1. This is very widely accepted theory of ascent of sap proposed by Dixon and Joly. 

2. It is based on principles of adhesion and cohesion of water molecules and transpiration by plants. 

3. A strong force of attraction existing between water molecules is cohesion and the force of attraction between water molecules and lignified walls of xylem elements is adhesion. 

4. Ascent of sap occurs through lumen of xylem elements. 

5. Owing to cohesive and adhesive forces a continuous water column is maintained in xylem from root to aerial parts i.e. leaves. 

6. Transpiration occurs through stomata and transpiration pull is developed in leaf vessels. 

7. This tension or pull is transmitted downwards through vein to roots which triggers ascent of sap.

8. In transpiration, water is lost in vapour form and this increases D.PD. of mesophyll cells that are near guard cells. 

9. Mesophyll cells absorb water from xylem in leaf and a gradient of D.PD. or suction pressure (S. E) is set. 

10. Owing to this gradient from guard cell to xylem in leaf, a transpiration pull or tension is created in xylem. 

11. Hence water column is pulled upward passively against gravity