Mar., 1921] KNUDSON AND GINSBURG — -OSMOTIC PRESSURE 
167 
the tube. The leaves were wrapped in several layers of washed muslin 
before being placed in the cylinder. The cylinder and piston apparatus 
may, of course, be constructed of any size which would be convenient. 
For applying pressure to the piston, use was made of materials-testing 
machinery of the College of Engineering. The one used gave a total pres- 
sure up to 50,000 pounds, while with the others available, pressures up to 
400,000 pounds could be obtained. The advantage of these machines is 
that absolutely known pressures can be obtained, and they are available in 
all engineering laboratories. 
The freezing points were determined by the Beckman apparatus. The 
sap was allowed to undercool to — 1.0° C, when it was inoculated by means 
of a platinum needle with crystals of hoar frost. Undercooling, therefore, 
was the same in all cases and practically negligible as to its influence. 
Experiments 
In this experiment, the influence of the method of freezing and of the 
amount of pressure applied to the tissue is noted. Leaves of Iresine were 
used. Total pressures of 50,000 and 10,000 pounds were used, and the 
tissue was frozen either by liquid air or by the salt-ice mixture. The data 
given in table i record the osmotic pressures as calculated from the depres- 
sion of the freezing point. 
Table i 
Method of Pressure Applied Osmotic Pressure 
Freezing to Leaves in Atmospheres 
Ice and salt 50,000 lbs. 5-674 
Liquid air 50,000 lbs. 5-714 
Ice and salt 10,000 lbs. 4-571 
Liquid air 10,000 lbs. 4-932 
It is apparent from the data that the amount of pressure applied is an 
important factor. The difference in the osmotic pressure determined from 
the sap expressed at 10,000 pounds and that expressed at 50,000 is with 
either method of freezing close to one atmosphere. On the other hand, the 
method of freezing the tissue previous to extraction of the sap shows but 
little difference in the osmotic-pressure values obtained. The difference is 
greater when a pressure of 10,000 pounds is used than when a pressure of 
50,000 pounds is employed. 
Similar data were obtained with the Iresine in other experiments and 
also with Zehrina pendula. 
Comparison of the plasmolytic and cryoscopic methods in determining os- 
motic pressure. Tables 2 and 3 give the osmotic pressures as determined 
by the plasmolytic and cryoscopic methods. Table 2 gives the data for 
Zehrina pendula, and table 3 for Iresine. The four series of determinations 
for each species were made on different days. For the cryoscopic deter- 
mination, the leaves were frozen by immersion in liquid air and extraction 
