With a suitable diagram explain the structure of an ovule.

Explanation

(i) Ovule is also called megasporangium and is protected by one or two covering called integuments.
(ii) A mature ovule consists of a stalk and a body. The stalk or the funiculus is present at the base and it attaches the ovule to the placenta.
(iii) The point of attachment of funicle to the body of the ovule is known as hilum.
(iv) In an inverted ovule, the funicle is adnate to the body of the ovule forming a ridge called raphe.

(v) The body of the ovule is made up of a central mass of parenchymatous tissue called nucellus which has large reserve food materials
(vi) The nucellus is enveloped by one or two protective coverings called integuments.
(vii) Integument encloses the nucellus completey except at the top where it is free and froms a pore called micropyle.
(viii) The ovule with one or two integuments are said to be unitegmic or bitegmic ovules respectively.
(ix) the basal region of the body of the ovule where the nucellus, the integument and the funicle meet or marge is called as chalaza.
There is a large, oval, sac-like structure in the nucellus toward the micropylar end called embryo sac or female gametophyte.
(xi) It develops from the functional megaspore formed within the nucellus.
(xii) In some species (unitegmic tenuinucellate) the inner layer of the integument may become specialized to perfom the nutritive function for the embryo sac and is called as endothelium or integumentary tapetum
Example : Asteracease.
(xiii) There are two types of ovule based on the position of the sporogenous cell.
(a) If the sporogenous cell is hypodermal with a single layer of nucellar tisuue around it is called tenuincuellate type. Normallytenuicellate ovules have very small nucellus.
(b) Ovules with subhypodermal sporogenous cell is called crassinucellate type. These ovules have fairly large nucellus.
(xiv) Group of cells found at the base of the ovule between the chalaza and embryo sac is called hypostage and the thick-walled cells found above the micropylar end above the embryo sac is called epistase.