When turgidity increases within the two guard cells flanking each stomatal aperture or pore, the thin outer walls and force the inner walls into a shape.

Options

A. bulge in, crescent
B. bulge out, crescent
C. bulge out, spindle
D. bulge in, spindle

Explanation

To solve the question regarding the shape of guard cells when turgidity increases, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Guard Cells**:
- Guard cells are specialized cells that flank each stomatal pore. They play a crucial role in regulating the opening and closing of stomata.

2. **Turgidity in Guard Cells**:
- Turgidity refers to the state of being swollen or firm due to the uptake of water. When guard cells take up water, they become turgid.

3. **Structure of Guard Cells**:
- Each guard cell has two types of walls:
- **Outer Walls**: These are thin and flexible.
- **Inner Walls**: These are thicker and more elastic.

4. **Effect of Increased Turgidity**:
- When turgidity increases in the guard cells, the thin outer walls bulge outwards due to the pressure from the internal water.

5. **Shape Formation**:
- As the outer walls bulge out, the thicker inner walls are forced into a crescent shape. This is because the inner walls are less flexible and cannot expand as much as the outer walls.

6. **Conclusion**:
- Therefore, when turgidity increases, the guard cells bulge outwards and the inner walls take on a crescent shape.

### Final Answer:
The correct answer is **bulge out crescent**.

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