Question

Which of the following statements is/are correct?

1. Monosporic development is the method of embryo sac formation from a single megaspore.

2. The egg apparatus consists of two synergids and one egg cell.

1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2

Answer 1

Correct Answer - Option 3 : Both 1 and 2

The correct answer is Both 1 and 2.

Female gametophyte

• In a majority of flowering plants, one of the megaspores is functional while the other three degenerate.
• Only the functional megaspore develops into the female gametophyte (embryo sac).
• This method of embryo sac formation from a single megaspore is termed monosporic development. Hence statement 1 is correct.
• The nucleus of the functional megaspore divides mitotically to form two nuclei which move to the opposite poles, forming the 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.
• It is of interest to note that these mitotic divisions are strictly free nuclear, that is, nuclear divisions are not followed immediately by cell wall formation.
• After the 8-nucleate stage, cell walls are laid down leading to the organisation of the typical female gametophyte or embryo sac.
• Six of the eight nuclei are surrounded by cell walls and organised into cells; the remaining two nuclei, called polar nuclei are situated below the egg apparatus in the large central cell.
• There is a characteristic distribution of the cells within the embryo sac.
• Three cells are grouped together at the micropylar end and constitute the egg apparatus.
• The egg apparatus, in turn, consists of two synergids and one egg cell. Hence statement 2 is correct.
• The synergids have special cellular thickenings at the micropylar tip called filiform apparatus, which play an important role in guiding the pollen tubes into the synergid.
• Three cells are at the chalazal end and are called the antipodals.
• The large central cell, as mentioned earlier, has two polar nuclei.
• Thus, a typical angiosperm embryo sac, at maturity, though 8-nucleate is 7-celled