Factors responsible for ascent of xylem sap in plants : Transpiration driven ascent of xylem sap is dependent chiefly on the physical properties of water that are following:
(a) Cohesion : The mutual attraction between water molecules.
(b) Adhesion : The attraction of molecules to polar surfaces e.g., the surface of tracheary elements.
(c) Surface Tension: The water molecules are attracted to each other in liquid phase more than to water in gas phase.
All these- properties provide water high tensile strength (an ability to resist a pulling force) as well as high capillarity (ability to rise in a thin tube). In plants, capillarity is aided by small diameter of both tracheary elements-tracheids and vessels