Most plants meet their water need by

Options

A. transpiration pul
B. root pressure
C. both 1 and 2
D. osmotic pressure

Answer

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks how most plants meet their water needs. We need to identify the primary mechanism through which water is transported in plants.

2. **Identifying the Mechanisms**: The options provided are:
- Transpiration pull
- Root pressure
- Both transpiration pull and root pressure
- Osmotic pressure

3. **Explaining Transpiration Pull**:
- Transpiration is the process where water evaporates from the surface of leaves. This creates a negative pressure (tension) in the leaf, which pulls water upward from the roots through the xylem.
- The cohesion between water molecules helps maintain a continuous column of water in the xylem vessels.

4. **Role of Root Pressure**:
- Root pressure is a positive pressure that develops in the roots due to the uptake of water from the soil. However, this mechanism is limited and can only push water up to small heights.

5. **Osmotic Pressure**:
- Osmotic pressure refers to the pressure required to prevent the flow of water across a semipermeable membrane. While it plays a role in water movement, it is not the primary driver for the upward movement of water in plants.

6. **Conclusion**:
- Given the explanations above, the primary mechanism through which most plants meet their water needs is **transpiration pull**. Therefore, the correct answer is the first option: **transpiration pull**.