Question 1

What is the role of the megaspore mother cell in an ovule?

Accepted Answer

The megaspore mother cell is a diploid cell differentiated in the micropylar region of the nucellus. It undergoes meiosis to produce four haploid megaspores arranged as a linear tetrad.

Question 2

Which megaspore usually develops into the embryo sac?

Accepted Answer

The functional megaspore is typically the chalazal-most megaspore. The other three megaspores degenerate, while the functional one undergoes three successive mitotic nuclear divisions.

Question 3

How do the 2-nucleate, 4-nucleate and 8-nucleate stages arise?

Accepted Answer

They arise from the first, second and third successive mitotic divisions of the functional megaspore nucleus. The resulting nuclei remain free initially, before wall formation and cellularisation produce the organised embryo sac.

Question 4

What makes up the egg apparatus?

Accepted Answer

The egg apparatus consists of one egg cell and two synergids at the micropylar end. The egg lies between the synergids, and both synergids bear a filiform apparatus near their micropylar walls.

Question 5

Why is the central cell important in the embryo sac?

Accepted Answer

The large central cell occupies the middle of the embryo sac and initially contains two distinct haploid polar nuclei. These polar nuclei may fuse to form a diploid secondary nucleus during embryo-sac maturation.

Embryo Sac 3D — Female Gametophyte Development

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