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Question
Read the following statements and find out the incorrect statements.
a. Ovules generally differentiate as a single megaspore mother cell (MMC) in the chalazal region of the nucellus.
b. The MMC undergoes reduction division and produces four megaspores.
c. In a majority of angiosperms, one of the megaspores is degenerated while the other three remain functional.
d. The nucleus of the functional megaspore divides mitotically three times and forms 2-nucleate, 4-nucleate and later 8-nucleate stages of the embryo sac.
e. These mitotic divisions are strictly free nuclear, that is, nuclear divisions are immediately followed by cell wall formation.
Read the following statements and find out the incorrect statements.
a. Ovules generally differentiate as a single megaspore mother cell (MMC) in the chalazal region of the nucellus.
b. The MMC undergoes reduction division and produces four megaspores.
c. In a majority of angiosperms, one of the megaspores is degenerated while the other three remain functional.
d. The nucleus of the functional megaspore divides mitotically three times and forms 2-nucleate, 4-nucleate and later 8-nucleate stages of the embryo sac.
e. These mitotic divisions are strictly free nuclear, that is, nuclear divisions are immediately followed by cell wall formation.
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Solution
The correct option is A b, c and d
Megaspore mother cell
Initially a hypodermal cell in the micropylar region of the nucellus differentiates as an archesporial cell. The archesporial cell may directly function as a megaspore mother cell (MMC) or divide to form an outer parietal cell and inner sporogenous cell. The primary parietal cell divides mitotically to form parietal tissue. The sporogenous cell acts as a megaspore mother cell. Thus, statement a is incorrect.
Reductional division in MMC
Megaspore mother cells divide by meiosis or reduction division to produce a tetrad of four megaspores.
Functional megaspore formation
Out of the four megaspores formed, usually three megaspores degenerate and disappear while one megaspore at the chalazal end remains functional. Thus, statement c is incorrect.
Embryo sac formation
1) The haploid nucleus of the functional megaspore divides mitotically to form two nuclei which move to the opposite poles to form a 2−nucleate embryo sac.
2) Two more mitotic nuclear divisions result in the formation of a 4−nucleate embryo sac followed by a final 8−nucleate embryo sac. It consists of 1 egg cell, 2 synergids, 3 antipodal cells and 1 central cell with 2 polar nuclei.
Nuclear divisions in the embryo sac
During megagametogenesis (formation of an embryo sac from the megaspore), the nuclear divisions (karyokinesis) are not followed immediately by cell-wall formation. Thus, the mitotic nuclear divisions are strictly free nuclear. Hence, statement e is incorrect.
Final answer
(D) a, c and e
Megaspore mother cell
Initially a hypodermal cell in the micropylar region of the nucellus differentiates as an archesporial cell. The archesporial cell may directly function as a megaspore mother cell (MMC) or divide to form an outer parietal cell and inner sporogenous cell. The primary parietal cell divides mitotically to form parietal tissue. The sporogenous cell acts as a megaspore mother cell. Thus, statement a is incorrect.
Reductional division in MMC
Megaspore mother cells divide by meiosis or reduction division to produce a tetrad of four megaspores.
Functional megaspore formation
Out of the four megaspores formed, usually three megaspores degenerate and disappear while one megaspore at the chalazal end remains functional. Thus, statement c is incorrect.
Embryo sac formation
1) The haploid nucleus of the functional megaspore divides mitotically to form two nuclei which move to the opposite poles to form a 2−nucleate embryo sac.
2) Two more mitotic nuclear divisions result in the formation of a 4−nucleate embryo sac followed by a final 8−nucleate embryo sac. It consists of 1 egg cell, 2 synergids, 3 antipodal cells and 1 central cell with 2 polar nuclei.
Nuclear divisions in the embryo sac
During megagametogenesis (formation of an embryo sac from the megaspore), the nuclear divisions (karyokinesis) are not followed immediately by cell-wall formation. Thus, the mitotic nuclear divisions are strictly free nuclear. Hence, statement e is incorrect.
Final answer
(D) a, c and e
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