SUGGESTIONS WITH RESPECT TO THE MEASUREMENT OF 
OSMOTIC PRESSURE 
L. Knudson and S. Ginsburg 
(Received for publication September 25, 1920) 
The cryoscopic method for measuring osmotic pressure in plant tissues is 
at present the only convenient and probably, if the proper precautions are 
taken, the only reliable method for determining the osmotic pressure of 
plant tissues. Some of its advantages and limitations have been discussed by 
various men, among them Dixon and Atkins (2), and the technique of the 
operation has been considered by a number of men in this country, parti- 
cularly by Gortner and Harris (4). While the methods of determining the 
freezing point of the expressed sap have been fairly well standardized and 
the necessary precautions have been generally followed by investigators, 
yet little attention has been given to the methods of freezing the tissue, or 
to the expression of the sap from tissues. It would be interesting also to 
have, if possible to obtain, data of osmotic pressures of tissues as determined 
by the freezing-point method and by the plasmolytic method. Dixon and 
Atkins have emphasized the necessity of freezing the tissue before expression 
of the sap, using liquid air to freeze the tissue. Gortner and Harris (4) 
considered the use of liquid air superfluous, but recently Harvey (5) has 
found that cabbage leaves, hardened to resist freezing, when frozen at 
— 5° C. yielded a sap with a freezing point of —1.160° C; when frozen with 
solid carbon dioxide, —1.630°, and when frozen with liquid air, —1.822° C. 
These differences are great enough to be significant. Harvey does not 
state specifically what pressures were used. He says that "pressures 
from 10-30 tons were used on a 2^ inch ram." 
In 1916, the authors began some work comparing the plasmolytic and 
the cryoscopic methods. Various circumstances have prevented thus far 
any continuation of the work, but the authors feel that the methods for 
expressing sap, and the relation of the pressure applied to the concentration 
of the expressed sap, make publication of the results desirable. This 
paper is concerned w^ith the effect of the temperature at which the tissue is 
frozen and of the pressure applied on the freezing point of expressed sap. 
The paper includes also data on the osmotic pressures as determined by the 
plasmolytic and cryoscopic methods; a special apparatus is described for use 
in expressing sap, and suggestions are made for applying pressures of known 
values in expressing the sap. 
Methods 
In the experiment here reported, two different species of plants were 
used: Zehrina pendula Schnizl, and Iresine Herhstii Hook. These plants 
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