Answer: Evaporation from mesophyll cells creating tension that pulls water up.
- A Root pressure from active ion accumulation in roots according to standard texts
- B Active pumping of water by living xylem parenchyma cells under typical physiological conditions
- C Evaporation from mesophyll cells creating tension that pulls water up
- D Osmotic flow of water moving from phloem into xylem in general clinical practice
Correct answer: C. Evaporation from mesophyll cells creating tension that pulls water up
Explanation: As water evaporates from leaf mesophyll cells during transpiration, it creates a negative pressure (tension) that is transmitted down the continuous water column in the xylem, pulling water upward from the roots.
Water can travel through cell walls (apoplast) or cell-to-cell through the cytoplasm (symplast) - but the Casparian strip at the endodermis blocks the apoplast route entirely, forcing everything through a selectively permeable membrane (symplast) before it can enter the xylem.
Concept context
Water and mineral absorption, the ascent of sap, transpiration, and phloem transport via the pressure flow hypothesis.