Megasporogenesis- Write short notes.
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Explanation:
Megasporogenesis
Megasporogenesis is the process of formation of megaspores from the megaspore mother cell. In the hypodermal region of nucellus towards the micropylar end develops a primary archesporial cell. It is distinguished from the other cells by its dense cytoplasm and a prominent nucleus.The archesporial initial either acts directly as a megaspore mother cell or divides periclinally into an outer primary parietal cell and the inner primary sporogenous cell which later functions as megaspore mother cell.
In the former case, there is no nucellar tissue between the megaspore mother cell and nucellar epidermis. Such ovules are called tenuinucellate and are characteristics of gamopetalous and dicotyledons with unitegmic ovules.
In the latter case where a primary parietal cell and a primary sporogenous cell is formed by archesporial initial, the parietal cell divides repeatedly as a result of which sporogenous cell becomes deep seated in the nucellar tissue. Such ovules are known as crassinucellate and are formed in polypetalous dicotyledons with bitegmic ovules.
The megaspore mother cell divides meiotically to form four haploid megaspores. These are arranged in a longitudinal row (linear tetrads). Occasionally, megaspore cells arrange themselves in T-shaped or inverted T-shaped tetrads. Megaspore is the first cell of the female gametophyte.
Among the linear tetrads, three megaspores towards the micropylar end degenerate. The lower most, i.e., the chalazal megaspore enlarges and remains functional. Its nucleus undergoes three divisions to produce eight nuclei. The chalazal megaspore later produces an embryo sac.