How does ascent of sap occur in tall trees ?

Explanation

In tall trees ascent of sap occurs due to transpiration. It depends mainly on the following physical properties of water.
1. Cohesion - mutual attraction between water molecules.
2. Adhesion - attraction of water molecules to polar surface (such as the surface of tracheary elements)
3. Transpiration pull - driving force for upward movement of water.
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.
Photosynthesis needs water and it is supplied by xylem vessels from roots to leaf veins.
Transpiration pull is created due to evaporation of water through the stomata.