R. G. Leavitt — Cause of Boot- Pressure. 381 



Art. XLIY. — A Preliminary Note as to the Cause of Root- 

 Pressure ; by Kobert G. Leavitt. 



The following suggestion is made in the hope of breaking 

 a way toward the proper understanding of the nature of so- 

 called root-pressure. The hypothesis here tentatively proposed 

 seems to coordinate certain of the results obtained by Sachs, 

 Clark and others, which after several years still stand in appar- 

 ent need of explanation. 



Yan 't Hoffs interpretation of the exact determinations of 

 osmotic pressures made by Pfeffer, de Vries, Tammann and 

 others, which interpretation is generally accepted by physicists 

 and by Pfeffer himself, though still neglected by the majority 

 of writers on vegetable physiology, is here assumed as the 

 basis of the discussion on the physical side. Unfamiliarity with 

 this theory on the part of many botanists may warrant a con- 

 densed statement of its central principles. 



Solutions (the term being applied to the dissolved sub- 

 stances, exclusive of the solvents) act in all ways, both qualita- 

 tively and quantitatively, as gases, exerting pressure against 

 containing water-, or other, surfaces by an inherent expansive 

 force. The laws of solutions are identical in form, if " solu- 

 tion " be substituted for " body of gas," with the laws of 

 Boyle, Charles and Avogadro for gases. Quantitatively con- 

 sidered, "dissolved substances exert the same pressure, in the 

 form of osmotic pressure, as they would exert were they 

 gasified, at the same temperature, without change of volume." 



The distention of a turgid cell is then due to the pressure of 

 organic matters in the cell sap, unable to escape or able to 

 escape only with difficulty. The entrance of water is a con- 

 sequence, not cause, of the osmotic pressure. A sufficient 

 supply of water at hand is, to be sure, a necessary condition 

 for allowing the osmotic force to act and push the cell-wall 

 outwards, since the boundaries of the solvent, equally with 

 definite membranes, limit the molecular wandering of the 

 solution. 



The movement of water up the stem of the thistle-tube, in 

 the familiar experiment, is caused by the upward pressure of 

 the solution against the free surface of the water ; the water 

 and the membrane below both being passive accessories in the 

 operation. If the stem were crossed by diaphragms per- 

 meable by both water and solution, these would not constitute 

 osmotic membranes in the sense of occasioning osmotic stress 

 and flow of water ; osmose and filtration of water would pro- 

 ceed through them. If the stem, however, were crossed by a 

 membrane permeable by water but not by the solution, both 

 would be stopped, and no exfiltration of the water by action 

 of the confined osmotic substance is conceivable. 





