_____ of the guard cell closes the stomatal aperture.

Explanation

**Step-by-Step Solution:**

1. **Understanding Guard Cells**: Guard cells are specialized cells that surround the stomata, which are small openings on the surfaces of leaves. These cells play a crucial role in regulating the opening and closing of the stomatal pores.

2. **Function of Stomata**: Stomata are essential for gas exchange in plants, allowing carbon dioxide to enter for photosynthesis and oxygen to exit as a byproduct. They also facilitate transpiration, the process of water vapor loss from the plant.

3. **States of Guard Cells**: Guard cells can exist in two states: turgid (swollen) and flaccid (shrunken). The state of these cells determines whether the stomatal aperture is open or closed.

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