The guard cell walls surrounding the aperture are thicker than outer wall.

Explanation

**Step-by-Step Solution:**

1. **Understanding Guard Cells**: Guard cells are specialized cells that surround the stomata (the pores on the leaf surface). They are typically kidney-shaped and play a crucial role in regulating gas exchange and transpiration in plants.

2. **Structure of Guard Cells**: Each guard cell has two walls: an inner wall that surrounds the stomatal aperture and an outer wall that faces the epidermis of the leaf.

3. **Comparison of Wall Thickness**: The inner wall of the guard cells, which directly surrounds the stomatal aperture, is thicker than the outer wall. This structural difference is important for the functioning of the guard cells.

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