Question

A rising column of water does not break its connection in the xylem vessels, despite negative pressure or tension due to

• A. Cohesion among water molecules
• B. Strong transpiration pull
• C. Adhesion
• D. Surface tension

Answer 1

The correct option is A Cohesion among water molecules

Cohesion-Tension or Cohesion Transpiration pull theory was put forward by Dixon and Jolly in 1894. It is also known as Dixon's theory of Ascent of Sap. The important points are:

1. Cohesion (attractive force between water molecules) and Adhesion (attractive force between water and xylem channels) Forces.

2. Continuous water columns.

3. Transpiration pull.
Water molecules are held together by strong cohesion force which is due to hydrogen bonds amongst them. It is due to these forces that water forms a continuous column in xylem elements. If there is an air bubble or air gap in this continuous column then water will not rise in the xylem elements. This break in rise of water is called as cavitation.

Thus, the correct answer is option A.