Transpiration process in plants creates a suction pressure which pulls up water from xylem of the roots to the stem and then to the leaves.
Xylem tissues are in the form of capillary tubes (tracheids and fibres) where narrower the diameter, greater will be the force.Whenever the xylem vessels lay empty, such as during the loss of water by transpiration, the water from below rises into them by a capillary force.
Due to transpirational pull more and more water molecules are pulled up due to their tendency of remaining joined (cohesion). Such pulling force created by the leaves is very important in the case of tall trees where an upward conduction of water takes place.
