Typical Angiosperm Embryo Sac at Maturity

Explanation:

The typical angiosperm embryo sac at maturity is 7-nucleate and 8-celled, which corresponds to option B.

Here is a detailed explanation of the structure and development of the embryo sac in angiosperms:

1. Megasporogenesis:
- The process of megasporogenesis occurs in the ovule, which is the female reproductive structure of the flower.
- A diploid cell called the megaspore mother cell undergoes meiosis to produce four haploid megaspores.
- Typically, three of these megaspores degenerate, and the remaining functional megaspore undergoes further development.

2. Megagametogenesis:
- The functional megaspore undergoes three rounds of mitotic divisions without cytokinesis, resulting in the formation of a 7-celled structure called the embryo sac.
- These mitotic divisions are characterized by the migration of nuclei to specific positions within the embryo sac.

3. Structure of the Embryo Sac:
- The mature embryo sac consists of three distinct regions: the chalazal region, the central cell, and the egg apparatus.
- The chalazal region contains two nuclei, the polar nuclei, which are located close to the chalaza (base) of the ovule.
- The central cell is located in the center of the embryo sac and contains two polar nuclei.
- The egg apparatus is situated at the micropylar end of the embryo sac and consists of three cells: the egg cell and two synergids.

4. Nuclei and Cells:
- The embryo sac is 7-nucleate, meaning it contains seven nuclei distributed among the eight cells.
- The three nuclei in the chalazal region are the polar nuclei, while the three nuclei in the egg apparatus include the egg nucleus and two synergids.
- The remaining two nuclei are located in the central cell and are the polar nuclei.

Therefore, option B (7-nucleate and 8-celled) is the correct answer.

Key Points:
- The typical angiosperm embryo sac at maturity is 7-nucleate and 8-celled.
- Megasporogenesis and megagametogenesis are the processes involved in the development of the embryo sac.
- The mature embryo sac consists of three regions: the chalazal region, central cell, and egg apparatus.
- The embryo sac is 7-nucleate, with three nuclei in the chalazal region, three in the egg apparatus, and two in the central cell.