Question

adhesion of water molecules to walls of xylem cells

Answer 1

Due to adhesion of water molecules to walls of xylem the capability of water chain molecules increases .
It helps in transpiration pull.

Answer 2

At night, when stomata close and transpiration stops, the water is held in the stem and leaf by the cohesion of water molecules to each other as well as the adhesion of water to the cell walls of the xylem vessels and tracheids. This is called the cohesion–tension theory of sap ascent.