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**Cohesion–tension theory** (Dixon and Joly, 1894) is the currently accepted explanation for ascent of sap. The 'pull' that drives water up is generated by:
ARoot pressure alone
BTranspiration from leaves — water loss from mesophyll cells creates tension that pulls the continuous water column upward through xylem
CCapillary action in xylem vessels alone
DActive pumping by xylem parenchyma
Answer & Solution
Correct answer: B. Transpiration from leaves — water loss from mesophyll cells creates tension that pulls the continuous water column upward through xylem
Cohesion–tension: transpiration creates negative pressure (tension) in xylem at the leaves. Water columns held together by cohesion (between water molecules) and adhesion (to xylem walls) transmit this tension down to the roots — pulling water up. Explains ascent in trees > 100 m tall.
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