OSMOTIC PROPERTIES OF THE CELL. 41 



the contrary took place; and again, that when an osmometer 

 containing a I per cent, solution of potassium chloride was 

 placed in distilled water the liquid in the tube rose i8mm., 

 whereas when a I per cent, solution of potassium carbonate 

 was used, it rose 439 mm. 



The absorption of water by the cell is doubtless subject to 

 the same conditions as those which we have found to deter- 

 mine the osmosis in the osmometer, the cell-sap corresponding 

 to the fluid contained in the osmometer. The question now 

 arises as to the determination of the osmotically active sub- 

 stance. It must be borne in mind that the concentration of 

 the cell-sap is slight, its specific gravity being but little higher 

 than that of distilled water ; the osmotically active substances 

 which it contains must therefore possess a strong affinity for 

 water. That sugar is not the substance which is active in the 

 process is evident; for, inasmuch as its affinity for water is 

 comparatively slight, a much larger quantity of it would have 

 to be present than can be inferred from the concentration of 

 the cell-sap, in order to account for the amount of water 

 absorbed. This objection applies also to the other organic 

 substances present which have only a slight affinity for water. 

 The mineral salts in the cell-sap have a much greater affinity 

 for water than sugar has, and they are doubtless osmotically 

 active. It must be remembered, however, that these salts are 

 not produced in the cell, but are absorbed from without; 

 hence there is a limit to their osmotic activity. The osmoti- 

 cally active substances which we seek must be such as have a 

 great affinity for water and are constantly being produced in 

 the metabolism of the cell. De Vries has pointed out that the 

 organic acids and acid salts fulfil these requirements. They 

 are present in all living cells which are capable of becoming 

 turgid, their formation is doubtless a necessary part of the me- 

 tabolism of the cell, and their affinity for water is considerable. 



Assuming then that the organic acids and acid salts are 

 the substances which bring about the absorption of water by 

 the cell, we may go on to enquire into the conditions upon 

 which the turgidity of the cell depends. A closed bladder 

 containing syrup will soon become turgid when immersed in a 



