The correct option is A is false but R is true
Explanation
Water transport in plants primarily occurs through the xylem tissue. This movement is driven by two distinct forces: root pressure (a positive pressure pushing water upwards from the roots) and transpiration pull (a negative pressure or suction pulling water upwards from the leaves). The efficiency of these forces varies based on the height of the plant.
Statement-wise Analysis
- Assertion (A): Incorrect. While root pressure does exist, the magnitude of this positive pressure is generally low. It is capable of pushing water up only to modest heights, such as in herbaceous plants or small shrubs. It is not sufficient to overcome gravity and transport water to the tops of tall trees (gymnosperms and angiosperms). In tall trees, the primary driving force is the transpiration pull (Cohesion-Tension Theory).
- Reason (R): Correct. Root pressure is generated through an osmotic mechanism. As mineral ions are actively absorbed from the soil into the vascular tissue (xylem) of the roots, the water potential within the xylem decreases. Consequently, water follows the potential gradient and enters the xylem via osmosis. This continuous influx of water creates a positive hydrostatic pressure, known as root pressure.
Key Takeaway
Root pressure provides a weak "push" effective only for small heights, whereas transpiration pull provides the strong "pull" required to transport water to the canopy of tall trees.