Q.In a fertilised embryo sac, the haploid, diploid and triploid structures are:

In angiosperms, fertilisation involves two distinct fusion events, collectively known as double fertilisation.

• One male gamete fuses with the egg cell, a process called syngamy, resulting in the formation of a diploid zygote.
• The other male gamete fuses with the two haploid polar nuclei located in the central cell. This fusion of three haploid nuclei (one male gamete and two polar nuclei) is termed triple fusion, leading to the formation of a triploid primary endosperm nucleus (PEN).
• Structures like synergids and antipodal cells, which are part of the embryo sac, remain haploid even after fertilisation, though they typically degenerate soon after. …

To understand the ploidy levels of structures within a fertilised embryo sac, we must first recall the structure of the unfertilised embryo sac and then the process of double fertilisation. The typical angiosperm embryo sac, or female gametophyte, is a seven-celled, eight-nucleate structure before fertilisation. All these cells and nuclei are haploid (n), meaning they contain a single set of chromosomes.

The cells present in an unfertilised embryo sac are:

• Egg cell: One, located at the micropylar end.
• Synergids: Two, flanking the egg cell at the micropylar end. They have filiform apparatus that guide the pollen tube.
• Antipodal cells: Three, located at the chalazal end.
• Central cell: One large cell containing two polar nuclei in its centre. Although the central cell is one cell, it contains two haploid polar nuclei, making it effectively diploid (2n) in terms of genetic content before fusion.

Fertilisation in angiosperms is unique because it involves a process called double fertilisation. This process involves two distinct fusion events:

1. Syngamy (Generative Fertilisation): One of the two male gametes (which are haploid, n), released from the pollen tube, fuses with the haploid (n) egg cell. This fusion results in the formation of a zygote, which is diploid (2n). The zygote will then develop into the embryo.

2. Triple Fusion (Vegetative Fertilisation): The second male gamete (haploid, n) fuses with the central cell's two polar nuclei (each haploid, n). Since the central cell already contains two haploid polar nuclei, the fusion of a third haploid nucleus (from the male gamete) results in a primary endosperm nucleus (PEN), which is triploid (3n). The PEN subsequently develops into the endosperm, which serves as nutritive tissue for the developing embryo.

Now, let's consider the ploidy of the structures in a fertilised embryo sac, as asked in the question:

• Synergids: These cells are originally haploid (n). After guiding the pollen tube, they typically degenerate. However, their ploidy remains haploid.
• Zygote: This structure is formed by the fusion of a haploid egg cell and a haploid male gamete. Therefore, the zygote is diploid (2n).
• Primary Endosperm Nucleus (PEN): This nucleus is formed by the fusion of one haploid male gamete with two haploid polar nuclei. Thus, the PEN is triploid (3n). …