Main driving force for ascent of sap through xylem is

Options

A. Root pressure
B. Transpiration pull
C. Cohesion - adhesion
D. Capillarity

Answer

To determine the main driving force for the ascent of sap through xylem, we need to analyze the processes involved in the upward movement of water and minerals from the roots to the upper parts of the plant.

### Step-by-Step Solution:

1. **Understanding Ascent of Sap**:
- The ascent of sap refers to the upward movement of water and minerals in the xylem tissue of plants. This process is crucial for transporting essential nutrients from the roots to the leaves and other parts of the plant.

**Hint**: Remember that the xylem is responsible for transporting water and minerals in plants.

2. **Identifying Driving Forces**:
- Several factors contribute to the ascent of sap, including root pressure, transpiration pull, cohesion, adhesion, and capillarity.

3. **Evaluating Root Pressure**:
- Root pressure is generated when water enters the roots from the soil, creating a pressure that pushes sap upward. However, this pressure is not the main driving force and is more significant under certain conditions (e.g., high soil moisture or low transpiration).

**Hint**: Root pressure can assist in sap ascent but is not the primary mechanism.

4. **Understanding Transpiration Pull**:
- Transpiration pull is the primary driving force for the ascent of sap. It occurs due to the evaporation of water from the leaves, creating a negative pressure (suction) that pulls water upward through the xylem. This process is explained by the cohesion-tension theory, which states that water molecules stick together (cohesion) and adhere to the walls of xylem vessels (adhesion), allowing for efficient movement against gravity.

**Hint**: Focus on how water loss from leaves creates a pulling force that drives sap upward.

5. **Cohesion and Adhesion**:
- While cohesion (attraction between water molecules) and adhesion (attraction between water molecules and xylem walls) play important roles in supporting the movement of water, they are not the main driving forces. They help maintain the column of water as it is pulled up.

**Hint**: Cohesion and adhesion are supportive forces, not the primary ones.

6. **Capillarity**:
- Capillarity refers to the ability of water to rise in narrow tubes due to surface tension. While it contributes to the movement of water in small xylem vessels, it is not the main driving force for the ascent of sap.

**Hint**: Capillarity aids in water movement but is limited compared to transpiration pull.

7. **Conclusion**:
- Based on the analysis, the main driving force for the ascent of sap through xylem is the **transpiration pull**. This process is essential for the upward movement of water and is supported by the cohesion-tension theory.

**Final Answer**: The main driving force for the ascent of sap through xylem is **transpiration pull**.