122 DIFFUSION AND OSMOTIC PRESSURE 



cell. This is just what probably occurs during the trans- 

 mission of substances from one cell to another by diffusion. 

 After a time, however, the artificial cell will come into 

 equilibrium. The diffusion tension of the sugar is then 

 just equaled by the resilience of the walls, and that of the 

 inclosed KNO 3 by the diffusion tension of the same salt in 

 the surrounding medium. No further changes of concen- 

 tration will occur until some alteration is made in the con- 

 ditions. Now let a few crystals of potassium nitrate be 

 added to the external solution. They dissolve immediately 

 and diffuse equally as far as the solution extends. But now 

 the diffusion tension of this salt has been raised in the sur- 

 rounding medium while it remains the same within the cell. 

 However, since the membrane is permeable to KNO 3 , this 

 condition cannot last long; inward diffusion of K and NO 3 

 ions will soon equalize the tension within and without. 

 Thus it is shown that a solute may diffuse not only out of 

 but also into a cell, the latter remaining turgid meanwhile, 

 through the action of another solute to which the osmotic 

 membrane is impermeable. 



Mass movements of the sap in stems, caused by changes 

 in temperature, mechanical bending (as by the wind), etc., 

 may aid very much in keeping the various solutes equally dis- 

 tributed throughout the inclosed solution. The mass move- 

 ment occasioned in the solutions by evaporation from above 

 (possibly also by sap pressure) must also aid in this. 

 Within the cells the streaming movements of the proto- 

 plasm must act in the same manner, and the protoplasmic 

 connections between adjacent cells probably sometimes set 

 up mass currents which aid in the transmission of solutes 

 from one cell to another. The latter consideration is prob- 

 ably of relatively great importance in the case of the trans- 

 mission of carbohydrates and other plastic materials through 

 the phloem region of stems. But by far the most impor- 



