Embryo Sac Development in Angiosperms

The correct answer is option 'A', which states that the functional megaspore in angiosperms develops into an embryo sac. An embryo sac, also known as a female gametophyte, is a structure found within the ovule of a flowering plant. It is responsible for the production of female gametes and plays a crucial role in the process of sexual reproduction in angiosperms.

Angiosperms, or flowering plants, are the most diverse group of plants on Earth. They reproduce sexually, with the male gametes (sperm) being produced in the pollen grains and the female gametes being produced in the embryo sacs.

The development of an embryo sac can be divided into several stages:

1. Megaspore Formation:
- Within the ovule, one of the cells in the megasporangium undergoes meiosis, producing four haploid megaspores.
- Out of the four megaspores, three degenerate, and only one functional megaspore survives.

2. Megagametogenesis:
- The functional megaspore undergoes several rounds of mitotic divisions, resulting in the formation of a multicellular structure called the embryo sac.
- The embryo sac consists of several cells, including the egg cell (located at the micropylar end), two synergids (located adjacent to the egg cell), three antipodal cells (located at the chalazal end), and a central cell with two polar nuclei.

3. Pollination and Fertilization:
- Pollination occurs when a pollen grain lands on the stigma of a flower.
- The pollen grain then germinates and forms a pollen tube, which grows through the style and reaches the ovule.
- Once the pollen tube reaches the embryo sac, two sperm cells are released into the sac.
- One sperm cell fuses with the egg cell, resulting in the formation of a zygote, which develops into an embryo.
- The other sperm cell fuses with the two polar nuclei in the central cell, resulting in the formation of endosperm, a nutrient-rich tissue that nourishes the developing embryo.

In summary, the functional megaspore in angiosperms develops into an embryo sac, which plays a crucial role in the production of female gametes and the process of sexual reproduction in flowering plants.