"ROOT-PRESSURE" — WHITE 495 



If, now, the control curve, recorded without imposed pressure, is 

 compared with the experimental one, obtained under pressures vary- 

 ing between and 6 atm., they are seen to be almost exactly alike. 

 Moreover, if the segment: 9 a. m. to noon, on the 3d day, is compared 

 with the corresponding segment on the 4th day, their slopes are seen 

 to be almost identical, although the second was obtained under zero 

 pressure and the first under 3 atm. This record represents one of 

 4 manometers set up in series on a single manifold. A second gave 

 an identical curve up to the afternoon of the 3d day when a bacterial 

 contamination of the culture solution set in and secretion stopped 

 abruptly. A third gave a similar curve but with a shallower slope, 

 possibly because the root used had a smaller diameter, and the fourth 

 developed a leak around the root which permitted air to escape. Six 

 atmospheres pressure, 4 times the greatest value that I have found 

 recorded elsewhere for root-pressure (the exudation pressures of 

 Boehm, Figdor, Molisch, and MacDougal were of quite a different 

 sort, in that they were obtained as a result of trauma) was not enough 

 to slow down secretion to a measurable extent. 



The sap movement which we are here studying is a matter of filtra- 

 tion through a membrane which we usually assume to be readily 

 permeable to water, and surely equally permeable in either direction. 

 The membrane can, therefore, be ignored in any calculation of the 

 forces causing filtration, at least so long as movement is slow. We 

 have been trying to balance a measured applied force against an 

 unknown secretion force. Secretion should stop when the external 

 and internal forces are equal, and water should flow back through 

 the membrane — the root — when the applied force exceeds the force 

 of secretion. Secretion should become slower and slower as the 

 external force approaches the value of the internal one. 



Six atmospheres external pressure did not bring about any observ- 

 able retardation of secretion from these roots. Only one conclusion 

 appears possible from this observation — 6 atmospheres must be so 

 small in comparison with the secretion pressure actually developed 

 by the roots as to be quite insignificant. It is the writer's opinion 

 that this secretion pressure cannot be less than 10 atmospheres and is 

 probably much more than that. In fact, it seems possible that it 

 may be limited only by the osmotic value of the cells themselves. 

 Attempts have been made to impose still higher pressures, but they 

 have met with mechanical difficulties that have not yet been over- 

 come. It is interesting that so far failures have all been due to flaws 

 in the apparatus. The roots have not failed to secrete liquid regularly 

 against all pressures to which they have been subjected. 



The old "root-pressure" theory of Hales has been disparaged by 

 modern plant physiology texts because it did not provide sufficient 

 force and because of the suspicion that it might be an artifact. These 



