CHAPTER-1 SEXUAL REPRODUCATION IN FLOWERING PLANTS.
Q. 6 With a neat diagram explain the 7-celled, 8-nucleate nature of the
female gametophyte.
Answer:
i) The megaspore mother cell in flowering plants undergoes reduction
division to form 4 haploid megaspores.
ii) Out of these 4 megaspores, 3 degenerate and the remaining one
functional megaspore undergoes three successive mitotic divisions and
form eight nucleate embryo sac.
iii) First mitotic division –
• It occurs in the megaspores and results in the formation of 2 nuclei
• One nucleus moves towards the micopylar end.
• Second nucleus moves towards the chalazal end.
• This forms two-nucleate embryo-sac.
iv) Second Mitotic division –
• The nucleus at the micopylar end divides mitotically.
• The nucleus at the chalazal end divides mitotically.
• This results in 2 nuclei at each end and four-nucleate embryo-sac
forms.
v) Third mitotic division –
• The 2 nuclei at each end re-divide mitotically
• This results in 4 nuclei at each end and thus forms eight-nucleate
embryo-sac.
vi) Out 4 nuclei at the micopylar end, only 3 differentiate. Two of them
form synergids and one forms egg cell. Together they are known as egg
apparatus.
vii) Out of 4 nuclei at the chalaza end, only 3 differentiate as antipodal.
viii) The remaining one nucleus from the micopylar end and one from
the chalazal end move towards the center and are known as polar nuclei,
which are situated in a large central cell.
ix) This results in the formation of the 7-celled, 8-nucleate embryo-sac.