Root-Pressure as a Factor in the Rise of Sap*

Abstract

THE history of our observations and theories—one can scarcely say our ‘knowledge’— concerning the movement of sap in plants has been a rather tortuous one. Beginning with the ‘vital’ explanations of Hales1 and Knight², in which the living cells were presumed to be the agents largely responsible for sap movement, our concepts have run the entire gamut to the ‘mechanical’ explanations of Dixon and Joly³ and Askenazy⁴ in which living cells were presumed to take no part in, or even to hinder, the movement of sap. TheDixon-Askenazy scheme is the one generally considered to-day to furnish the most satisfactory explanation of sap movement, while ‘root-pressure’, first observed by Hales in 1727, is looked upon as a transitory phenomenon, involving negligible forces (maximum value recorded, about 1·5 atm.), the occurrence of which in uninjured plants was subjected to doubt by Molisch5, and hence probably unimportant.