Explain about Cohesion - Tension Theory .

Explanation

Cohesion - tension theory was originally proposed by Dixon and Jolly (1894) and again put forward by Dixon (1914 , 1924) . This theory is based on the following features :
I . Strong cohesive force or tensile strength of water : Water molecules have the strong mutual force of attraction called cohesive force due to which they cannot be easily seperated from one another . Further , the attraction between a water molecule and the wall of the Xylem element is called adhesion . These cohesive and adhesive force works together to form an unbroken continuous water column in the xylem . The magnitude of the cohesive force is much high (350 atm) and is more than enough to ascept sap in the tallest trees .
ii. Continuity of thewater column in the introduction of air bubbles in the xylem . Gas bubbles expanding and displacing water within the xylem element is called cavitation or embolism . However , the overall continuity of the water column remains undisturbed since water diffuses into the adajacent xylem elements for continuing ascent of sap .
iii. Transpiration pull or Tension in the unbroken water column : The unbroken water column from leaf to root is just like a rope . If the root is just like a rope . If the rope is pulled from the top , the entire rope will move upward. In plants , such a pull is generated by the process of transpiration which is known as transpiration pull. Water vapour evaporates from mesophyll cells to the intercellular spaces near stomata as a result of a active transpiration . The water vapours are then transpired through the stomatal pores . Loss of water from mesophyll cells causes a decrease in water potential . So water moves as a pull from cell to cell along the water potential gradient . This tension , generated at the top (leaf ) of the unbroken water column , is transmitted downwards from petiole , stem and finally reaches the roots . The cohesion theory is the most accepted among the plant physiologists today .