Answer in one or two word:
A chemical used to prevent excessive transpiration in plants.

Explanation

**Step-by-Step Text Solution:**

1. **Understanding the Question**: The question asks for a chemical that prevents excessive transpiration in plants.

2. **Identifying the Process**: Transpiration is the process by which water vapor is lost from the plant, mainly through small openings called stomata. Excessive transpiration can lead to water loss and stress in plants.

3. **Finding the Chemical**: To prevent excessive transpiration, certain chemicals can be used that work by closing the stomata, thereby reducing water loss.

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