Transpiration Pull Drives Water Uptake

Steps:

• Water evaporates from moist walls of mesophyll cells in the leaf.
• This evaporation creates tension in the xylem vessels due to water's cohesive properties.
• The tension pulls a continuous column of water upward from roots through xylem.
• This process, known as transpiration, maintains the water flow to leaves.

Why C is correct:

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

Why the others are wrong:

• A: Active transport requires energy for ions, not bulk water movement in xylem, which is passive.
• B: Epidermis evaporation is minimal due to the waxy cuticle; main loss occurs internally via mesophyll.
• D: Photosynthesis uses minimal water compared to transpiration rates, insufficient to drive bulk flow.

Final answer: C