Explain the transpiration pull theory for ascent of sap.

Hint: The pulling force that is responsible for lifting the water column as water is lost in the form of water vapour to the atmosphere from the mesophyll is called transpiration.

Complete answer:
Transpiration is the process by which water is carried through plants from roots to small pores on the underside of leaves, where it changes to vapour and is released to the atmosphere.Intercellular spaces present amongst mesophyll cells of the leaves are always saturated with water vapors.The intermolecular spaces of mesophyll cells are connected to the outside air through stomata. Outside air is seldom saturated with water vapour. It has a low water potential than the moist air present inside the leaf.

Therefore, the water vapor diffuses out of the leaf. The mesophyll cells continue to lose water to enter cellular spaces. As a result, curvature of the meniscus holding water increases resulting in increased surface tension and decrease in water potential sometime 30 bars. The mesophyll cell withdraws the water from the deeper cell as its molecules are held together by the hydrogen bonds.The deeper cells intern obtain water from the tracheary elements. The water in the tracheary elements would come under tension.

Note: Transpiration rate is much greater than that of exudation. Transpiration is responsible for absorption of major quantities of water by roots by way of passive mechanism without requiring energy in the form of ATP.