Define wall pressure.

Explanation

**Step-by-Step Text Solution:**

1. **Definition of Wall Pressure**: Wall pressure is defined as the pressure exerted by the cell wall on the contents of a plant cell.

2. **Relationship with Turgor Pressure**: This pressure is equal and opposite to the turgor pressure, which is the pressure of the fluid (usually water) inside the cell that pushes against the cell wall.

3. **Mechanism of Action**: When a plant cell is turgid (filled with water), the fluid contents exert outward pressure on the cell wall. This outward pressure is referred to as turgor pressure.

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