THE OSMOTIC CHARACTERS OF THE CELL 17 



to gases, and since the kinetic theory of solutions and of osmotic pressure is 

 based experimentally on Pfeffer's researches in vegetable physiology, it will 

 be appropriate to consider this theory briefly here. 



In the course of his investigations on osmosis, Pfeffer, in 1877, demon- 

 strated that the high osmotic pressures whose existence in certain vegetable 

 cells had been proved by him a short time previously, were due to the presence 

 of crystallizable bodies, such as cane sugar, potassium nitrate, and other sub- 

 stances with relatively small molecules, although, until that date, physicists 

 had held that high osmotic pressures could be attained only by colloids, such 

 as proteids, gums, &c., with large molecules. Previously to Pfeffer's experiments 

 only such materials as parchment and animal bladder had been used as separa- 

 ting membranes in the osmometer, and with colloids these gave higher osmotic 

 pressures than crystalloids, the reason being that the crystalloid diffused with 

 great readiness. Pfeffer showed that by using semipermeable precipitation 

 membranes instead of such materials, high pressures could easily be obtained 

 with the aid of crystalloids. The artificial eel ! employed by Pfeffer was 

 a porous pot in the wall of which a copper-ferrocyanide membrane had been laid 

 down, as described above, and using sugar solutions of different concentrations he 

 obtained the following pressures as indicated by heights in centimetres of 

 mercury : — 



Sugar in weight %. Height in cm. due to pressure. 



I 53-8 



a IOI-6 



4 2082 



6 307-5 



I 53-5 



We may note, in the first place, that the osmotic activity which brings 

 about the pressure is almost exactly proportional to the degree of concentration. 

 An average of several experiments gives us 50-5 cm. of mercury for a i per cent, 

 solution, and thus a 34-2 per cent, solution ( = i G.M.) will give a pressure of 

 1,727 cm., or 22.7 atmospheres. This number forms the basis on which Van't 

 Hoff founds his comparison of osmotic pressure with the pressure of gases. 

 Boyle's law, governing the pressure of gases, states that the product of the 

 volume and of the pressure of a gas is a constant quantity ; if the volume of 

 the gas decreases, then the pressure must increase correspondingly. Under 

 ordinary atmospheric pressure i G.M. of oxygen (32 g.) or i G.M. of carbon- 

 dioxide (44g.) occupies 22-4 litres. To compress the gas until the G.M. occupies 

 only one litre will necessitate a pressure of 22-4 atmospheres. The pressure 

 of I G.M. of gas in the litre corresponds exactly, as one would expect, to the 

 pressure of i G.M. of cane sugar in Pfeffer's osmotic cell, and on this corre- 

 spondence is based Van't Hoff's theory, i. e. that osmotic pressure arises from 

 the impacts at the same time of the molecules dissolved in water and of the 

 ions on the wall of the cell. However, botanical investigations on the amount 

 of osmotic pressure are entirely independent of this physical theory. 



As it is quite immaterial in the case of gases whether the separate mole- 

 cules in the vessel are all of one kind, or are chemically different, so in con- 

 sidering the amount of osmotic pressure in the plant cell it is immaterial whether 

 the cell-sap contains only cane sugar, as we have hitherto assumed, or whether 

 it contains a complex mixture. So long as these bodies do not react on each 

 other, so long as the number of molecules and ions remains unaltered, and so 

 long as the protoplasm remains impermeable, so long does the osmotic pressure 

 remain unaltered, and we can always estimate its amount by the plasmolytic 

 method whether we know the nature of the materials which produce it or not. 

 If we find that a 3-5 per cent, solution of cane sugar will bring about the first 

 trace of plasmolysis in certain plant cells, while a 3 per cent, solution 

 produces no effect, it follows that the cell-sap is approximately isosmotic 

 with a 3-5 per cent, solution of cane sugar, and that the cell exercises the same 



JOST Q 



