Ascent Of Sap


 
 
Concept Explanation
 

Ascent Of Sap

Translocation of water or ascent of sap is usually upwards from the roots toward the top of the plant via stem i.e. to the leaves and growing points or apical meristems and other aerial plant parts. It occurs through the tracheary elements of xylem. Many theories have been put forward to explain the upward movement of water.

Two of them are given below :-

Root Pressure :

  • It is believed that all plants absorb excess of water by an active process and tends to build up a positive hydrostatic pressure within the root system called root pressure.
  • Due to this pressure, the water is pushed upward along the length of the stem to a small height. The pressure inside the xylem is caused due to diffusion pressure gradient and is maintained by the activity of living cells.
  • Transpiration Pull

  • Plants themselves have a continuous water column in their xylem channels that starts at the base(i.e. roots) and continues up to leaves from where water is lost through the process of transpiration. Thus, despite the absence of circulatory system in plants achieves fairly high rates i.e. up to 15 metres per hour.
  • This transpiration pull occurs when water is lost from mesophyll cells to the intercellular spaces as a result of transpiration which develops a strong negative water potential. This negative pressure in the water column , exerts an upward pull over the water column. The pull is known as transpirational pull. This tension or pull is transmitted up to the roots in search for more water.  The water column now moves upward under the influence of transpirational pull.
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