Question

The most widely accepted theory that explains the ascent of sap in tall trees is

• A. Root pressure theory
• B. Cohesion tension transpiration theory
• C. Mass flow hypothesis
• D. Both a and b

Answer 1

The correct option is B Cohesion tension transpiration theory

Several theories were put forward to explain the ascent of sap. Out of them, two are very significant to understand the ascent of sap.
Root pressure theory and transpiration pull theory.
The positive hydrostatic pressure developed in the xylem of roots due to the entry of water is called root pressure and it is responsible for the ascent of sap. But root pressure can push the water column only upto a small height and cannot explain the ascent of sap in tall trees.

Cohesion tension transpiration theory was proposed by Dixon and Jolly which explains the ascent of sap in tall trees. According to this theory, the upward movement of water driven by transpiration depends on certain physical properties of water such as cohesion, adhesion and surface tension.

Mass flow hypothesis explains the movement of photosynthates (sugars) through phloem.