Line 1: In most flowering plants, only one megaspore out of the four formed during megasporogenesis is functional. The remaining three degenerate.
Explanation: This line describes the outcome of megasporogenesis, the process in which a single functional megaspore develops from the four megaspores produced. The degeneration of the non-functional megaspores ensures that only one megaspore continues to develop into the female gametophyte.
Line 2: This single functional megaspore matures into the female gametophyte, also known as the embryo sac.
Explanation: The functional megaspore undergoes further development to become the female gametophyte, which is commonly referred to as the embryo sac. This structure plays a central role in the sexual reproduction of flowering plants.
Line 3: This process of embryo sac formation from a single megaspore is termed monosporic development.
Explanation: Monosporic development refers to the formation of the embryo sac from a single megaspore. It contrasts with other modes of development, such as bisporic or tetrasporic, where multiple megaspores contribute to the formation of the female gametophyte.
Line 4: Let’s examine the ploidy (number of chromosome sets) of different cells involved:
Explanation: This line introduces the concept of ploidy and sets the stage for understanding the genetic characteristics of various cells involved in the development of the female gametophyte.
- Nucellus cells: Diploid (2n) – They maintain the same ploidy level as the somatic cells of the plant.
- Megaspore Mother Cell (MMC): Diploid (2n) – It inherits the diploid condition from the parent sporophytic tissue.
- Functional Megaspore: Haploid (n) – Meiosis in the MMC halves the chromosome number, resulting in haploid megaspores.
- Female Gametophyte (Embryo Sac): Mostly haploid (n) with a brief diploid stage – The functional megaspore undergoes mitotic divisions (without cell wall formation) to create an 8-nucleate embryo sac. However, most of the cells remain haploid. The exception is the central cell, which becomes briefly diploid after the fusion of polar nuclei (explained later).
Line 5: The functional megaspore undergoes its first mitotic division, resulting in two nuclei.
Explanation: Following the formation of the functional megaspore, it undergoes mitotic division to produce two nuclei, marking an early stage in the development of the embryo sac.
Line 6: Cell wall formation doesn’t occur immediately after this nuclear division. This is called free nuclear division.
Explanation: During free nuclear division, the nuclei divide without the formation of cell walls, leading to the presence of multiple nuclei within a shared cytoplasmic space.
Line 7: A large central vacuole (fluid-filled sac) appears between the two daughter nuclei.
Explanation: The emergence of a central vacuole between the nuclei contributes to the structural organization of the developing embryo sac, marking a key stage in its morphological development.
Line 8: The vacuole expands, pushing the nuclei towards opposite poles of the developing embryo sac.
Explanation: As the vacuole enlarges, it exerts pressure on the nuclei, causing them to migrate towards opposite ends of the embryo sac, contributing to its spatial organization.
Line 9: Each of the two nuclei divides mitotically twice, leading to four nuclei at each pole (total of eight nuclei).
Explanation: Following the migration of nuclei to opposite poles, they undergo further mitotic divisions, resulting in the formation of four nuclei at each end of the embryo sac, totaling eight nuclei.
Line 10: It’s important to note that these are still free nuclear divisions – the cytoplasm remains undivided.
Explanation: Despite the nuclear divisions, the cytoplasm of the embryo sac remains undivided, emphasizing the unique mode of cellular organization during this stage of development.
Line 11: After the 8-nucleate stage, cell walls are finally laid down, leading to the formation of distinct cells within the embryo sac.
Explanation: Following the completion of free nuclear divisions, cell walls begin to form around individual nuclei, giving rise to distinct cellular compartments within the embryo sac.
Line 12: Observe the distribution of these cells in Figure 2.8b and 2.8c.
Explanation: Visual representations provided in Figures 2.8b and 2.8c depict the spatial arrangement of cells within the mature embryo sac, aiding in the understanding of its structural organization.
Line 13: Six of the eight nuclei become surrounded by cell walls, forming individual cells.
Explanation: At this stage, most of the nuclei within the embryo sac are enclosed by cell walls, delineating distinct cellular entities within the structure.
Line 14: The remaining two nuclei, called polar nuclei, are situated below the egg apparatus in a large central cell.
Explanation: Two nuclei, known as polar nuclei, occupy a central position within the embryo sac, coexisting with the egg apparatus in a specialized cellular environment.
Line 15: The mature embryo sac exhibits a characteristic distribution of cells:
Explanation: This line summarizes the cellular arrangement within the mature embryo sac, highlighting the distinct cell types and their spatial organization.
- Micropylar End (Three Cells): These constitute the egg apparatus (Figure 2.8c).
- Two Synergids: They have specialized cell wall thickenings at the tip (filiform apparatus) that guide pollen tubes (Figure 2.8c).
- One Egg Cell: This is the female gamete involved in fertilization (Figure 2.8c).
- Chalazal End (Three Cells): These are called antipodal cells, but they usually degenerate before or shortly after fertilization (Figure 2.8c).
- Large Central Cell: This cell houses the two polar nuclei (Figure 2.8c).
Line 16: Most cells in the mature embryo sac are haploid (n) – the egg cell, synergids, and antipodals.
Explanation: The majority of cells within the mature embryo sac exhibit a haploid chromosome complement, reflecting their role in sexual reproduction and gamete formation.
Line 17: The central cell is a temporary exception. It becomes briefly diploid (2n) after the fusion of the two polar nuclei during double fertilization (a later stage in the reproductive process).
Explanation: Although most cells within the mature embryo sac are haploid, the central cell undergoes a transient diploid phase following the fusion of polar nuclei during double fertilization, facilitating the formation of endosperm.
Line 18: Three Antipodal Nuclei: These are typically enclosed within cell walls. Each antipodal nucleus is located within its own antipodal cell, and these cells are generally bounded by cell walls.
Explanation: The antipodal nuclei, situated at the chalazal end of the embryo sac, are often surrounded by cell walls, contributing to the structural integrity of these cells within the reproductive structure.
Line 19: Two Polar Nuclei: These eventually fuse to form the secondary nucleus, and are not typically enclosed within individual cell walls. Instead, they are found within the central cell, which itself is enclosed by a cell membrane but may not have distinct cell walls surrounding the individual nuclei. Synergids and Egg Cell: These also lack cell walls surrounding their nuclei. They are individual cells, but they have very thin cell wall structures or may lack them altogether. This allows for easier movement of materials and interaction between these cells.
Explanation: The nuclei of the polar nuclei and the gametes within the egg apparatus and synergids exhibit unique structural characteristics, including the absence or minimal presence of cell walls, facilitating their interactions during fertilization and subsequent reproductive processes.
Additional Notes:
The provided information describes the Polygonum type of embryo sac development, which is the most common type found in flowering plants. Refer to Figure 2.8b and 2.8c (assuming they depict the stages of embryo sac development) for visual representation alongside this explanation. We’ve covered the basic structure and development of the embryo sac. However, the story doesn’t end here! The next steps involve: Pollen Tube Growth: Pollen grains germinate and produce pollen tubes that travel down the style towards the ovule. Fertilization: The pollen tube delivers sperm cells to the embryo sac, where one sperm fertilizes the egg cell (giving rise to the embryo), and the other sperm fuses with the two polar nuclei in the central cell (forming the endosperm, a nutritive tissue for the developing embryo). These processes are crucial for successful seed formation in flowering plants.