Megaspores are produced by meiosis in megaspore mother cells. Megaspores germinate into a female gametophyte, which produces female gametes or eggs. Megaspores are the larger of two spores produced by heterosporous plants.
Some pteridophytes, gymnosperms and angiosperms are heterosporous, i.e. they produce two kinds of spores. The larger megaspores and smaller microspores, which give rise to female and male gametophytes, respectively. Female gametophytes produce eggs, and male gametophytes produce sperms.
Megasporangium
Megaspores are produced inside megasporangium. In flowering plants or angiosperms, megasporangium is better known as ovules. Megaspore mother cells get differentiated in the nucellus region of ovules. The nucellus is surrounded by one or two protective layers, called integuments, except at the tip called the micropyle.
The mass of cells of the nucellus store food reserves. The functional megaspore produces a female gametophyte or an embryo sac in the nucellus.
Let us now see how megaspores are produced.
Megasporogenesis
The formation of megaspores is known as megasporogenesis. Megaspores are produced by meiosis or reduction division in the megaspore mother cell. A single cell of the nucellus differentiates as the megaspore mother cell or MMC at the micropylar region. The cell grows in size and contains dense cytoplasm and nucleus. It divides by meiosis to give rise to four haploid megaspores.
Generally, one of the four megaspores is functional, and it germinates into a female gametophyte or embryo sac. This is known as monosporic development. The nucleus of the functional megaspore undergoes mitotic divisions to produce a 7-celled, 8-nucleate embryo sac. The central cell of the embryo sac contains two polar nuclei.
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