Explain cohesion tension theory

Cohesion-tension theory was proposed by Dixon and Jolly in 1894.

Water molecules have a strong mutual force of attraction called cohesive forcedue to which they cannot be easily separatedfrom one another. The attractionbetween a water molecule and the wall of the xylem element is called adhesion. These cohesive and adhesive force works together to form an unbroken continuous water column in the xylem.

The upward movement of water is mainly due to the creation of a negative force or tension attributed to the continuous evaporation of water at the surfaces of leaves in the process of transpiration. As molecule after molecule of water evaporates through the stomata, it creates a pulling action on the next molecule of water in the transpiration stream. This pulling force,referred to as transpiration pull, is strong enough to overcome the force of gravity which is responsible for the tendency of water to move downward.

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