Discuss the factors responsible for ascent of xylem sap in plants.

Transpirational pull is responsible for the ascent of water in the xylem. This ascent of water is dependent on the following physical factors:

• Cohesion – Mutual attraction between water molecules
• Surface tension – Responsible for the greater attraction between water molecules in liquid phase than in gaseous phase
• Adhesion – Attraction of water molecules to polar surfaces
• Capillarity –Ability of water to rise in thin tubes

These physical properties of water allow it to move against gravity in the xylem.

Cohesion: Mutual attraction between water molecules is called cohesion.

Adhesion: Attraction of water molecules to polar surfaces is called adhesion.

Surface Tension: Any liquid has a tendency to occupy the least possible surface area. This property is called surface tension.

The above mentioned properties impart high tensile strength to water. The high tensile strength imparts an ability to resist a pulling force and high capillarity. The ability to rise in tubes is called capillarity. The thin tubes of xylem work like capillary tubes.

The transpiration driven ascent of xylem sap depends mainly on the following physical properties of water.

• Cohesion – mutual attraction between water molecules.
• Adhesion – an attraction of water molecules to polar surfaces.
• Surface Tension – water molecules are attracted to each other in the liquid phase more than to water in the gas phase

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.