MICHIGAN ACADEMY OF SCIENCE. 47 



A SIMPLIFICATION OF THE PKESENT FREEZING POINT 



METHOD FOR THE DETERMINATION OF THE 



OSMOTIC PRESSURE OF PLANT SAP. 



(Abstract.) 



BY 0. E. HARRINGTON AND R. V. HIBBARD. 



The paper describes a simplification of the usual freezing-point 

 method of determining osmotic pressure of plant sap. By this 

 modification, instead of extracting the sap from the material and 

 determining the freezing point of the extract, a determination is 

 made directly upon the tissue which is in the form of a pulp. 



The following are the essential features of the method: The 

 material to be used is first carefully frozen, then ground in an ordi- 

 nary food grinder, mixed, samples taken and placed in the freez- 

 ing tubes and allowed to reach a temperature about one degree 

 below its freezing point. Solidification is then brought about by 

 turning the thermometer a few times to create a disturbance in 

 the pulp. 



Comparisons have been made between results, obtained in this 

 way, with those obtained in the usual way upon the same material 

 and it has been found that tlie closeness with which the results 

 compare depends upon the thoroughness of the freezing and the 

 thoroughness with which the sap is extracted. When a large press 

 is used, with which it is possible to extract practically all of the 

 sap, the results of the two methods check within the range of ex- 

 perimental error. 



There are two special advantages of this modification. First, 

 less time transpires from the time the preparation of the material 

 begins to the end of the process, thus reducing the possibility of 

 change in the composition of the material; second, a more accu- 

 rate sample of the original material is used than is the case when 

 the sap is extracted. 



In as much as the results obtained by the tAvo methods check 

 within the range of experimental error when great care is exer- 

 cised in the extraction of the sap, and in as much as the difference 

 in results is greater when the sap is extracted less carefully, it is 



