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Describe in detail the mechanism of the ascent of sap in trees.

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Mechanism of Ascent of Sap: 

To explain the upward rise of water and dissolved minerals against gravitational force, several theories have been put forwarded. These theories have been divided into three categories: 

1. Vital Force theories 

2. Root Pressure theory 

3. Physical Force theories 

1. Vital Force Theories

1. According to these theories ascent of sap is the result of force developed by activities of living cells. 

2. Opinions expressed by some scientists in this regard are as follows: 

  • Relay Pump Theory: 

According to Godlewski (1884), the living cells of xylem parenchyma and medullary rays show a periodic change in their osmotic pressure. According to this theory, the upward movement of water is due to the force created by the pumping activity of wood parenchyma and cells of medullary rays. 

3. Strasburger(1891): 

Experimentally proved that ascent of sap is not due to the activity of living cells and the process is independent of the living cells of xylem tissue. However, their presence may provide favourable conditions for the process of the ascent of sap. 

4. Pulsation Theory: 

  • Sir J.C. Bose (1923) strongly advocated a vital force theory for the ascent of sap, which is known as pulsation theory. 
  • According to this theory, the cells of the innermost layer of cortex, just outside the endodermis show rhythmic pulsation which causes pumping of water from cell to cell in an upward direction.
  • J.C. Bose experimented on Indian telegraph plant (Desmodium gyrans) to demonstrate rhythmic pulsation activity of the cells of the innermost layer of the cortex. 

2. Root Pressure Theory: 

  • The positive pressure found in the sap of xylem vessels of the root is called root pressure. 
  • The inward flow of water in the parenchyma cells of cortex of roots creates tension on the elastic wall of these cells. 
  •  Due to the accumulation of water, hydrostatic pressure is developed in these cells.
  • When the walls tend to return to their original state, some amount of sap is forced into the tracheary elements of xylem. 
  • On account of this, a pressure develops on the water column of xylem due to which it rises up. 
  • This positive pressure developed on the sap of the tracheary elements is called root pressure. 
  • Root pressure can be demonstrated and may be measured with the help of manometer.
  • The value of root pressure in plants is up to 2.0 atm.
  • Water may rise up to 20.0-meter height by 2.00 atm.

  • In very tall woody trees a pressure of 12 atm. is required for an ascent of sap.
  •  In no plant under any condition, such a high value of root pressure has been seen to develop. 
  • Gymnosperms include some of the tallest trees and root pressure has not been observed in gymnosperms. 
  • Hence the theory of root pressure has some limitations because it can be of significance in some plants under some conditions only

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