Essential Parts of Flower: Gynoecium - Megasporogenesis

Megasporogenesis:

• The process of development of a megaspore from a megaspore mother cell is called megasporogenesis.
• As the ovule develops, a single hypodermal cell in the nucellus becomes enlarged and functions as an archesporium. In some plants, the archesporial cell may directly function as a megaspore mother cell.
• The archesporial cell may undergo a transverse division to form outer primary parietal cell and the inner primary sporogenous cell.
• The parietal cell may remain undivided or divided by a few periclinal and anticlinal divisions to embed the primary sporogenous cell deep into the nucellus. The primary sporogenous cell functions as a megaspore mother cell. The megaspore mother cell (MMO) undergoes meiotic division to form four haploid megaspores.
• Based on the number of megaspores that develop into the Embryo sac, we have three basic types of development: monosporic, bisporic, and tetrasporic. The megaspores are usually arranged in a linear tetrad. Of the four megaspores formed, usually, the chalazal one is functional and the other three megaspores degenerate.

1. The functional megaspore forms the female gametophyte or embryo sac. This type of development is called monosporic development. Example: Polygonum
2. Of the four megaspores formed if two are involved in Embryo sac formation, the development is called bisporic. Example: Allium.
3. If all the four megaspores are involved in Embryo sac formation the development is called tetrasporic. Example: Peperomia

• An ovule generally has a single embryo sac.