Explanation

The most accepted theory of ascent of sap is transpiration pull theory or cohesion-tension theory proposed by Dixon and Jolly (1894). This theory states that water forms a continuous column from root to leaf through xylem ducts. The loss of water from mesophyll by transpiration creates transpiration pull or tension which is transmitted downwards. The column of xylem resists breaking due to force of cohesion between water molecules and adhesion between water and the wall.