PHTSIOLOGT 



If a 5 per cent sugar solution extracts a little 

 water from the cell its osmotic pressure must be 

 a little greater than that of the solution in the 

 cell. Since the osmotic pressure of a 5 per cent 

 sugar solution can be determined, the osmotic 

 pressure within the cell can be estimated. 



In this way de Vries not only measured the 

 osmotic pressure of a large number of plant cells 

 but he performed the remarkable feat of de- 

 termining by means of such cells the molecular 

 weight of a compound which baffled the best 

 efforts of chemists. This substance is a sugar, 

 raffinose, for which three different formulae 

 were proposed. Although the chemists were 

 unable to decide which formula was correct it 

 was for de Vries a simple matter. The three 

 formulae gave the molecular weights 396, 594, 

 and 1 1 88. If raffinose has the same molecular 

 weight as cane sugar (342) a 5 per cent solution 

 will extract water from the plant cell to the 

 same extent as a 5 per cent solution of cane 

 sugar. But if its molecular weight is only half as 

 great the 5 per cent solution will contain twice 

 as many molecules and its power to draw out 



