question_answer 27)

  From the statements given below choose the option that are true for atypical female gametophyte of. (i) It is eight-nucleate and seven-celled at maturity. (ii) It is free-nuclear during the development. (iii) It is situated inside the integument, but outside the nucellus. (iv)It has an egg apparatus situated at the chalazal end. (a) (i) and (iv)                      (b) (ii) and (iii)                  (c) (i) and (ii)                       (d) (ii) and (iv)

Answer:

                  (c) The female gametophyte or embryo sac is located inside the nucellus, enclosed within the integuments. In a majority of flowering plants, one of the megaspore is functional while the other three degenerate. Three repeated mitotic divisions of the functional megaspore results in the formation of seven-celled or eight-nucleate embryo sac. Six of the eight nuclei are organized at the two poles. Three cells grouped at micro pylarend forms egg-apparatus and 3 at the chalazal end forms antipodal cells. The large central cell at the centre has two polar nuclei. The meiotic divisions in the formation of embryo sac are strictly free nuclear, that is nuclear divisions are not followed immediately by cell-wall formation. Gametophyte is situated at micropylar end not at chalazal end.