Question

Long answer question.

Explain cohesion theory for translocation of water.

Answer 1

1. This theory was put forth by Dixon and Jolly (1894).
2. This is presently a widely accepted theory explaining the ascent of sap in plants.
3. This theory is based on two principles i.e. Cohesion and adhesion, and transpiration pull.
4. Cohesion and adhesion:
   1. A strong force of attraction between water molecules is called cohesive force.
   2. While a strong force of attraction between water molecules and the lignified wall of the lumen of the xylem vessel, is called adhesive force.
   3. Due to combined cohesive and adhesive forces a continuous water column is developed (formed) in the xylem right from root up to the tip of the topmost leaf in the plant.
5. Transpiration pull:
   1. The transpiration pull developed in the leaf vessel is transmitted down to the root and thus accounts for the ascent of sap.
   2. Excess water is lost in the form of vapour, mainly through the stomata found on a leaf.
   3. This water loss increases the D.P.D. of mesophyll cells. These cells withdraw water ultimately from the xylem in the leaf.
   4. In other words, due to continuous transpiration, a gradient of suction pressure (i.e. D.P.D.) is developed right from guard cells up to the xylem in the leaf. This will create a tension (called a negative pull or transpiration pull) in the xylem.
   5. Consequently, the water column is pulled out of xylem. Thus, water is pulled upwards passively against the gravity leading to the ascent of sap.