Water transport from roots to leaves is explained by
(a) the pressure flow theory
(b) differences in source and sink solute concentrations
(c) the pumping force of xylem vessels
(d) the cohesion-tension theory

Options

A. the pressure flow theory
B. differences in source and sink solute concentrations
C. the pumping force of xylem vessels
D. the cohesion-tension theory

Answer

To solve the question regarding the mechanism of water transport from roots to leaves in plants, we need to analyze the given options and identify the correct theory that explains this process.

### Step-by-Step Solution:

1. **Understanding Water Transport in Plants**: Water is absorbed by the roots and needs to travel upward to the leaves where it is used for photosynthesis and other metabolic processes.

2. **Reviewing the Options**:
- **(a) The Pressure Flow Theory**: This theory primarily describes the movement of sugars in phloem, not water transport in xylem.
- **(b) Differences in Source and Sink Solute Concentrations**: This concept relates to the movement of nutrients and sugars, not specifically water transport.
- **(c) The Pumping Force of Xylem Vessels**: Xylem does not pump water; it relies on passive transport mechanisms.
- **(d) The Cohesion-Tension Theory**: This theory explains how water moves from roots to leaves through transpiration and the cohesive properties of water molecules.

3. **Identifying the Correct Theory**: The correct explanation for water transport from roots to leaves is the **Cohesion-Tension Theory**. This theory was proposed by Dixon and Jolly and describes how water is pulled up through the plant due to the evaporation of water from the leaves (transpiration), creating a negative pressure (tension) that pulls water upward. The cohesion between water molecules helps maintain a continuous column of water in the xylem vessels.

4. **Conclusion**: Based on the analysis, the correct answer is **(d) the cohesion-tension theory**.

### Final Answer:
The correct explanation for water transport from roots to leaves is (d) the cohesion-tension theory.

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