Megasporangium
Gynoecium: The gynoecium represents the female reproductive part of the flower.
Monocarpellary: When the gynoecium consists of a single pitil, it is called monocarpellary.
Multicarpellary: When the gynoecium consists of more than one pistil, it is called multicarpellary.
Syncarpous: When there are more than one pistil, and they are fused together, it is called syncarpous condition.
Apocarpous: When the multiple pistils are free, it is called apocarpous condition.
Each pistil has three parts, viz. stigma, style and ovary. The topmost flat portion of the pistil is called stigma. Stigma serves as the landing platform for pollen grains. The elongated slender part of the pistil is called style. The basal swollen part of the pistil is called ovary.
The Megasporangium (Ovule)
The ovule is a small structure attached to the placenta by means of a stalk called funicle. The body of the ovule fuses with funicle in the region called hilum. Thus, hilum represents the junction between ovule and funicle. Each ovule has one or two protective envelopes called integuments. Integuments encircle the ovule except at the tip where a small opening called the micropyle is present. Opposite the micropylar end, is the chalaza. The chalaza represents the basal part of the ovule.
A mass of cells; called nucellus; is enclosed within the integuments. The cells of nucellus have abundant reserve food material. Embryo sac or female gametophyte is located in the nucellus. An ovule generally has a single embryo sac. The embryo sac is formed from a megaspore through meiosis.
Megasporogenesis
The process of formation of megaspores from the megaspore mother cell is called megasporogenesis. Ovules generally differentiate a single megaspore mother cell (MMC) in the micropylar region of the nucellus. MMC a large cell containing dense cytoplasm and a prominent nucleus. The MMC undergoes meiotic division which results in formation of four megaspores.
Female gametophyte: In a majority of flowering plants, one of the megaspores is functional while the other three degenerate. Only the functional megaspore develops into the female gametophyte (embryo sac). This method of embryo sac formation from a single megaspore is termed monosporic development.
Formation of Embryo Sac
The nucleus of the functional megaspore undergoes meiosis to form 2 nuclei. These nuclei move to the opposite poles; to form 2-nulceate embryo sac. This is followed by two more sequential mitotic nuclear divisions. These divisions result in formation of the 4-nucleate and later 8-nulceate stages of embryo sac. Cell walls are formed after the 8-nucleate stage. Six of the eight nuclei are surrounded by cell walls. Remaining two nuclei; called polar nuclei; are situated below the egg apparatus in the large central cell.
Egg Apparatus: Three cells which are grouped together at the micropylar end make the egg apparatus. The egg apparatus consists of two synergids and one egg cell. The synergids have special cellular thickenings at the micropylar tip; called filiform apparatus. The cellular thickenings play important role in guiding the pollen tubes into the synergid.
Antipodals: Three cells at the chalazal end are called antipodals.
A typical angiosperm embryo sac, at maturity, though 8-nucleate is 7-celled.