How does ascent of sap occur in tall trees ?

Explanation

Water is mainly .pulled through the plant, and that the driving .force for this process is transpiration from the leaves. This is referred to as the cohesion - tension -transpiration pull model of water transport. 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 surfaces (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 infrom roots fo leaf veins. Transpiration pull is created, due to evaporation of water through the stomata.