Explanation

Transpiration pull. If the stem of plant is cut under a state of tension in xylem sap, the air will be pulled into the xylem and the transport of water (xylem sap) remains in continuity. However, it can be discontinued with the introduction of air bubble in the xylem. Copeland (1902) believed that air bubbles enter into the xylem and break the tensile strength or cohesion force between the water molecule