Evaporation of the water is the process that exerts the pull on water molecules that is relayed from leaf to root via cohesion. The pressure of the water present in the leaves becomes less due to evaporation of water. The pressure of water in the roots and lower part of the shoot is high. This gradient of pressure creates a pull on the water molecules present in the roots to move upwards to the leaves to occupy the vacant space. The whole process is called transpiration.
