Development of female gametophyte
The carpels of angiosperm is known as megasporophyll. It is differentiated into three regions-ovary, style and stigma. The ovary contains ovules or megasporangium.
An ovule or megasporangium may arise from the inner surface of the base of an ovary. Each ovule is attached to the placenta by a stalk called funicle. The point of attachment of the ovule to the funicle is known as hilum. The funicle continues beyond the hilum along the body of the ovule and forms a ridge called raphe.
The body of the ovule consists of a parenchymatous mass of tissue called nucellus. The nucellus is surrounded by one or two coverings called integuments. The integuments do not completely cover the nucellus, but leaves a small opening at the tip called micropyle.
Usually a single hypodermal initial known as primary archesporial cell is differentiated at the apex of the nucellus. The primary archesporial cell divides periclinally into outer primary parietal cell or primary wall cell and inner primary sporogenous cell.
The primary parietal cell may or may not divide. The primary sporogenous cell directly behave as megaspore mother cell. The megaspore mother cell undergoes meiotic division to form four megaspores. The four megaspores thus formed are arranged in an axial row forming a linear tetrad. Usually only one megaspore of the tetrad is functional and grows at the expense of other three, which degenerate. The functional megaspore enlarges and forms the embryosac.
The embryosac has three protoplasts of the egg-apparatus towards the micropylar end. Of the three cells constituting the egg-apparatus, one is the egg cell (female gamete) and the other two are known as the synergids. The egg cell, which is enlarged lies below the synergids. At the chalazal end are three antipodal cells. These antipodal cells have no definite function and soon getsdisorganized. In the centre of the embryosac is the secondary nucleus.