Selaginella sperms swim towards archegonia. The phenomenon is

Options

A. Chemotaxis
B. Chemotropism
C. Chemonasty
D. Thigmonasty

Answer

To answer the question regarding the phenomenon of Selaginella sperm swimming towards archegonia, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Context**:
- Selaginella is a genus of plants that reproduces via spores and has a unique reproductive structure. The male gametes (sperms) swim towards the female reproductive structure (archegonia).

2. **Identifying the Phenomenon**:
- The movement of Selaginella sperm towards archegonia is a response to a chemical signal. This indicates that the sperm can detect and respond to certain chemicals released by the archegonia.

3. **Explaining Chemotaxis**:
- The specific phenomenon where organisms move in response to chemical stimuli is known as **chemotaxis**. In this case, the archegonia releases a chemical attractant that guides the sperm towards it.

4. **Differentiating Other Terms**:
- **Chemotropism** refers to growth movement in response to chemical stimuli, which is not applicable to the swimming sperm.
- **Chemonasty** is a type of movement that is not directed towards a stimulus but rather a general response to it, which again does not fit this scenario.
- **Thigmonasty** refers to movement in response to touch or vibration, which is unrelated to the swimming of sperm towards archegonia.

5. **Conclusion**:
- Based on the definitions and the context of Selaginella's reproductive process, the correct answer to the question is **chemotaxis**.

### Final Answer:
The phenomenon is **chemotaxis**.