Question

With a neat diagram explain the 7-celled, 8-nucleate nature of the female gametophyte
 

Answer 1

The female gametophyte(embryo sac)develops from a single functional megaspore.
This megaspore undergoes three successive miotic divisions to form eight nucleate
embryo sacs.
The first mitotic division in the megaspore forms two nuclei.One nucleus moves
towards the micropylar end while the other nucleus moves towards the chalazal end.
Then,these nuclei divide at their respective ends and re-divide to form eight
nucleate stages.As a result,there are four nuclei each at both the ends i.e.,at the
mycropylar and the chalazal end in the embryo sac.At the mycropylar end,out of the
four nuclei only three differentiate into synergids and one egg cell.Together they
are known as the egg apparatus.Similarly,at the chalazal end,three out of four
nuclei differentiates as antipodal cells.The remaining two cells(of the mycropylar and
the chalazal end)move towards the centre and are known as the polar nuclei,which
are situated in a large central cell.Hence,at maturity,the female gametophyte
appears as a 7-celled structure,thought it has 8 nucleate.