TRANSLOCATION OF WATER ANd mINERALS (ASCENT OF SAp)
The upward movement of sap (water with dissolved ingredients) from the root towards the top of the plant is known as
ascent of sap. Roots are organs concerned with the absorption of water. If the plant is to survive and grow, this water
absorbed by roots must reach the top of plants to replace the water lost in transpiration and to be used in photosynthesis
as a raw material.
The upward movement of water occurs through the tracheary elements (tracheids and vessels) of xylem. Sap is lifted from
near the root tip to the shoot tip against the force of gravity, sometimes to great heights. Some trees are much taller,
often attaining a height upto 110-130 m, e.g., Sequoia sempervirens (height 110 m), Picea sitchensis (height 95-100 m),
Pseudotsuga menziesii (height 100 m), Eucalyptus (height above 130 m), etc. The force required to move water to such
heights are substantive. The rate of translocation is 25–75 cm/min (15–45 m/hr).
Several theories have been put forward to explain the mechanism of ascent of sap. The four main theories are: vital force
theory, relay pump theory, root pressure theory, capillary force theory and transpiration pull theory.
Vital force theory
A common vital force theory about the ascent of sap was put forward by Sir J.C. Bose in 1923. It is called pulsation
theory and it believes that the innermost cortical cells of the root absorb water from the outer side and pump the same
into xylem channels.
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