Question

Which of these factors do not influence the ascent of sap in trees?
A) Continuity of water column
B) Cohesion and adhesion of water molecules
C) Transpiration pull
D) Pressure of tracheary elements

Answer 1

The upward movement of water and dissolved mineral salts from roots upwards to the leaves and other aerial parts is called Ascent of Sap. This water is conducted upwards to the leaves through xylem.

Complete answer:
The ascent of sap can be explained by transpiration pull theory. It was originally proposed by Dixon and Joly and Dixon further elaborated it later. This theory is most convincing and has now been widely supported. According to it, following features are responsible for ascent of sap:

Cohesion among water molecules
Adhesive properties of water molecules forming a continuous water column.
Transpiration pull
Water molecules stay together due to presence of H-bonds causing cohesion between them. The attraction between the water molecules and the walls of xylem called adhesion further ensures the continuity of the water column in xylem. At the upper regions of the plant, when transpiration takes place in leaves, water is evaporated from the leaves to the outer atmosphere through the stomata. The lost water is replaced by more water from mesophyll cells into the intercellular spaces from the mesophyll cells which causes more water to enter mesophyll cells from the xylem of the leaf.
This builds up tension in water present in the xylem elements of the leaves which is transmitted downwards till the xylem elements of roots due to which water is pulled upward in the form of continuous water column to reach the apex of the plant.

Thus, the correct option is- D. Pressure of tracheary elements.

Note: For ascent of sap, transpiration pull acts majorly along with adhesive and cohesive nature of water molecules. Water can move as high as the apex of the plants, which can even be up to several hundred feet tall.