The theory for the ascent of sap is known as the cohesion-tension-transpiration pull theory. It has two main points-1.The water molecules are attracted to each other due to cohesion which forms a continuous channel. Water molecules are also attracted by xylem elements due to adhesion. So cohesive and adhesive forces are responsible to form a continuous channel between roots and the leaves.2.Due to loss of water by transpiration, the pulling force(transpiration pull) is developed that causes water to move in the upward direction.