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What is the ascent of sap? Explain the cohesion-tension theory for it.

Last updated date: 13th Jun 2024
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Hint: Plants consist of xylem and phloem as their vascular elements which transfer water and minerals from the soil and food from leaves to all parts of the plant respectively. This transfer is aided with the help of some mechanisms which are forced by tension.

Complete answer:
The upward movement of water and minerals from the roots to the top of the plants via xylem is known and the ascent of sap. This is a phenomenon that is dependent on the loss of water during transpiration from the leaves. The loss of water is compensated with the upward movement of water present in the xylem vessels of the plant by the tension created known as transpiration pull.
Cohesion tension theory was developed by an Irish botanist Henry H. Dickson in 1894 and this work was published in 1914. This theory states that there are several factors responsible for the ascent of sap like – transpiration pull which is referred to as tension, cohesive property of water and adhesion or hydration of the cell walls. This theory states the following assumptions –
-Cohesion – water molecules are attracted to each other and this is the property of cohesion that makes the water column from the roots to the topmost tips of the plant in the xylem tubes.
-Tension – Due to transpiration water is lost from the leaves thus creating a negative pull for the water molecules which are present in the xylem tubes forming a water column with the help of hydrogen bonds present in between them. These water molecules move to the place from where water has been lost thus an upward movement of water takes place. This is known as transpiration pull.
-Adhesion – the water molecules present in the xylem tubes adhere to the walls of xylem cells and keep the continuity of the water column.

Note: Though the cohesion - tension theory is the most accepted theory to explain the process of ascent of sap but it faced objections as the gases dissolved in the water produce air bubbles which create discontinuity in the water column. This discontinuity produced in the water column by air bubbles is known as cavitation.