Question

Explain 'Dixon-Jolly' of ascent of sap with diagram.

Answer 1

'Dixon-Jolly' theory: This theory was proposed by Dixon and Jolly in 1894. According to this theory, water molecules are in a continuous movement and are strongly attracted to each other to form a continuous column of water xylem elements and water is pulled upwards by transpiration.

The essential features of this theory are as follows-

(1) Strong cohesion force of Tensile strength of water: The principle is mainly based on facts that: (i) Water molecules have a strong mutual attraction (cohesion) with each other, and cannot be easily separated from one another. (ii) An attraction also exists between water molecules and the wall of xylem elements(cohesion). The cohesive and adhesive attractions together maintain water column in xylem tracheids.

(2) Continuity of water column in the plant: A strong objection against the theory has been the evidence for the occurrence of air bubbles in the water column of xylem elements. They disturb the continuity of the water column.

(3) Transpiration pull or tension on the unbroken water column: As a result of transpiration, water is lost from the mesophyll cells to the intercellular spaces. The water vapor moves out of the plant through stomata. Hence the DPD of mesophyll increases. With the increase in DPD, these cells absorb water from adjoining cells. Ultimately the water is absorbed from xylem elements of vascular bundles of the leaf. Since the xylem elements are filled with continuous water column this tension of pull is transmitted down from roots leading to upward movement of water.

Thus, water ascends in the plants because of transpiration pull and the column of water remains continuous because of cohesive force of water molecules. This theory is the most accepted theory at the present time.