Question

In angiosperms, the female gametophyte develops from a single cell of megasporangium.

(i) Name that cell of megasporangium and describe the sequential changes it undergoes to form a mature female gametophyte.

(ii) Draw a well labelled diagram of mature female gametophyte. Label any four parts.

Hint:

The megaspore mother cell makes four megaspores by meiosis. One survives and divides three times to form a 7-celled, 8-nucleate embryo sac.

Answer 1

Step 1: Understanding the Question:
The female gametophyte of an angiosperm is the embryo sac. It forms inside the ovule, in the nucellus (megasporangium).
Part (i) asks for the starting cell and the steps. Part (ii) asks for a labelled diagram.

Step 2: Part (i): the cell:
The cell is the megaspore mother cell (MMC), also called the megasporocyte.
It is a large diploid cell (\( 2n \)) near the micropylar end of the nucellus.

Step 3: Part (i): meiosis and the functional megaspore:
The MMC divides by meiosis. This gives a linear tetrad of four haploid megaspores (\( n \)).
Three megaspores degenerate. Usually the one at the chalazal end stays alive and is the functional megaspore.

Step 4: Part (i): free nuclear divisions:
The functional megaspore grows and its nucleus divides by mitosis. The two nuclei move to opposite poles.
Each nucleus divides twice more by mitosis. Now there are four nuclei at each pole, so eight nuclei in all. This is the 8-nucleate stage.

Step 5: Part (i): cell wall formation:
Cell walls form. Three cells at the micropylar end make the egg apparatus: one egg cell and two synergids.
Three cells at the chalazal end become antipodals.
One nucleus from each pole moves to the centre. These two polar nuclei lie in the large central cell.
So the mature embryo sac is 7-celled and 8-nucleate.

Step 6: Part (ii): the diagram:
A neat labelled sketch is expected. It is an oval embryo sac, with the micropylar end up and the chalazal end down. Label any four of these:
1. Egg cell and two synergids at the micropylar end, together forming the egg apparatus.
2. Filiform apparatus on the synergids, which guides the pollen tube.
3. Central cell with the two polar nuclei (they fuse to form the secondary nucleus).
4. Three antipodal cells at the chalazal end.

Final Answer:
The starting cell is the megaspore mother cell. It gives four megaspores by meiosis, one survives, and three mitoses make an 8-nucleate, 7-celled embryo sac with a labelled diagram as described.