Question

Upward movement of water through xylem is best explained by

• A. Cohesion tension theory
• B. Pulsation theory
• C. Capillarity theory
• D. Root pressure theory

Answer 1

Answer: (A)

Cohesion tension theory

The upward movement of water is best explained by the cohesion-tension theory, also known as transpiration pull theory. The theory was given by Dixon and Jolly in 1894. The theory is based on following features:

1. Cohesive and adhesive properties of water molecules to form an unbroken continuous water column in the xylem.

2. Transpiration pull or tension created in this water column.

The water is filled in the xylem capillaries and due to cohesion and adhesion properties of water, it forms a continuous water column. The water column cannot be broken or pulled away from the xylem walls due to cohesion and adhesion of water. The water column is subjected to various forces which try to break it. These forces are the weight of column itself and resistance put to it during translocation. Since the magnitude of the cohesive force is much high (up to 350 atm.) the column is not broken. The water column is just like a steel rope which is extended from substomatal cavities in leaves to roots. If the rope is pulled from the top, the entire rope will move upward. In plants, the pull is generated by the process of transpiration which is known as transpiration pull.

Thus, the correct answer is option A.