Question

A column of water within xylem vessels of tall trees does not break under its weight because of

• A. lignification of xylem vessels
• B. positive root pressure
• C. dissolved sugars in water
• D. tensile strength of water

Answer 1

The correct option is D tensile strength of water

Transpiration is the major driving force for the ascent of sap. Transpiration creates a tension that causes the pulling of the water column up against gravitational force.

The water column moves up against gravity due to 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 other polar surfaces such as 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 mentioned properties of water give high tensile strength and capillarity. Tensile strength gives water the ability to resist the pulling force and capillarity allows water to move up through thin tubes of tracheary elements.