Megasporogenesis – Class 12 | Chapter – 2 | Biology Short Notes Series PDF

Megasporogenesis: Megasporogenesis is the process of creating haploid megaspores inside the megasporangium by meiotically dividing diploid megaspore mother cells (MMC) (ovule). To create the embryo sac, the haploid megaspore goes through several mitotic divisions through a process called megagametogenesis. Female reproductive organs are where megaspores develop. After fertilisation, the ovules become seeds, and the ovary becomes a fruit. The haploid male gamete, which is found inside the pollen grain, and the haploid female gamete, which is found inside the ovule, combine to form the diploid zygote during fertilisation.

• A layer of sporogenous tissue is present in the ovules of the female flowering plant.
• The megaspore mother cells (MMC) are developed from the sporogenous tissue. It is a large diploid cell with a prominent nucleus and dense cytoplasm.
• The MMC undergoes meiotic nuclear divisions to produce four haploid megaspores.
• The megaspores are arranged axially, called the linear tetrad.
• Out of the four, only one megaspore is functional and the rest three degenerate.
• The megaspore at the chalazal end is functional, and the three towards micropyle degenerate.
• The one functional megaspore divides mitotically to form the embryo sac, a process called megagametogenesis.
• It divides multiple times to produce eight haploid nuclei for the embryo sac, which is also called a megagametophyte.

Structure of Ovule or Megasporogenesis

• Gynoecium represents the female component of a flower. It may consist of one or a number of carpels.
• Each carpel represents a megasporophyll which has three parts, i.e., stigma, style and ovary.
• An ovary may contain one or many ovules, and each ovule encloses an embryo sac. Ovule or megasporangium develops from the inner wall of the ovary.
• As the ovule develops, it gets raised from the ovary wall by a short stalk called a funicle through which food and water are transported to the ovule.
• The body of the ovule fuses with the funicle in the region called the hilum, which represents the junction between the ovule and the funicle.
• The multilayered body of the ovule is called nucellus which is enclosed by one or two protective layers called integuments, except for a small pore at one end called the micropyle.
• Within the nucellus is present one megaspore mother cell (or embryo-sac mother cell).
• The other end of the ovule, i.e., opposite the micropylar end, where the funicle joins with the nucellus and integument, is called the chalaza or chalazal end. It represents the basal part of the ovule.

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By Team Learning Mantras