USA Biology Olympiad (USABO) Practice Exam

Which structure is part of the egg-producing system in flowering plants?

1. Microgametophyte

2. The megagametophyte

3. Ovary wall

4. Seed coat

The correct answer is: The megagametophyte

The megagametophyte is indeed a crucial structure in the egg-producing system of flowering plants. In flowering plants, the megagametophyte is a haploid structure that develops within the ovule, which is found in the ovary of the flower. This stage follows the process of megasporogenesis, where a single megasporangium undergoes meiosis to produce four megaspores, typically leading to the formation of one functional megaspore. This functional megaspore then undergoes mitotic divisions to form the megagametophyte, which contains the egg cell that is essential for fertilization. The presence of the megagametophyte is central to reproduction in angiosperms because it provides the environment where fertilization occurs, leading to the development of the zygote. This structure is characterized by specific cells, including the egg cell and synergids that assist in the fertilization process. Other structures mentioned in the options play different roles within the plant's reproductive cycle. The microgametophyte refers to the pollen grain, which is involved in producing sperm cells, while the ovary wall is part of the fruit that develops after fertilization. The seed coat protects the developing embryo but does