Sequence the following steps in formation of female gametophyte of flowering plants
A. Cell walls are laid down.
B. Formation of seven cells with eight nuclei.
C. Meiosis in megaspore mother cell and formation of megaspore tetrad.
D. Functional megaspore undergoes three mitotic divisions, results in formation of eight nuclei.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → C, D, A, B
C. Meiosis in megaspore mother cell and formation of megaspore tetrad. — first. The megaspore mother cell divides by meiosis into four megaspores; three degenerate.
D. Functional megaspore undergoes three mitotic divisions, results in formation of eight nuclei. — second. The divisions are free nuclear: no walls form, giving 2, 4 and then 8 nuclei.
A. Cell walls are laid down. — third. Only after the 8-nucleate stage do walls form, around six of the nuclei.
B. Formation of seven cells with eight nuclei. — last. The mature embryo sac has 3 antipodals, the egg apparatus (2 synergids + egg) and a central cell with 2 polar nuclei.
NCERT Extract:
The Pistil, Megasporangium (ovule) and Embryo Sac – “… The nucleus of the functional megaspore divides mitotically to form two nuclei which move to the opposite poles, forming the 2- nucleate 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. Observe the distribution of cells inside the embryo sac (Figure 1.8b, c). Six of the eight nuclei are surrounded by cell walls and organised into cells; the remaining two nuclei, called polar nuclei are situated below the egg apparatus in the large central cell.”
The Pistil, Megasporangium (ovule) and Embryo Sac – “… Thus, a typical angiosperm embryo sac, at maturity, though 8-nucleate is 7-celled.”