1. Development:
- The embryo sac develops from a single megaspore within the ovule, which is located inside the ovary of the flower.
- Through a series of mitotic divisions, the megaspore differentiates into a seven-celled, eight-nucleate structure.
2. Gamete Production:
- One of these cells, the egg cell, is the female gamete.
- Two other nuclei, known as polar nuclei, fuse to form a single diploid nucleus, called the central cell nucleus.
3. Fertilization:
- During pollination, pollen grains land on the stigma of the flower.
- A pollen tube grows down through the style and reaches the embryo sac.
- One sperm nucleus from the pollen grain fuses with the egg cell, forming the zygote, which will develop into the embryo.
- The other sperm nucleus fuses with the central cell nucleus, forming the endosperm, which provides nourishment for the developing embryo.
4. Development of the seed:
- After fertilization, the embryo sac develops into the seed.
- The zygote develops into the embryo, the endosperm provides nourishment, and the ovule's integuments (outer layers) become the seed coat.
In summary, the embryo sac is crucial for:
* Producing the egg cell (female gamete)
* Facilitating fertilization by guiding the pollen tube and allowing the sperm to reach the egg cell
* Developing into the seed that contains the embryo and endosperm
The embryo sac's role in sexual reproduction ensures the continuation of the plant species by producing seeds that can grow into new plants.
