'ROOT-PRESSURE"— AN UNAPPRECIATED FORCE IN SAP 



MOVEMENT J 



By Philip R. White 



[With one plate] 



The long and tortuous history of our knowledge — or better our 

 observations and theories — on the movement of sap in plants is famil- 

 iar to most botanists. From the time of Stephen Hales, 200 years ago, 

 up to the middle of the nineteenth century, the movement of water 

 through the plant was supposed to be brought about primarily through 

 the activity of living cells, either by the agency of unidentified tissues 

 of root and stem, as Hales believed (1727), or the medullary rays, as 

 suggested by Knight (1801). The "drawing power" of the leaves was 

 recognized by both Hales and Knight, but, since it was well known 

 that a suction pump would not lift water higher than about 30 feet, 

 this was considered of secondary importance. Beginning about 1860, 

 along with the rise of mechanistic theories in other fields, evidence 

 began to accumulate which indicated that living cells might not be 

 necessary for the rise of sap. This changing viewpoint found strong 

 support in Molisch's demonstration (1902) of the traumatic nature of 

 many of the bleeding phenomena upon which Hales' vital theory 

 rested and finally culminated in the development of the Dixon- 

 Askenazy cohesion theory of sap movement (Dixon and Joly, 1895; 

 Askenazy, 1895). This theory, which takes account of the enormous 

 suctions developed at evaporating leaf surfaces and of the fact that in 

 capillary tubes water possesses a great tensile strength capable of 

 transmitting these suctions through a plant stem to the soil, seemed to 

 deal with forces more nearly commensurate with the needs of tall 

 trees than had those demonstrated by Hales and his successors. The 

 result was that most plant physiologists completely abandoned the 

 vital theories in favor of the mechanical ones. In spite of the objec- 

 tions of Priestley (1935), Ursprung (1906), Heyl (1933), and others, 

 that is approximately where the situation rests today. 



The cohesion theory certainly has some very serious flaws which are 

 rather well outlined in Priestley's paper of 2 years ago (1935). It has 



1 Paper from the Department of Animal and Plant Pathology of the Rockefeller Institute for Medical 

 Research, Princeton, New Jersey, presented at the Indianapolis meeting of the American Association for 

 the Advancement of Science, December 1937. The American Association prize was awarded to Dr. White 

 for his noteworthy contribution to science presented at the annual meeting. Reprinted by permission 

 from the American Journal of Botany, vol. 25, No. 3, March 1938. 



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