Q.Draw the diagram of a mature embryo sac and show its 8-nucleate, 7-celled nature. Show the following parts: antipodals, synergids, egg, central cell, polar nuclei.

The male gametophyte in flowering plants is the pollen grain, which carries the male gametes. The female gametophyte, also known as the embryo sac, is found within the ovule and contains the egg cell.

A mature embryo sac is typically 8-nucleate and 7-celled. Its structure is organised as follows:

• At the micropylar end, there is one egg cell flanked by two synergids. The synergids possess a filiform apparatus at their micropylar tip, which guides pollen tube entry.
• At the chalazal end, there are three antipodal cells.
• In the centre, there is a large central cell, which contains two polar nuclei.

The diagram of a mature embryo sac should depict an oval-shaped structure with:

• The egg cell and two synergids positioned together at one end (micropylar end). …

In flowering plants, sexual reproduction involves the fusion of male and female gametes. The female gametophyte, also known as the embryo sac, is a crucial structure where the egg cell is housed and fertilisation takes place. It develops within the ovule, which is located inside the ovary. Understanding its structure is fundamental to comprehending the process of fertilisation and subsequent seed development.

The development of the embryo sac, a process called megagametogenesis, typically begins from a single functional megaspore. This megaspore, which is haploid (n), undergoes three successive free nuclear mitotic divisions. "Free nuclear" means that the nuclear divisions occur without immediate cytokinesis (cell wall formation).

These divisions proceed as follows:

• The first mitotic division of the megaspore nucleus produces two nuclei. These two nuclei move to opposite poles of the embryo sac.
• The second mitotic division occurs in both nuclei, resulting in four nuclei (two at each pole).
• The third mitotic division again occurs in all four nuclei, leading to a total of eight nuclei (four at each pole).

Following these three mitotic divisions, cell walls are laid down, leading to the organisation of the embryo sac into a 7-celled structure, even though it contains 8 nuclei. This specific arrangement is characteristic of the Polygonum type of embryo sac, which is the most common type found in angiosperms.

Let's detail the structure of this mature, 8-nucleate, 7-celled embryo sac:

• At the Micropylar End: Three cells are grouped together to form the egg apparatus.

  • Egg Cell: This is the single female gamete, typically larger than the synergids and located centrally within the egg apparatus. It is the cell that will fuse with one of the male gametes during fertilisation.
  • Synergids: Flanking the egg cell are two synergid cells. These cells have special cellular thickenings at the micropylar tip called the filiform apparatus. The filiform apparatus plays a crucial role in guiding the pollen tube into the synergid and releasing the male gametes. Synergids are short-lived and degenerate soon after the pollen tube enters.

• At the Chalazal End: Three antipodal cells are present. These cells are typically small and are believed to provide nourishment to the embryo sac or play a role in early embryo development, though their exact function can vary. They usually degenerate before or after fertilisation.

• In the Centre: The largest cell of the embryo sac is the central cell.

  • Central Cell: This cell is significantly larger than the others and contains two polar nuclei. These two polar nuclei are initially separate but often fuse before fertilisation to form a diploid secondary nucleus. This central cell will fuse with the second male gamete during double fertilisation to form the primary endosperm nucleus, which develops into the endosperm, providing nourishment to the developing embryo.

A diagram of a mature embryo sac would clearly show its 8-nucleate, 7-celled nature and the specified parts: …