Question

Most accepted theory for ascent of sap is

• A. capillarity theory
• B. root pressure theory
• C. pulsation theory
• D. transpiration pull

Answer 1

The Correct Option is D

Transpiration pull is the tension which develops due to transpiration. It has been demonstrated and evidenced, that rate of water absorption and hence, ascent of sap closely follows the rate of transpiration and hence, ascent of sap closely follows the rate of transpiration. Cohesion and transpiration pull theory, given by Dixon and Jolly (1894) is the most accepted theory for ascent of sap. Root Pressure theory (Priestley; 1916) is a manifestation of active water absorption. It was objected, as it has not been applicable to all plants (eg, gymnospermous tallest trees). Moreover, this pressure is low to raise the sap to the top of trees and also many reasons which this theory was unable to proove. Pulsation theory or vital force theory (J C Bose; 1923) was also discarded because living cells do not seen to be involved in the ascent of sap as water continuous to rise upward in the plant in which roots have been cut or living cells of stem are killed by poison and heat (Strassburger; 1891). Capillarity theory (Bohm; 1863), laid stress on capillarity movement of water due to adhesion and cohesion forces balanced by downward pull of gravity. It was also discarded, as the value of capillary is too small and applicable only to small sized plants and tall plants with narrow vessels.