Explain the transpiration pull theory of ascent of sap.

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Hint: The phenomenon of losing water to the surroundings in the form of water vapours from the exposed surfaces of plants parts is known as transpiration. 80-90% of transpiration occurs mainly through stomata which are specialised minute epidermal structures generally present on leaf epidermis.

Complete answer:
Water and minerals from the soil enter the plant through the epidermis of roots, moving radially across the root cortex, they pass into the xylem. From there the xylem sap, the water and dissolved minerals in the xylem, moves upward. It is vitally important for a plant to transport water and minerals from the soil to its uppermost leaves. Water and minerals move upward from root to aerial parts against the gravitational force. This movement is known as the ascent of sap.
Scientists have agreed that there is a ‘pull’ that leads to the movement of water even to the highest tops of trees and the driving force behind this pull’ is transpiration from the leaves. Transpiration generates pressure in the xylem of leaves to pull the water upward. Dixon and Jolly’s cohesion- tension theory or transpiration pull is the most accepted theory for upward movement of water in the xylem. Some physical properties of water such as cohesion (the tendency of water molecules to cling to one another), adhesion (attraction of water molecules with the walls of xylem)and surface tension support the formation of the water column in the xylem vessel and their upward movement.
These properties give water a high tensile strength i.e the strength to withstand a pulling force and high capillarity i.e the capability to rise in a thin tube. In plants, the small diameter of tracheids and vessel elements is the reason for the high capillarity. When water evaporates through the stomata, one continuous thin film of water over the cells is created. Water is pulled, molecule to molecule, into the leaf from the nearest xylem. Another factor is because of the lower concentration of water vapour in the atmosphere as compared to the substomatal and intercellular space in the leaves, water diffuses into the surrounding air and thus creates a pull.

 -Various external factors such as temperature, humidity, light influence the rate of transpiration. With the increase in temperature, water evaporates rapidly and hence the plant transpires rapidly. Light induces the opening of stomata as well as warms the leaf, which is enough to increase the transpiration rate.
-If the surrounding environment has more water vapour i.e the weather is humid, then less water is lost due to transpiration. Also, if the wind is present, then it carries away humid air that gets replaced by drier air, thus, again increasing the rate of transpiration.