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Solution


Q.

In angiosperms, the correct sequence of events in formation of female gametophyte in the ovule is                    [2023]

(A) 3 successive free nuclear divisions of functional megaspore.
(B) Degeneration of 3 megaspores.
(C) Meiotic division in megaspore mother cell.
(D) Migration of 3 nuclei towards each pole.
(E) Formation of wall resulting in seven celled embryo sac.

Choose the correct answer from the options given below.
1 (A), (B), (C), (D), (E)  
2 (C), (E), (A), (D), (B)  
3 (B), (C), (A), (D), (E)  
4 (C), (B), (A), (D), (E)  

Ans.

(4)

The sequence of events in formation of female gametophyte in the ovule in an angiosperm is as follows:
(i) Megaspore mother cell or megasporocyte is a diploid (2n) cell which undergoes meiosis to form a linear tetrad of four haploid megaspores (n).

(ii) Normally, the chalazal megaspore is the functional megaspore while the other three degenerate. The functional megaspore is the first cell of female gametophyte.

(iii) The nucleus of the functional megaspore divides mitotically to form two nuclei which move to the opposite poles, forming 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. These mitotic divisions are strictly free nuclear, which means nuclear divisions are not followed immediately by cell wall formation.

(iv) Out of 8 nuclei, 4 nuclei are arranged at each pole. One nucleus from each pole migrates to the centre to form two polar nuclei. Cell walls are laid down leading to the organisation of the typical female gametophyte or embryo sac with 7-cells.