Question

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

• A. If both assertion and reason are true and reason is the correct explanation of assertion
• B. If both assertion and reason are true but reason is not the correct explanation of assertion
• C. If assertion is true but reason is false
• D. If both assertion and reason are false

Answer 1

Answer: (B)

Transpiration pull or cohesion - tension theory is based upon the cohesive and adhesive properties of water molecules, which forms the unbroken continuous water column in the xylem.
Water molecules are held together by strong cohesion force which is due to hydrogen bonds amongst them. There is another force of adhesion which holds water to the walls of xylem vessels. Water molecules are attracted to one another more than the water molecules in the gaseous state. It produces surface tension that accounts for high capillarity through tracheids and vessels. Water column is present in tracheary elements. There is a continuous column of water from roots through the stem and into the leaves. These tracheary elements form this continuous system through their unthickened areas. Since, a large number of tracheary elements are present together, no breakage in the continuity of water occurs even if there is a blockage of one or few of them.