One or many ovules are present inside the ovary. Each ovule is attached to the placenta by a stalk known as funicle. The point of attachment of the funicle with the main body of the ovule is called hilum. Sometimes funicle gets fused with the body of the ovule along one side and forms a ridge known as raphe. The basal region of the ovule is known as chalaza.
Main body of an ovule is called nucellus (megasporangium) which consists of a mass of parenchymatous tissue.
In some ovules e.g., Zostera, a group of thickened cells is found in the chalazal region just below the embryosac, called hypostase. When a similar group of specialized cells is found in the nucellar region just above the embryo sac, it is called epistase.
Depending upon the development of nucellus, ovules are of two types-
(i) Crassinucellate. The nucellus is well developed e.g., Polypetalae.
(ii) Tenuinucellate. The nucellus is poorly developed e.g., Gamopetalae.
Structure of an anatropous ovule containing developed embryo sac (female gametophyte)
The nucellus is invested all around by one or two layered covering called integuments, except at the apex where a small passage is formed known as micropyle. On the basis of number of integuments, ovules are of following types-
(i) Unitegmic : Ovules with one integument e.g., members of gamopetalae and gymnosperms.
(ii) Bitegmic : Ovules with two integuments e.g., members of polypetalae and monocots.
(iii) Ategmic : Ovules are without integument e.g., Santalum, Loranthus (Parasites) and Liriosoma.
Third integument in the form of aril develops from base of ovule or funicle in many plants e.g., Litchi, Asohodelus and Inca dulce_ In litchi and Inga dulce aril is fleshy and edible.
In castor bean (Ricinus), proliferation of the outer integumentary cells at micropylar region is called caruncle or strophiole . It performs two functions :
(i) It acts as water absorbing pad.
(ii) It is made up of sugary substances, that sugar attract ants and helps in the seed dispersal by ants (myrmecochory).
The placental or funicular outgrowth present at the micropylar end is called obturator. It directs the passage of pollen-tube towards the ovule.