Upward movement of water through xylem aganist gravitational force is called accept 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 palar 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 creates 'transpiration pull'. The forces generated by transpiration can creates pressures sufficient to lift a xylem sized column of water over 130 metres high.
