THE INFLUENCE OF OSMOTIC PRESSURE n 



used and the plant examined. Kohl is therefore incorrect in supposing 

 the increased velocity to be due to a diminution of the pressure of the 

 cell-sap against the lining layer of protoplasm. As a matter of fact 

 the internal pressure does not decrease, but increases by an amount 

 corresponding to the osmotic concentration of the external solution, and 

 the external friction takes place between the rotating layers and the outer 

 non-moving layers of ectoplasm, not between the latter and the cell-wall. 



Fluid friction differs markedly from solid friction in one important 

 respect. Thus the flow of an incompressible liquid through a tube of 

 uniform bore with smooth walls depends solely on its viscosity, upon the 

 diameter of the tube, and upon the difference of pressure at its two ends, 

 but is not influenced by the lateral pressure on the containing walls. 

 Thus pressures at the two ends corresponding to o cm. and 20 cm. of 

 mercury respectively will produce the same rate of flow as pressures 

 of 200 and 220 cm. respectively. 



Protoplasm, like other substances capable of imbibition, contains, 

 however, a large quantity of water which can be driven from it by 

 pressure 1 . As the percentage of imbibed water decreases, the pressure 

 required to squeeze out more water increases very greatly. 



Now the pressure of the cell-sap on the protoplasm varies from 

 10 to 25 atmospheres in ordinary turgid cells, and its percentage of water 

 is usually about 70. Hence a rise of at least 3 to 6 atmospheres (i to 2 per 

 cent. KNO 9 ) in the osmotic pressure would be necessary to squeeze out 

 any appreciable quantity of water from it, even supposing it held no 

 osmotic substances in solution, and exerted no appreciable osmotic pressure 

 itself. Any withdrawal of water, however, increases the viscosity, hence, 

 ceteris paribuS) diminishing the velocity of streaming. It follows, therefore, 

 that whenever a solution whose osmotic concentration is less than i per 

 cent. KNO 3 markedly retards (or accelerates) streaming, even when the 

 shock of sudden immersal is avoided by a gradual increase of concentra- 

 tion, it does so, not necessarily because of any osmotic action, but probably 

 because the substance in question acts as a chemical stimulus. Dilute 

 salt solution acts very largely in this way on cells of Chara and Nitella, 

 but in most cases its action is mainly physical, and hence fairly strong 

 solutions are necessary to produce a pronounced effect. Similarly when- 

 ever a very dilute solution causes a gradual decrease in the velocity of 

 streaming, its action is that of a retarding chemical stimulus. 



A rise of temperature of i5C. causes an increase of 5 per cent, in 

 the osmotic pressure of the cell-sap, but will not perceptibly affect the 

 percentage of water in the protoplasm, even when it contains very much 

 imbibed water and its inherent osmotic pressure is low. On the other 



Cf. Reinke, Nachr. d. Ges. d. Wiss. zu Gottingen, 1894, p. 54. 



