The ovule is a part of a plant which also refers to as “Megasporangium” that forms megaspores which finally develops into a seed after the process of fertilization. The megasporangium is present inside the base of the ovary which extends and give rise to the stalk-like structure called “Style” and finally opens into the Stigma.
The megasporangium starts to swell and develops into a seed after the fertilization and the ovary that surrounds the ovule develops into a fruit.
The number of ovules can differ in different plants. As the number of the ovule in the ovary can be one like in Avocado, mango etc. and can be more than one like kiwi, apple etc.
Definition of Ovule
The ovule also refers as “Megasporangium” that forms megasporocytes which then yields megaspores. Therefore megasporangium can define as a part of female reproductive part or female gametophyte which refers as “Gynoecium”, which converts into a seed after the fertilization.
The megasporangium is a structure that is present inside an ovary of the flower and is found attached to the parenchymatous cushion which is known as “Placenta” that provides nourishment to the ovary.
The anatomy of ovule mainly comprises of a stalk and the main body. Let us study the detailed structure of an ovule.
The stalk consists of the following elements:
The stalk-like structure in the megasporangium which arises through the placenta refers to as “Funiculus”.
Inside the funiculus, the vascular strand is present which supplies nutrients from the placenta to the megasporangium.
“Hilum” acts as the junction element which connects the stalk (funicle) with the main body.
The main body comprises the following elements:
The raphe is an outgrowth of the funicle which is present beyond the hilum and is ridge-like.
The major part of the megasporangium or ovule is composed of nucellus tissue which also refers as “Megasporium proper”. Nucellus tissue is a type of parenchymatous tissue whose quantity differs in different plants. Based on the quantity of nucellus tissue, there are two types of an ovule.
- Crassinucleate ovule: This consists of a massive nucleus.
- Tenuinucleate ovule: This consists of the less developed nucleus.
Sometimes nucellus comes out of the micropyle is known nucellus beak that occurs in the members belongs to Euphorbiaceae, Polygonaceae etc. family.
Integuments acts as “Protective layer or envelope” which surrounds the nucellus. Based on the number of integuments, ovule can be of the following types:
- Unitegmic: It contains a single integument. In this type, the nucellus disintegrates early due to which the cells of single integument modified to give nutrition to the megasporangium which refers as “Integumentary tapetum”. The unitegmic megasporangium occurs in the members of the Compositae family.
- Bitegmic: It contains two integuments namely inner and outer. The inner integument forms first then the second integument forms. The bitegmic megasporangium occurs in monocots and primitive dicots.
- Ategmic: It contains no integument and occurs in the species which belong to the genus of Santalam, Lorenthus and Olax etc.
- Aril: It consists of the third integument. The third integument arises from the base of the megasporangium and occurs in the species of litchi, pithcoelobium etc.
The micropyle is a region which lacks integument through which the pollen tube enters the megasporangium. Obturator forms by the placenta and funiculus is an ovular structure that gives chemical guidance to the polar tube so that it can enter into the megasporangium through the micropyle.
The embryo sac is the female gametophyte that is embedded in the nucleus and forms by functional megaspore.
A typical embryo sac is seven-celled and contains eight nuclei. It is the part where the fertilization occurs after which a seed forms.
Types of Ovule
The ovule is of mainly six classes based on their position inside the ovary, which are as follows:
Orthotropous Ovule: It also refers as “Atropous ovule” which is erect or straight in a position where the hilum, chalaza and micropyle lie in the same alignment.
Example: Polygonum sp.
Anatropous Ovule: It is just opposite to the orthotropous ovule. In anatropous type, the whole body of the megasporangium is inverted in position at an angle of 180 degrees due to which the micropyle lies very close to hilum.
Example: Gamopetalae members
Hemitropous Ovule: Hemianatropous type is transverse in position at an angle of 90 degrees due to which the micropyle and chalaza lie in the same alignment.
Example: Ranunculus sp.
Campylotropous Ovule: In this type, the main body of a megasporangium is curved in position. Due to this curvature, the micropyle and the chalaza do not lie in the same alignment.
Example: Leguminosae members.
Amphitropous Ovule: It is a horseshoe-shaped, where the embryo sac becomes curved.
Example: Allismaceae, Butomaceae sp.
Circinotropous Ovule: In this type, first the main body is at an angle of 180 degrees, but then it moves at an angle of 360 degrees, where the micropyle again points upward.
Example: Opuntia sp.
Therefore after the fertilization, the megasporangium converts into seed, the ovary converts into fruit and the wall of ovary develops into a fruit wall or pericarp.