Question

Ascent of sap through xylem due to transpiration pull depends upon

• A. mutual attraction between water molecules
• B. attraction of water molecules to other polar molecules
• C. surface tension
• D. all of the above

Answer 1

The correct option is D all of the above

The major force responsible for the ascent of sap is transpiration pull.

As the water is lost from an actively transpiring leaf, it creates a tension in the plant due to which the water column is pulled up from the roots causing ascent of sap. This tension caused by transpiration is called transpirational pull.

Cohesion-tension-transpiration pull theory was put forward by Dixon and Jolly to explain the ascent of sap. According to this theory, the upward movement of water driven by transpiration depends on certain physical properties of water like:
Cohesion, adhesion and surface tension.

The mutual attraction between water molecules is called cohesion.

The attraction of water molecules to the surface of xylem elements is called adhesion of water molecules.

Surface tension: Water has high surface tension due to hydrogen bonding in water molecules and in the liquid phase, there is greater attraction between water molecules than in gaseous phase.

All the above said properties of water give high tensile strength and capillary action. Tensile strength gives the ability to resist the pulling force caused by transpiration and capillary action allows water to move up through thin tubes of tracheary elements.