Transpiration Pull in Plants MCQ - Practice Questions with Answers

• According to these theories, the ascent of sap is due to some physical forces.
• Boehm’s Concept: It suggests that capillary action and atmospheric pressure causes ascent of sap. As per this concept, water can rise up to 32 ft.
• Sach’s Concept: It suggests that the force of imbibition causes ascent of sap.
• The most widely accepted theory called Cohesion of Water Theory or Transpiration Pull Theory was given by Dixon and Jolly in 1894.

The cohesion of Water Theory or Transpiration Pull Theory:

• This theory is based on the following three assumptions:
  • Force of cohesion between the water molecules
  • Continuity of water column
  • Transpiration pull
• The hydrogen bonding between the water molecules causes a strong force of cohesion between the water molecules. 
• The force of cohesion is responsible for the high tensile strength of water.
• There exists a force of adhesion between the water column and walls of xylem tracheary elements.
• The forces of adhesion and cohesion maintain the column of water such that it is not broken or pulled away from the walls of tracheary elements.
• The magnitude of cohesion is too large to be broken by resistances like the weight of water column, resistance during translocation of water and air spaces in the intercellular spaces.
• Transpiration increases the osmotic pressure of the cells of leaves.
• These cells draw water from the cells having low osmotic pressure. 
• Water is drawn from the veins of the leaves.
• Veins, in turn, draw water from the xylem of the main stem.
• Thus, negative or pulling pressure is generated in the shoot system.
• This causes rapid passive absorption of water by the roots. 
• It is strong enough to raise the column of water to great heights.
• Water potential of -30 Bar has been recorded in the leaves that are present on the top of the trees and are transpiring actively.