This can be explained with the help of cohesion and the transpiration pull theory. According to the theory put forward by Dixon and Joly. The strong force of cohesion and the continuity of the water column in the xylem vessel elements and the transpiration pull are responsible for the ascent of sap. The cohesive and the adhesive property of the water which builds up an unbroken continuous column of water.
The water potential of the dry air is lower than the intercellular space of the mesophyll cells since it is accumulated with the water vapor. Thus the water vapor diffuses out through the stomata and the process is known as the transpiration. Thus due to transpiration the water potential of the surrounding cells get reduced so that the water is drawn from the adjacent cells and then ultimately from the xylem cells so as to meet the demand of the loss of water through transpiration. Due to transpiration continuous flow of water occurs and a tension is created inside the xylem cylinder and this pull or tension is known as the transpiration pull. The transpiration pull exerted on the continuous water column lifts the water to the tips of the tallest trees. This tension in the xylem vessel element is transmitted to the root tip which is in contact with the water present in the soil.