Root pressure forces the water up from below. Further, it has been proved experimentally that sap moves up the stem through the lumen of xylem vessels and tracheids and not through their walls. The ascent of sap takes place in the lumen of vessels and tracheids situated between the parenchyma root hair and cortex of the root below, and the mesophyll tissue of the leaf above. A number of theories have been put forward from time to time to account for the ascent of sap. Live pump theories: Plant physiologists of 1880’s like Westermeir, Godlewski, Sir J. C. Bose (1920s), Priestly and others opined that the ascent of sap or water is due to pump like activity of living cells found among the dead xylem elements. Root pressure theory. Typical red wood tree: About 350 ft. Physical force theories . THEORIES EXPLAINING ASCENT OF SAP. They are as follows: Vital theories. Various theories have been put forward by different scientists to explain the process of ascent of sap. This water is conducted upwards to the leaves through xylem. They are, A. Explain mechanism of ascent sap - 2433421 Hey, friend, The fluid that moves upward in the stem is not pure water. Mechanism of ascent of sap. Vital force theories, B. Root pressure theory, and C. Physical force theory. Mechanism of transportation of water in plants. 1. Theories of ascent of sap : The various theories put forward to explain the mechanism of ascent of sap in plants can be placed in following three categories : (1) Vital force theories Vital theories 2. 2. Various theories based upon biological and physical phenomena were suggested to explain the mechanism of rapid translocation of water from roots to the uppermost leaves. Mechanism of ascent of sap. (a) ROOT PRESSURE The hydrostatic pressure generated in the root which forces the water upward in the stem is called root pressure. 1. A number of theories have been put forward to explain the mechanism of ascent of sap as follows. (It is called the sap as it contains many dissolved minerals). Physical theories. Xylem is a complex tissue consisting of living and non-living cells. (a) ROOT PRESSURE. According these theories, the living cells of the plants are responsible for the ascent of sap… Here are following theories which explain the ascent of sap in plants: a) Root pressure (b) Capillarity (c) Vital theory and (d) Cohesion-tension theory. Many theories have been proposed to explain the mechanism of ascent of sap. Root pressure forces the water up from below. The hydrostatic pressure generated in the root which forces the water upward in the stem is called root pressure. 1. But, the most widely accepted theory is Dixon’s theory of ascent of sap. The conducting cells in xylem are typically non-living and include, in various groups of plants, vessels members and tracheids. mechanism of ascent of sap 1. Vital theories. It is dilute solution of mineral ions absorbed from the soils. Vital Force Theories Although a mechanism of ascent of sap is not well understood, a number of theories have been put forward to explain it. Australian Eucalyptus: About 450 ft. In ascent of sap, the biggest challenge is the force required to lift the water to the top of the tallest trees. Vital theories. Ascent of sap involves root pressure and transpiration pull. The upward movement of water and dissolved mineral salts from roots upwards to the leaves and other aerial parts is called Ascent of Sap. Root pressure theory 3. The ascent of sap in the xylem tissue of plants is the upward movement of water and minerals from the root to the crown. According to this theory the water present in the leaves is continuously evaporated by sunlight falling over them. Mechanism of Ascent of Sap . Vital theories: Westermaier in the year 1883 for the first time suggested that living cells take part in ascent of sap. 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