Embryo Sac Development Types

The embryo sac, also known as the female gametophyte, is a structure within the ovule of flowering plants where fertilization occurs. The development of the embryo sac from a megaspore mother cell involves meiosis and subsequent mitotic divisions and organization. Based on how many of the four megaspores produced after meiosis contribute to the embryo sac, development is categorized into three main types:

• Monosporic: One functional megaspore develops into the embryo sac.
• Bisporic: Two megaspore nuclei, contained within a functional dyad, contribute to the embryo sac.
• Tetrasporic: All four megaspore nuclei contribute to the embryo sac, usually within a single coenocytic structure before cellularization.

Allium Type Embryo Sac Development

The question specifically asks for bisporic, 8-nucleate, bipolar embryo sac development. This description precisely matches the Allium type of embryo sac development.

Here’s how the Allium type development proceeds:

1. A diploid megaspore mother cell ($\text{2n}$) undergoes Meiosis I, resulting in a dyad (two cells), each containing a haploid nucleus ($\text{n}$).
2. Meiosis II occurs, typically without cell wall formation immediately. In the Allium type, either the micropylar dyad cell or the chalazal dyad cell degenerates. The remaining functional dyad contains two haploid nuclei. Both of these nuclei will contribute to the embryo sac.
3. The two nuclei in the functional dyad undergo three rounds of mitotic divisions.
   • After the first mitosis, there are 4 nuclei, two at each pole.
   • After the second mitosis, there are 8 nuclei, four at each pole.
   • After the third mitosis, the eight nuclei are ready for organization.
4. These 8 nuclei then organize into the typical 7-celled structure of the mature embryo sac:
   • Three cells differentiate at the micropylar end to form the egg apparatus (one egg cell and two synergids).
   • Three cells differentiate at the chalazal end to form the antipodal cells.
   • The remaining two nuclei migrate to the center to form the polar nuclei, which often fuse to form a single diploid secondary nucleus.

Thus, the Allium type embryo sac develops from a structure with two nuclei (bisporic), contains 8 nuclei when mature, and these nuclei are typically organized at two poles (bipolar).

Comparison of Embryo Sac Types in Options

Let's examine the embryo sac development types for the plants listed in the options:

• Oenothera: Has monosporic, 4-nucleate development (Oenothera type). Only one megaspore functions, and it undergoes two mitotic divisions, resulting in a 4-nucleate embryo sac (often lacking antipodal cells).
• Penaea: Has tetrasporic, 16-nucleate development (Penaea type). All four megaspore nuclei contribute, and subsequent divisions lead to a 16-nucleate embryo sac with a complex structure.
• Plumbago: Has tetrasporic, 8-nucleate development (Plumbago type). All four megaspore nuclei contribute and undergo one further mitotic division, resulting in an 8-nucleate embryo sac where the polar nuclei are derived from different sets of megaspore nuclei than the egg or antipodals.
• Allium: Has bisporic, 8-nucleate development (Allium type), as described above.

Here is a summary in a table:

Conclusion

Comparing the characteristics of embryo sac development in the given options, Allium is the plant that exhibits a bisporic, 8-nucleate, bipolar embryo sac development.