What may be the possible advantage occuring out of the presence of antheridia and archegonia on the underside of a fern prothallus?

Options

A. They are protected from direct rays of the sun.
B. Capillary water accumulates on the underside of prothallus between its lower surface and the soil surface sex organs projecting in this water can be readily fertilised by the cilliated sperms which are chemotactically attracted by the archegonia
C. The sex organs remain protected from wind
D. Nutrients manufactured by the green prothallus can radily seep downwards to the sex organs due to the action of gravity.

Answer

### Step-by-Step Solution:

1. **Understanding the Structure**: The fern prothallus is a small, heart-shaped structure that is part of the gametophyte generation in ferns. It is typically bisexual, meaning it contains both male (antheridia) and female (archegonia) reproductive organs.

2. **Location of Reproductive Organs**: The antheridia (male organs) and archegonia (female organs) are located on the underside of the prothallus. This positioning is crucial for the reproductive process.

3. **Function of Archegonia and Antheridia**: The archegonia produce eggs, while the antheridia produce sperm. For fertilization to occur, sperm must swim to the eggs.

4. **Role of Water**: The presence of these reproductive organs on the underside allows them to be in close contact with water. Ferns require water for fertilization because the sperm are motile and need to swim to reach the eggs.

5. **Capillary Action**: When rain or moisture accumulates, it collects on the underside of the prothallus due to capillary action. This water creates a suitable environment for the sperm to swim towards the eggs.

6. **Protection and Fertilization**: The location of the reproductive organs also offers some protection from environmental factors. The water that accumulates helps in the fertilization process as the ciliated sperm are attracted chemically to the archegonia, facilitating successful fertilization.

7. **Conclusion**: The advantage of having antheridia and archegonia on the underside of the fern prothallus is that it ensures a moist environment for sperm motility, enhances the chances of fertilization, and provides some protection to the reproductive structures.