In plant physiology, the 'Transpiration pull' theory, also known as the cohesion-tension theory, was proposed to explain the ascent of sap in tall trees. The key forces involved are:
A. Root pressure and capillarity
B. Cohesion between water molecules, adhesion to xylem walls, and tension created by transpiration
C. Osmotic pressure in the leaves
D. Active transport by xylem parenchyma
Answer: Option B
Solution (By JKSSB Mock Tests)
The cohesion-tension theory, primarily proposed by Dixon and Joly (and later supported by Denison), is the most widely accepted mechanism for the ascent of sap in tall trees. It relies on the physical properties of water. Transpiration from the leaves creates a negative pressure (tension) in the xylem. This tension pulls the water column upwards. The water column remains unbroken due to the high cohesive force between water molecules (hydrogen bonding) and the adhesive force of water to the hydrophilic walls of the xylem vessels and tracheids. This creates a continuous, passive pull from the top.
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