57.

Give an account of the cohesion and transpiration pull theory of ascent of sap.

Cohesion is the phenomenon of attraction between similar molecules. The water molecules remain attracted by the cohesive force and cannot be easily separated from one another. Further, there is attraction between water molecules and the inner wall of xylem ducts because of the forces known as adhesion forces. Then water column cannot be pulled away from the wall of xylem ducts due to strong adhesive and cohesive properties of water and the continuity of water column is maintained from roots to leaves.

According to this theory, evaporation of water from the leaf to the atmosphere results in a decrease in the water potential of the epidermal cells, which are in direct contact with the atmosphere. The water lost from epidermal cells is replaced by the water moving from the adjacent cells along a water potential gradient. The water lost from the leaf cells is replaced by the water from the xylem elements in leaf veins. Due to this pattern of water movement, a tention is created in water in the xylem elements of the leaves. This tention is transmitted downward in water in xylem elements of the roots through the xylem of petiole and stem and hence, the water is pulled upward in the form of continuous unbroken water column to reach the transpiring surface upto the top of plants.