Question

Assertion: Cohesion, adhesion, and surface tension give high tensile strength to water.
Reason: Capillarity is aided by the small diameter of the tracheary elements.

A) Both assertion and reason are correct and reason is the correct explanation for assertion.
B) Both assertion and reason are correct but reason is not the correct explanation for assertion.
C) Assertion is correct but reason is incorrect.
D) Both assertion and reason are incorrect.

Answer 1

Hint: Cohesion are the attractive forces between the same types of the molecules. Adhesions are the attractive forces between the different types of the molecules and when cohesive force between the molecules causes the liquid surface to contract to the smallest area is called surface tension.

Complete Answer:
- Dixon in 1914 introduced the cohesion-tension theory or transpiration pull. This theory is based on the adhesive and cohesive properties of the molecules of water that form an unbroken continuous column of water in the xylem.
- As we know that molecules of water are held together by the strong cohesive forces due to the high hydrogen bonding among them. The other force is adhesive force which helps the water to hold on to the walls of xylem vessels. Water molecules are attracted to each other more than in the gaseous state.
- Then, it induces surface tension that provides high capillarity through tracheid and other vessels. There is a formation of continuous water columns from roots to stems and then into leaves. Capillarity through tracheary elements form a continuous system by their unthicken area.

Thus, the correct answer is option B. i.e., Both assertion and reason are correct but reason is not the correct explanation for assertion.

Note: When there are a large number of tracheary elements present. Then there will be no breakage of water continuity. The basic features of cohesion-tension theory: cohesive and adhesive properties, transpiration pull and continuous water column.