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Megagametogenesis is the process of maturation of the female gametophyte, or megagametophyte, in plants. [1] During the process of megagametogenesis, the megaspore, which arises from megasporogenesis, develops into the embryonic sac, in which the female gamete is housed. [2] These megaspores then develop into the haploid female gametophytes. [2]
In the process of double fertilization, two sperm nuclei from a pollen grain (the microgametophyte), rather than a single sperm, enter the archegonium of the megagametophyte; one fuses with the egg nucleus to form the zygote, the other fuses with two other nuclei of the gametophyte to form 'endosperm', which nourishes the developing embryo.
Two distinct processes are involved in producing the megagametophyte from the megaspore mother cell: Megasporogenesis, formation of the megaspores in the megasporangium (nucellus) by meiosis Megagametogenesis; development of the megaspore(s) into the megagametophyte(s) which contains the gametes .
The egg-producing gametophyte is known as a megagametophyte, because it is typically larger, and the sperm producing gametophyte is known as a microgametophyte. Species which produce egg and sperm on separate gametophytes plants are termed dioicous , while those that produce both eggs and sperm on the same gametophyte are termed monoicous .
The female gametophyte, the megagametophyte, that participates in double fertilization in angiosperms which is haploid is called the embryonic sac. This develops within an ovule, enclosed by the ovary at the base of a carpel. Surrounding the megagametophyte are (one or) two integuments, which form an opening called the micropyle.
One cell is responsible for drilling down through the integuments, and creating a conduit for the two sperm cells to flow down. The megagametophyte has just seven cells; of these, one fuses with a sperm cell, forming the nucleus of the egg itself, and another joins with the other sperm, and dedicates itself to forming a nutrient-rich endosperm ...
The microspores of both exosporic and endosporic species are free-sporing, distributed by wind, water or animal vectors, but in endosporic species the megaspores and the megagametophyte contained within are retained and nurtured by the sporophyte phase. Endosporic species are thus usually dioecious, a condition that promotes outcrossing.
The female gametophyte — specifically termed a megagametophyte — is also called the embryo sac in angiosperms. The megagametophyte produces an egg cell for the purpose of fertilization. The ovule is a small structure present in the ovary. It is attached to the placenta by a stalk called a funicle. The funicle provides nourishment to the ovule.