A mature embryo sac of an angiosperm is:
8 nucleate, 7 celled
Understanding the Mature Embryo Sac
In angiosperms, the mature embryo sac is the female gametophyte. It is located within the ovule, which is inside the ovary of a flower. This structure is crucial for sexual reproduction in flowering plants as it contains the egg cell, which will be fertilized by a male gamete.
Development of the Embryo Sac
The process of embryo sac formation from a megaspore is called megagametogenesis. In most angiosperms, a single functional megaspore undergoes three successive mitotic divisions. Let's trace the nuclear divisions:
- The first mitotic division in the functional megaspore results in two nuclei, which move to opposite poles.
- The second mitotic division occurs at both poles, forming two nuclei at each pole (a total of four nuclei).
- The third mitotic division also occurs at both poles, resulting in four nuclei at each pole (a total of eight nuclei).
These eight nuclei then organize into cells. Cell walls are laid down around six of the eight nuclei, forming distinct cells. The remaining two nuclei, one from each pole, move towards the center of the embryo sac and fuse (or remain adjacent) to form the central cell.
Structure of a Mature Angiosperm Embryo Sac
A mature angiosperm embryo sac, typically of the Polygonum type, consists of seven cells and eight nuclei. These cells are arranged in a specific manner:
- At the micropylar end (the end facing the micropyle of the ovule), there is the egg apparatus, which consists of three cells:
- One egg cell: This is the female gamete.
- Two synergid cells: These flank the egg cell and have special cellular thickenings called filiform apparatus at the micropylar tip, which guide pollen tube entry.
- At the chalazal end (the opposite end), there are three antipodal cells. Their function is not entirely clear but they are believed to play a role in nutrition or degeneration after fertilization.
- In the center of the embryo sac is a large central cell. This cell contains two polar nuclei, which originated from the opposite poles of the embryo sac after the third mitotic division. These two polar nuclei typically fuse before fertilization to form a diploid (2n) secondary nucleus, or they may remain unfused until the second male gamete reaches the central cell. However, the structure is described as having two polar nuclei before fertilization is complete.
Let's count the cells and nuclei in a mature embryo sac:
| Cell Type | Number of Cells | Number of Nuclei per Cell | Total Nuclei |
|---|---|---|---|
| Egg cell | 1 | 1 | 1 |
| Synergid cells | 2 | 1 | 2 |
| Antipodal cells | 3 | 1 | 3 |
| Central cell | 1 | 2 (Polar Nuclei) | 2 |
| Total | 7 | 8 |
As shown in the table, there are a total of 7 cells (1 egg + 2 synergids + 3 antipodals + 1 central cell) and a total of 8 nuclei (1 in egg + 2 in synergids + 3 in antipodals + 2 polar nuclei in central cell) in a mature embryo sac.
Analyzing the Options
Based on the structure described above, let's evaluate the given options:
- 7 nucleate, 7 celled: This option is incorrect because a mature embryo sac has 8 nuclei, not 7.
- 8 nucleate, 7 celled: This option correctly describes the number of nuclei and cells found in a typical mature angiosperm embryo sac.
- 6 nucleate, 7 celled: This option is incorrect because a mature embryo sac has 8 nuclei, not 6.
- 4 nucleate, 7 celled: This option is incorrect because a mature embryo sac has 8 nuclei, not 4.
Therefore, a mature embryo sac of an angiosperm is typically 8 nucleate and 7 celled.
Revision Table: Embryo Sac Structure
| Feature | Description |
|---|---|
| Location | Inside the ovule within the ovary |
| Total Cells | 7 |
| Total Nuclei | 8 |
| Cells at Micropylar End | Egg cell (1), Synergids (2) |
| Cells at Chalazal End | Antipodals (3) |
| Cell in the Center | Central cell (1) with 2 polar nuclei |
| Role | Female gametophyte, contains egg cell for fertilization |
Additional Information: Megagametogenesis Insights
The development of the embryo sac (megagametogenesis) can vary among different plant species. The most common type, described above, is the Polygonum type, which develops from a single functional megaspore (monosporic development) and involves three mitotic divisions leading to an 8-nucleate structure before cellularization into a 7-celled structure.
Other types of embryo sac development exist, such as bisporic and tetrasporic development, where two or all four megaspores participate in the formation of the embryo sac. However, the final mature structure in many cases still results in an 8-nucleate, 7-celled organization, while variations like 4-nucleate or 16-nucleate embryo sacs also occur in some species. The question refers to a typical mature embryo sac of an angiosperm, which implies the Polygonum type.
Important Questions from Sexual Reproduction in Flowering Plants
Which one of the following is not associated with megasporangium?
In diploid organisms, which of the following undergoes meiosis?
In a velvet grass seed, the cotyledon is called:
Cells present in the mature pollen grains are ____________.
Match List-I with List-II:
List-I (Structures)
List-II (Functions)
List-I List-II (A) Filiform apparatus (I) Made up of sporopollenin (B) Tapetum (II) Attachment of ovule to the placenta (C) Exine (III) Guides pollen tube into the synergid (D) Funicle (IV) Nourishes the pollen grain Choose the correct answer from the options given below:
