At what stage does cell wall formation occur during the development of the embryo sac?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → After the 8-nucleate stage
The embryo sac forms from the functional megaspore. Its nucleus divides by mitosis three times, giving the 2-nucleate, 4-nucleate and 8-nucleate stages. These divisions are free nuclear: the nuclei divide, but no cell walls form between them. Walls are laid down only after the 8-nucleate stage, and the 7-celled embryo sac is then organised.
Option (1) At 2-nucleate stage — Incorrect. At the 2-nucleate stage the first two nuclei have just moved to the opposite poles. The division is free nuclear, so no wall forms yet.
Option (2) At 4-nucleate stage — Incorrect. The 4-nucleate stage is also reached by a free nuclear division. Two more nuclei are still to come, and walls are not laid down at this stage.
Option (3) At 8-nucleate stage — Incorrect. The third mitotic division produces the 8 nuclei, but this division is free nuclear as well. Walls come after it, not during it.
Option (4) After the 8-nucleate stage — Correct. Once all eight nuclei are present, cell walls are laid down. Six nuclei are walled into cells (egg apparatus and antipodals), and the two polar nuclei stay in the large central cell.
NCERT Extract:
The Pistil, Megasporangium (Ovule) and Embryo Sac – “… Two more sequential mitotic nuclear divisions result in the formation of the 4-nucleate and later the 8-nucleate stages of the embryo sac. It is of interest to note that these mitotic divisions are strictly free nuclear, that is, nuclear divisions are not followed immediately by cell wall formation. After the 8-nucleate stage, cell walls are laid down leading to the organisation of the typical female gametophyte or embryo sac.”
Figure 1.8 (b) 2, 4, and 8-nucleate stages of embryo sac and a mature embryo sac