Model Answer
0 min readIntroduction
The embryo sac, also known as the female gametophyte, is a crucial structure in angiosperms responsible for housing the egg cell and facilitating fertilization. Its development is a complex process involving meiosis and mitosis, resulting in a characteristic eight-nucleate structure. While the typical seven-celled, eight-nucleate Polygonum type is most common, variations exist across different angiosperm families. These variations, termed different types of embryo sacs, reflect adaptations to diverse pollination mechanisms and fertilization strategies. Understanding these variations is fundamental to comprehending plant reproductive biology. This answer will detail the known types of embryo sacs, their formation, and provide examples for each.
Types of Embryo Sacs in Angiosperms
Several types of embryo sacs have been identified in angiosperms, categorized based on the number of nuclei and cellular organization. The primary types include:
1. Polygonum Type (Typical Embryo Sac)
This is the most common type, found in approximately 70% of angiosperms.
- Formation: It develops from a megaspore mother cell (MMC) through meiosis, resulting in four haploid megaspores. Three megaspores degenerate, and the remaining functional megaspore undergoes three successive mitotic divisions without cytokinesis, forming an eight-nucleate cell. Subsequently, cellularization occurs, resulting in a seven-celled embryo sac. These cells include the egg apparatus (egg cell and two synergids), three antipodal cells, and a central cell containing two polar nuclei.
- Structure: Seven cells, eight nuclei (1 egg, 2 synergids, 3 antipodals, 2 polar nuclei in the central cell).
- Example: Polygonum (Knotweed), Beta vulgaris (Sugar beet)
2. Fritillaria Type
This type is characterized by the functional megaspore undergoing two rounds of mitosis, resulting in four nuclei. Then, cellularization occurs, leading to a four-celled and four-nucleate embryo sac.
- Formation: The functional megaspore divides mitotically twice, forming four nuclei. Cellularization then occurs, resulting in four cells, each with one nucleus.
- Structure: Four cells, four nuclei (one egg, one synergid, two polar nuclei). Antipodal cells are absent.
- Example: Fritillaria (Lily of the valley)
3. Allium Type
Similar to the Fritillaria type, but with a slightly different pattern of cellularization.
- Formation: The functional megaspore undergoes two mitotic divisions, resulting in four nuclei. Cellularization occurs, but the two polar nuclei remain in a single cell.
- Structure: Three cells, four nuclei (one egg, one synergid, two polar nuclei in a single cell). Antipodal cells are absent.
- Example: Allium cepa (Onion)
4. Drusa Type
This type is found in plants of the family Berberidaceae.
- Formation: The functional megaspore undergoes mitosis, resulting in four nuclei. These nuclei do not undergo further division or cellularization, remaining within a single large cell.
- Structure: One cell, four nuclei. No cellularization occurs.
- Example: Berberis (Barberry)
5. Peperomia Type
This type exhibits an unusual development where the functional megaspore undergoes mitosis, but the nuclei do not migrate to their typical positions.
- Formation: The functional megaspore divides mitotically, but the resulting nuclei remain scattered within the megaspore wall.
- Structure: A single cell containing multiple nuclei.
- Example: Peperomia (Radiator plant)
The formation of these different embryo sac types highlights the plasticity of plant reproductive development and the evolutionary adaptations that have occurred within the angiosperms.
| Embryo Sac Type | Number of Cells | Number of Nuclei | Antipodal Cells | Example |
|---|---|---|---|---|
| Polygonum | 7 | 8 | Present (3) | Polygonum |
| Fritillaria | 4 | 4 | Absent | Fritillaria |
| Allium | 3 | 4 | Absent | Allium cepa |
| Drusa | 1 | 4 | Absent | Berberis |
| Peperomia | 1 | Multiple | Absent | Peperomia |
Conclusion
In conclusion, while the Polygonum type is the most prevalent, the existence of diverse embryo sac types – Fritillaria, Allium, Drusa, and Peperomia – demonstrates the remarkable adaptability of angiosperm reproduction. These variations in development, stemming from differences in mitotic divisions and cellularization, reflect evolutionary pressures and contribute to the success of flowering plants. Further research into the genetic and molecular mechanisms governing these developmental pathways will continue to refine our understanding of plant reproduction and its implications for crop improvement and conservation.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.