Transpiration pull drives water ascent in xylem

Steps:

• Water evaporates from mesophyll cell walls in leaves, creating tension.
• This tension pulls on water molecules in xylem via cohesion.
• Continuous column of water rises due to adhesion to xylem walls.
• Replaces lost water from roots, enabling transport to leaves.

Why C is correct:

• Evaporation from mesophyll cell walls initiates the cohesion-tension mechanism, pulling water upward as per the transpiration theory.

Why the others are wrong:

• A: Active transport involves energy for ions, not bulk water movement in xylem.
• B: Mesophyll walls lack direct evaporation sites; it's the cell walls that lose water.
• D: Photosynthesis uses minimal water compared to transpiration losses.

Final answer: C