Mechanism of Ascent of Sap

Mechanism of Ascent of Sap:

In plants, water, sometimes, has to be raised to such heights as 300 ft or more to the top of tallest trees like Sequoia and Eucalyptus, and the maximum rate at which sap can be translocated has been measured as high as 75 cm per minute. This reflects that some elaborate and highly specialized mechanism exists for the ascent of sap in plants.

Several theories have been proposed to explain the mechanism of transport of water. These can be classified and studied in three categories:

(a) Vital Force Theories.

(b) Root Pressure Theory.

(c) Physical Theories.

Vital Force Theories- According to these theories, the metabolic activity of living cells is responsible for the ascent of sap. A number of such theories have been proposed by the scientists. Important among them are the Relay-Pump theory of Godlewski (1884) and the Pulsation theory of Sir J. C. Bose. None of these theories is acceptable for want of experimental evidence.

Root Pressure Theory- Root pressure under certain conditions exudates water from cut stems. This prompted the view that root pressure causes the movement of water through the xylem vessels. But it is of negligible importance to plants because of the following facts.

(1) The magnitude of root pressure never exceeds 2-3 atmospheres. As such it can force the water only up to a short distance whereas a pressure of about 16-20 atmospheres is required to raise water to the top of trees reaching heights of over 80 metres.

(2) Root pressure shows seasonal fluctuations being the highest in spring and the lowest in summer.

(3) Water continues to rise even in the absence of root pressure.

(4) No root pressure has been observed in Gymnosperms.

Physical Theories- Several theories involving physical forces such as imbibition, capillarity, atmospheric pressure, etc., were suggested but only capillarity needs to be mentioned here.

The uptake of water through xylem vessels is possible in small-sized plants by capillarity. By capillary action, water can rise in tubes of small diameter kept in a vessel containing water. This rise in water is due to adhesive and cohesive forces. Adhesive forces attract molecules of different kinds, whereas cohesive forces attract molecules of the same kind.

According to the theory of capillary action, water is first taken in by the walls of xylem vessels and tracheids due to the force of adhesion. Now the cohesive forces between molecules pull the water upward. This upward pull of water continues until the forces of adhesion and cohesion are balanced by the downward force of gravity.

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