CBSE Class 10 Answered

As the water is lost from leaf surface by transpiration, more water molecules (along with dissolved substances i.e. sap) are pulled up through continous column of xylem present from leaf to root. This is due to the tendency of water molecules to remain joined to each other (cohesion) and to the xylem tissues (adhesion). 

Transpiration pull also develops a negative pressure. Loss of water from mesophyll cell of leaf results in diffusion of water from adjecent vascular tissue (xylem). This tension is transmitted all the way down to unbroken column of water to roots, which ultimatly cause water (sap) absorption  

There is thus a continuous flow of water ( and sap) from root to leaf.