Question

Who proposed the cohesion theory of 'ascent of sap'?
A. Strasburger
B. Goldweski
C. Western
D. Dixon and Jolly

Answer 1

Hint:This theory suggests that negative pressure or pull or suction is produced in the leaves as a result of the transpiration that forces the water upwards.

Complete answer:
The flow of water and dissolved mineral salts from roots to the leaves and other areas of plants is called ascent of sap.This water is taken to the leaves by xylem.

The Vital Theory was first suggested by Godiewski. According to this theory, the flow of water is due to the pumping action of the wood parenchyma and medullary ray cells.

Henry H. Dixon, an Irish botanist, and John Joly, a physicist, conceived the concept of a cohesion-tension system for the ascent of sap in 1894.

The theory explains that the ascent of sap is due to the transpiration pull or tension, water cohesion and hydration of cell walls (i.e. adhesion). The movement of water through the xylem is a bulk flow caused by the pressure difference from one end of the system to the other. Negative pressure or tension due to transpiration is conveyed to the root by a constant column of water within the xylem. Breaking the water column is avoided by cohesion between neighboring water molecules and adhesion between water molecules and cell walls.

Thus, the correct answer is option D i.e., Dixon and Jolly

Note:Water molecules are bound to each other by hydrogen bonding, which is why water forms a chain of molecules when moving towards xylem. The molecules of water bind together and get pushed up by the force called the tension. This force is exerted on the surface of the leaf due to evaporation.