12. Discuss the factors responsible for the 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 the liquid phase than in the 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.