that develops into a
seed after fertilization. It is located inside the ovary of the flower in angiosperms
and on the surface of megasporophylls in gymnosperms.
Ovule /Megasporangium
Gynoecium is the female reproductive organ.
The free unit of gynoecium is called pistil or carpel.
Structure of ovule
Thestalk is called funicle. One end of the funicle is attached to placenta and
the other end to the body of the ovule.
The point of attachment of funicle with the body is called hilum.
Funicle sometimes extends up to the base of ovule (i.e. chalaza). The ridge thus
formed is called raphe.
The body of the ovule shows two ends- the basal end, often called the chalaza
and the upper end is called the micropylar end.
The main body of the ovule is covered with one or two envelops called
integuments.
These leave an opening at the top of the ovule called micropyle.
The integuments enclose a large parenchymatous tissue known as nucellus.
In the centre of the nucellus is situated a female gametophyte known as
embryo sac.
Ovule structure
4.
Following are someof the conditions seen in ovule in relation to
integuments.
Unitegmic Ovule with a single integument; e.g. gamopetalous dicotyledons.
Bitegmic Ovule with two integuments as in polypetalous dicotyledons and
monocotyledons.
Aril This is a collar-like outgrowth from the base of the ovule and forms third
integument. Aril is found in litchi, nutmeg, etc.
Caruncle It is formed as an outgrowth of the outer integument in the
micropylar region. Caruncle is common in the ovules of Euphorbiaceae e.g.
castor.
Ategmic In some parasites like Loranthus, Viscum, Santalum, etc. there is no
integument, such an ovule is called ategmic.
Integument types
There are sixtypes of ovules in Angiosperms.
These are classified on the basis of mutual relationship between funicle,
chalaza and micropyle
Typeof ovule
1. Atropous or orthotropous : In this type, funicle, chalaza and micropyle
are situated on one vertical axis with micropyle directed upwards.
Hence it is also known as straight ovule; e.g. Polygonum, Piper etc.
2. Anatropous : These ovules are also called inverted (180° curved)
because the body of the ovule is so bent that micropyle is now directed
downwards and lies close to the hilum. It is the most common type of
ovule found in angiosperms, e.g., castor.
3. Campylotropous : This is an ovule placed transversely on the funicle
(90° curved). The body is bent in such a way that micropyle gets
directed downwards and is not in straight line with chalaza. It is found in
gram, bean, members of Cruciferae like mustard etc.
9.
4. Hemianatropous :In this type, the body of the ovule is situated at right
angles to the funicle e.g. Ranunculus.
5. Amphitropous : The ovule is curved (at 180°) and embryo sac also becomes
horse shoe-shaped. The micropyle is directed downwards and does not lie on
straight line with chalaza. Amphitropous ovules occur in poppy, members of
Alismaceae, Butomaceae, etc.
6. Circinotropous : The ovule is initially orthotropous but becomes anatropous
due to unilateral growth of funicle. The growth continues till the ovule once
again becomes orthotropous. As a result funicle completely surrounds the
body of the ovule; e.g. Opuntia (prickly pear).
