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

### Step-by-Step Solution:

1. **Understanding Guard Cells and Stomata**:
- Guard cells are specialized cells that flank each stomatal pore. They play a crucial role in regulating the opening and closing of the stomata, which are small openings on the surface of leaves that facilitate gas exchange and transpiration.

2. **Role of Turgidity**:
- Turgidity refers to the pressure exerted by the fluid inside the guard cells. When water enters the guard cells, they become turgid (swollen), which affects their shape.

3. **Changes in Guard Cell Shape**:
- As turgidity increases in the guard cells, the thin outer walls of the guard cells bulge outward. This is due to the pressure of the fluid within the cells.

4. **Formation of Crescent Shape**:
- The inner walls of the guard cells, which are thicker and more elastic, are forced into a crescent shape as the outer walls bulge out. This change in shape is what allows the stomatal pore to open.

5. **Conclusion**:
- Therefore, when turgidity increases within the guard cells, the thin outer walls bulge out, forcing the inner walls into a crescent shape. This mechanism is essential for the opening of the stomata, allowing for gas exchange and transpiration.

### Final Answer:
When turgidity increases within the two guard cells flanking each stomatal aperture or pore, the thin outer walls bulge out and force the inner walls into a crescent shape.

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