Assertion: Dried specimens are poisoned by `HgCl_2`.
Reason: It prevents the specimen from moisture

Answer

### Step-by-Step Solution:

1. **Understanding the Assertion**: The assertion states that "Dried specimens are poisoned by HgCl2." This refers to the practice of using mercury chloride (HgCl2) in the preservation of dried plant specimens, commonly used in herbarium techniques.

2. **Understanding the Reason**: The reason provided is that "It prevents the specimen from moisture." This implies that the purpose of using HgCl2 is to protect the specimens from absorbing moisture.

3. **Evaluating the Assertion**: The assertion is true. In herbarium techniques, dried specimens are indeed treated with HgCl2 to preserve them. This helps in preventing decay and degradation of the specimens.

4. **Evaluating the Reason**: The reason is false. The primary function of HgCl2 is not to prevent moisture but to act as an insecticide. It protects the specimens from insect damage, which is crucial for their preservation.

5. **Conclusion**: Since the assertion is true and the reason is false, we conclude that the correct relationship is that the assertion is true, but the reason is false.

### Final Answer:
- Assertion: True
- Reason: False