Explain the Transpiration pull theory of ascent of sap. Add a note on its merits of demerits.

Explanation

It is one of the most successful physical theories of ascent of sap. Dixon and Jolly proposed this theory. According to this theory, ascent of sap is due to three factors namely,
(a) Cohesive force of water
(b) Adhesion of water molecules to the walls of the xylem vessel
(c) Transpiration pull
A strong inter molecular force of attraction exists between water molecules. Thus water molecules are bound to each other forming a continuous column of water.
There is a force of attraction between water molecules on the inner walls of xylem elements. Therefore water molecules are attached to the wall of xylem. It is called adhesion.
Due to adhesive and cohesive properties of water, a contiunuous column of water is present in xylem. Transpiration at the leaf surface force the mesophyll cells of the leaves to draw water from the neighbouring xylem elements. This creates a suction force known as Transpiration pull. This section force pulls the water column in the xylem upwards. Thus ascent of sap is due to combined effect of adhesive, and cohesive properties of water and transpiration pull.
Merits:
1. The ascent of sap is directly proportional to the rate of transpiration.
2. It is a physical process, and does not need energy.
3. Strong cohesive and adhesive forces are sufficient to prevent the rupture of water column in xylem vessels.
Demerits:
1. Presence of air bubble breaks the continuity of water column.
2. Ascent of sap continuous even in the absence of transpiration, as at night times.
3. Strength of water column is thoughtful against two opposing forces such as gravitations force and transpiration tension.