The transpiration driven ascent of xylem sap depends mainly on the following physical properties of water:
- Cohesion – the mutual attraction between water molecules.
- Adhesion – the attraction of water molecules to polar surfaces (such as the surface of tracheary elements).
- Surface Tension – water molecules are attracted to each other in the liquid phase more than to water in the gas phase.
These properties give water high tensile strength, i.e., an ability to resist a pulling force, and high capillarity, I ‘.e., the ability to rise in thin tubes. In plants capillarity is aided by the small diameter of the tracheary elements – the tracheids and vessel elements.
