Explain the cohesion tension theory.

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Hint: The cohesion tension theory explains the ascent of sap through the Xylem tissues. The water and other nutrients are absorbed by the root and are transported to the leaves by the xylem. The water is transported by the process of osmosis.

Complete answer:
Ascent of sap is the translocation of water molecules from the root to the leaves. It is an upward movement.
Henry H. Dixon and John Jolly put forward the cohesion-tension mechanism of ascent of sap in 1894.
The ascent of sap occurs between the root which is the region of positive pressure and the shoot which is the region of negative pressure.

The upward movement of water is due to the strong force of attraction between the water molecules which cannot be broken easily. This is called force of cohesion. There exists a force of adhesion between the water and the walls of the xylem. These two forces together are responsible for the continuous movement of sap in the xylem vessel.
The upward force is generated by the transpirational pull which is generated through loss of water through stomata. This creates a pulling action called the transpirational pull. This force is strong enough to pull water against the gravity.

Note: The transport of water through the xylem vessels is termed as passive transport. The other forces responsible for the ascent of sap is the capillary action, root pressure and other passive forces that develop in the plants. Also certain environmental factors like sunlight, moisture is also responsible.