Upward movement of water through xylem is best explained by
(a) Cohesion theory
(b) Pulsation theory
(c) Capillarity theory
(d) Root pressure theory

Options

A. Cohesion theory
B. Pulsation theory
C. Capillarity theory
D. Root pressure theory

Answer

To solve the question regarding the upward movement of water through xylem, we need to analyze the options provided and determine which theory best explains this phenomenon.

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks for the best explanation of the upward movement of water through xylem in plants.

2. **Reviewing the Options**:
- **(a) Cohesion theory**: This theory suggests that water molecules are attracted to each other (cohesion) and to the walls of the xylem vessels (adhesion). This creates a continuous column of water that can be pulled upward due to transpiration at the leaves.
- **(b) Pulsation theory**: This theory, proposed by J.C. Bowles, suggests that water absorption is due to metabolic activities in living cells. However, it does not specifically explain the upward movement of water through xylem.
- **(c) Capillarity theory**: This theory states that water moves upward in narrow tubes (like xylem vessels) due to capillary action. While it contributes to water movement, it does not fully explain the upward movement against gravity.
- **(d) Root pressure theory**: This theory explains that osmotic pressure in the roots pushes water upward. However, it does not account for the primary mechanism of water transport in tall plants.

3. **Identifying the Best Explanation**: Among the options, the **Cohesion theory** is the most comprehensive explanation for the upward movement of water through xylem. It accounts for both the cohesive properties of water and the adhesion to xylem walls, allowing for effective transport against gravity.

4. **Conclusion**: Therefore, the correct answer is **(a) Cohesion theory**.

### Final Answer:
**(a) Cohesion theory**

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