Cohesion-Tension Theory of Sap Ascent
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Cohesion-tension theory explains sap ascent: leaf evaporation creates tension pulling water upward through the xylem, held continuous by cohesion and adhesion to vessel walls.
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A redwood pulls water nearly 116 meters upward on this tension alone, no pump organ expending energy on the lift.
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This lift needs an unbroken water column — an air bubble, an embolism, can block the flow.