What is the function of a guard cell?

Explanation

### Step-by-Step Solution

1. **Definition of Guard Cells**:
- Guard cells are specialized epidermal cells located on the upper and lower surfaces of leaves. They are responsible for forming stomata, which are small openings that facilitate gas exchange.

2. **Structure of Stomata**:
- A pair of guard cells surrounds each stoma (singular of stomata). The shape and turgidity of these cells determine whether the stomata are open or closed.

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