This water is conducted upwards to the leaves through xylem. The ascent of sap in the xylem tissue of plants is the upward movement of water and minerals from the root to the crown. Various theories have been put forward by different scientists to explain the process of ascent of sap. But, the most widely accepted theory is Dixon’s theory of ascent of sap. The upward movement of water and dissolved mineral salts from roots upwards to the leaves and other aerial parts is called Ascent of Sap. 1. mechanism of ascent of sap 1. Physical theories. 2. Vital force theories, B. Root pressure theory, and C. Physical force theory. (a) ROOT PRESSURE The hydrostatic pressure generated in the root which forces the water upward in the stem is called root pressure. It is dilute solution of mineral ions absorbed from the soils. Xylem is a complex tissue consisting of living and non-living cells. Root pressure forces the water up from below. 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 THEORIES EXPLAINING ASCENT OF SAP. 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. Many theories have been proposed to explain the mechanism of ascent of sap. They are as follows: Vital theories. Root pressure theory 3. Physical force theories . 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. Mechanism of transportation of water in plants. Although a mechanism of ascent of sap is not well understood, a number of theories have been put forward to explain it. They are, A. Ascent of sap involves root pressure and transpiration pull. 1. Root pressure forces the water up from below. Typical red wood tree: About 350 ft. Vital theories. 1. A number of theories have been put forward from time to time to account for the ascent of sap. Mechanism of Ascent of Sap . A number of theories have been put forward to explain the mechanism of the ascent of sap. Root pressure theory. 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. The conducting cells in xylem are typically non-living and include, in various groups of plants, vessels members and tracheids. In ascent of sap, the biggest challenge is the force required to lift the water to the top of the tallest trees. Vital theories 2. (It is called the sap as it contains many dissolved minerals). (a) ROOT PRESSURE. 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. The hydrostatic pressure generated in the root which forces the water upward in the stem is called root pressure. Vital theories: Westermaier in the year 1883 for the first time suggested that living cells take part in ascent of sap. Mechanism of ascent of sap. Australian Eucalyptus: About 450 ft. Mechanism of ascent of sap. Vital Force Theories According to this theory the water present in the leaves is continuously evaporated by sunlight falling over them. 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. According these theories, the living cells of the plants are responsible for the ascent of sap… A number of theories have been put forward to explain the mechanism of ascent of sap as follows. 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