what are the main Avidances to support transpiration pull theory
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Answer:
Cohesion and Transpiration Pull Theory was first proposed by Dixon and Joly (1894) and is based on the following features:
(i) Cohesion and Adhesion: Mutual attraction between water molecules is called cohesion. The walls of tracheids and vessels of xylem are made-up of lignin and cellulose and have a strong affinity for water (adhesion).
(ii) Tension: Transpiration pull develops a negative pressure or tension in xylem sap which is transmitted down to the root.
The moist walls of mesophyll cells in leaf lose water vapour through stomata and cuticular transpiration. Sufficient quantity of water is transpired through intercellular spaces of the mesophyll cells. This is because the dry air outside the leaf has lower water potential than the moist air of leaf. As a result, water diffuses out of stomata and diffusion pressure deficit (DPD) increases. As a result, water is sucked from adjoining mesophyll cells and ultimately from vascular tissue. This tension is transmitted to the roots. The molecules of water show cohesion and molecules of water and vessel wall show adhesion. Because of cohesive and adhesive forces, water column does not break and is pulled upward by the force called as transpiration pull.
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