Megasporogenesis → embryo sac (step by step)गुरुबीजाणुजनन → भ्रूणकोश (चरणबद्ध)
NCERT Fig 1.8a, b · Class 12 Biology · Chapter 1 · interactive diagram
Parts and their functions
- Nucellus (बीजांडकाय) — Tissue of the ovule in which the megaspore mother cell differentiates (micropylar region).
- Megaspore mother cell (MMC) (गुरुबीजाणु मातृ कोशिका) — A single, large cell with dense cytoplasm and a prominent nucleus in the micropylar region of the nucellus; diploid (2n).
- Dyad (द्विक) — Two cells formed after the first meiotic division of the MMC.
- Functional megaspore (क्रियाशील गुरुबीजाणु) — MMC undergoes meiosis → 4 megaspores (tetrad). In most flowering plants only ONE is functional; it forms the embryo sac. Formation of the embryo sac from a single megaspore = monosporic development.
- Degenerating megaspores (अपह्रासित गुरुबीजाणु) — The other three megaspores degenerate.
- Free-nuclear divisions (मुक्त केंद्रकी विभाजन) — The nucleus of the functional megaspore divides mitotically: 1 → 2 (one moves to each pole) → 4 → 8 nuclei. These are free-nuclear divisions — nuclear divisions NOT followed immediately by cell wall formation.
- Mature embryo sac (परिपक्व भ्रूणकोश) — After 8 nuclei form, cell walls are laid down: 7-celled, 8-nucleate embryo sac (female gametophyte).
- Antipodals (3) (प्रतिमुख कोशिकाएँ) — Three cells at the chalazal end.
- Polar nuclei (2) (ध्रुवीय केंद्रक) — The remaining two nuclei, in the large central cell.
- Egg apparatus (अंड उपकरण) — 2 synergids + 1 egg cell at the micropylar end.
Steps
- A megaspore mother cell (MMC, 2n) forms in the micropylar region of the nucellus.
- Meiosis I → dyad.
- Meiosis II → tetrad of 4 megaspores (n). Only one is functional; three degenerate.
- Mitosis 1: functional megaspore nucleus → 2 nuclei, one moves to each pole (2-nucleate embryo sac).
- Mitosis 2: 4-nucleate stage (free-nuclear).
- Mitosis 3: 8-nucleate stage — 4 nuclei at each end.
- Walls form: 7-celled, 8-nucleate mature embryo sac (3 antipodals, central cell with 2 polar nuclei, egg apparatus).
Original diagram drawn by Simple Learning Hub, based on NCERT Fig 1.8a, b. Report a mistake