Question

Physical force theory explains
A. Non- living cells are not essential for the ascent of sap
B. Living cells are not essential for sap
C. The ascent of sap may occur in both living and non-living cells
D. Both B and C

Answer 1

Hint:-
The physical force theory explains that the ascent of sap in plants occurs through the help of physical force. Various known physical force theories are the imbibition theory, cohesion of water and transpiration pull theory (most widely accepted), atmospheric theory, capillary force theory, and Jamin’s chain theory.

Complete step-by-step solution:- According to the physical force theory, there is a requirement of only physical forces for the ascent of sap. This process does not require the help of plant protoplast, hence living cells are not essential. The ascent of sap only occurs through non- living cells. The ascent of sap is the upward movement of water and minerals from the roots to other parts of the plant. It takes place with the help of xylem tracheids and vessels.
Cohesion tension theory also called cohesion of water and transpiration pull theory was proposed by Dixon and Joly in 1894. According to this theory, the upward transport of water is aided through transpiration and capillary action of water. This movement is against gravity and can occur due to the cohesion of water i.e. attraction between water molecules. This cohesion produces surface tension and water gets pulled upwards from the roots. Water rises due to the suction effect of the transpiration pull. Adhesion of the xylem vessels due to the presence of lignin helps with the continuous flow of water without breaking off the water column.
The correct answer is B) living cells are essential for sap. This is because the only xylem which is a dead tissue is responsible for it.

Note:-
Phloem cells are not responsible for water transport as they are living cells. They have protoplasts and proteins that aid in transport. These cells are responsible for the movement of food for plants. The movement in the phloem is bidirectional but the movement in the xylem is unidirectional. The ascent of sap is possible but there is no descent of sap.