Question: 119: The ascent of xylem sap in plants is mainly accomplished by the :
(1) size of the stomatal aperture
(2) distribution of stomata on the upper and lower epidermis
(3) cohesion and adhesion between water molecules
(4) root pressure
Answer: Option (3)
Explanation:
The ascent of xylem sap refers to the upward movement of water and dissolved minerals from roots to aerial parts of the plant.
This process mainly occurs due to the cohesion–tension mechanism.
Cohesion is the force of attraction between water molecules, which keeps them united in a continuous column inside the xylem vessels.
Adhesion is the force of attraction between water molecules and the walls of xylem vessels,
which helps water adhere to the vessel walls.
Due to transpiration from leaves, a negative pressure or tension is created at the top of the xylem.
This tension pulls the continuous column of water upward, made possible by cohesion and adhesion forces.
Stomatal aperture size and distribution affect the rate of transpiration but do not directly cause ascent of sap.
Root pressure plays only a minor role and is insufficient to account for the ascent of sap in tall plants.
Therefore, the ascent of xylem sap is mainly accomplished by cohesion and adhesion between water molecules.