OSMOSIS 193 



raffinose. The three possibilities of the chemical constitution 

 of raffinose as given by the chemists being now considered, 

 one of these may be observed to have a molecular weight of 594, 

 viz., the second formula, Ci8H320i6-5H20, the slight difference of 

 2 being due to experimental error. 



A botanist like deVries would be more interested in making 

 the plant cell serve as his osmotic system rather than setting up 

 an apparatus in the laboratory. Furthermore, deVries, in using 

 plant cells, could perform the entire experiment before a physical 

 chemist could get his apparatus assembled. DeVries first found 

 that concentration of sucrose which was isotonic with cells; 

 then he did the same for raffinose. The two sugars, being iso- 

 tonic with the same osmotic system (that of the cells), were 

 obviously isotonic with each other and could then be compared. 



Here and there on the foregoing pages care has been taken to 

 emphasize the fact that comparison between the concentrations 

 of solutions from the point of view of their osmotic behavior 

 can be made only of nonionizable substances such as sugars. 

 It is obvious that if equimolecular solutions have the same 

 osmotic pressure, this can be true only if the substances stay 

 not only equimolecular but of equal particle number, whatever 

 these particles may be. Consequently, if a solution of sodium 

 chloride is equimolecular with one of sugar, it will not be of the 

 same osmotic pressure if each salt molecule breaks down (dis- 

 sociates) into two particles, or ions. Each ion will possess osmotic 

 activity just as does a molecule; therefore, if dissociation is 

 complete, a monovalent ionized salt should have twice the osmotic 

 pressure as the unionized sugar of equimolecular concentration. 

 This is true except for the discrepancy introduced by the fact 

 that the salt either does not dissociate fully or some other factor 

 enters in which makes it appear that the salt has not fully 

 dissociated (page 298). That monovalent salts, each molecule 

 of which dissociates into two ions, do not have twice the osmotic 

 pressure of equimolecular concentrations of sugars, and that 

 divalent salts, with their three ions, do not have thrice the 

 osmotic activity of sugar led Arrhenius to postulate the incom- 

 plete dissociation of electrolytes. 



The Plant Cell as an Osmotic System. — Plant cells usually 

 contain a large central vacuole or cavity filled with a solution. 

 The vacuole and the surrounding protoplasm are each enclosed 



