Ascent of sap in tall trees is helped by

Explanation

Upward movement of water through xylem against gravitational force is called ascent of sap. The transpiration driven ascent of xylem sap depends on (a) Cohesion - mutual attraction between water molecules (b) Adhesion - attraction of water molecules to polar surfaces (c) transpiration pull - driving force for upward movement of water. These properties give water high tensile strength and high capillarity. In plants capillarity is aided by the small diameter of the tracheary elements. As water evaporates through the stomata, Since the thin film of water over the cells is continuous, it results in pulling of water, molecule by molecule into the leaf from the xylem. Also, because of lower concentration of water vapour in the atmosphere, water diffuses into the surrounding air. This creats transpiration pull. The forces generated by the transpiration can create pressure sufficient to lift a xylem sized column of water over 130 metres high.