Please explain Coehsion-Tension theory.
And also tell about adhesion in moving the water against gravity?

Dear Student. 
 
1) The theory of  Cohesion - Adhesion was proposed by Dixon and Jolly. According to this theory, a large tension (negative hydrostatic pressure) is developed at the top of a plant, due to which water is pulled from the roots through the xylem. That is why water moves upwards.

It is also called ascent of sap. It has two main features:
a) Cohesion of water and adhesion between water and xylem - Water molecules are attracted to each other hence remains as a continuous channel. Also water molecules remain attracted by adhesive forces to the xylem elements. Both these forces play a role in maintaining the continuity of water column from the root to the leaves.

b) Transpiration pull - It is the pulling force generated in the xylem elements due to loss of water by transpiration. the loss of water creates a negative hydrostatic pressure that exerts pulling force.

The transpiration pull exerts a pulling force on whole of the water column in the xylem of the plant and pushes the water column in the upward direction called ascent of sap.


2) The primary reason behind the transportation of water is the tension which is created in the plant due to transpiration, thus due to pressure differences water is moved upwards. Similarly, adhesion of water molecules is another reason due to water can move up against the gravity. Majority of water is transported through transpiration pull. Root Pressure Positive pressure created inside the xylem when water follows the ions transported along the concentration gradients into the vascular system .This can be observed when a freshly cut root stump continues to exude sap from its xylem vessels. The mechanism probably depends on active secretion of salts or other solutes into the xylem sap, thus lowering its water potential. Water then moves into the xylem by osmosis from neighbouring root cells. Root pressure can transport water and dissolved mineral nutrients from roots through the xylem to the tops of relatively short plants when transpiration is low. 


Regards