Question

What is Megasporogenesis? Describe the development of zygote up to formation of mature ovule in an Angiospermic flower.

Hint:

Megasporogenesis = MMC (2n) → meiosis → 4 megaspores → 1 functional megaspore → embryo sac (7-celled, 8-nucleate).

Answer 1

Megasporogenesis: It is the process of formation of haploid megaspores from a diploid megaspore mother cell (MMC) in the ovule of flowering plants (angiosperms).

Step 1: Formation of Megaspore Mother Cell (MMC). \[\begin{array}{rl} \bullet & \text{In the ovule, a single hypodermal cell differentiates into an MMC (2n).} \\ \bullet & \text{The MMC is large with dense cytoplasm and a prominent nucleus.} \\ \end{array}\]

Step 2: Meiosis in MMC. \[\begin{array}{rl} \bullet & \text{The MMC undergoes meiosis to form a linear tetrad of four haploid megaspores.} \\ \bullet & \text{Usually, three megaspores degenerate and one functional megaspore survives.} \\ \end{array}\]

Step 3: Development of Female Gametophyte (Embryo Sac). \[\begin{array}{rl} \bullet & \text{The functional megaspore undergoes three mitotic divisions → 8 nuclei.} \\ \bullet & \text{Arrangement: 3 cells at micropylar end (egg apparatus: 1 egg + 2 synergids), 3 cells at chalazal end (antipodals), and 2 polar nuclei in the center (fuse to form secondary nucleus).} \\ \end{array}\] Final Structure: The mature ovule contains a 7-celled, 8-nucleate female gametophyte (embryo sac).