Mar., 1921] KNUDSON AND GINSBURG OSMOTIC PRESSURE 
165 
were employed because a large supply was at hand and all were growing 
under the same environmental conditions and the plants of each species 
were of equal age. Furthermore, the presence of pigments in the cell sap 
makes easier plasmolytic determinations. 
Plasmolytic determinations were made of the pigmented mesophyll cells 
of Iresine and of the pigmented cells of the lower epidermis of Zebrina. In 
making the determinations, free-hand cross sections of the leaves were used. 
In the plasmolytic determinations, two solutions were used: calcium 
chloride and sucrose. Calcium chloride was used in preference to other 
salts for the reason that permeability, as shown by Osterhout (9, 10), 
True and Bartlett (12), and others, is generally decreased by calcium chlor- 
ide. No correction was made for shrinkage. Shrinkage of cells would tend 
to make the osmotic pressure determination higher than the actual. Renner 
(11) believes that values obtained by the plasmolytic method would generally 
be lower than those obtained by the cryoscopic method, since in the latter, 
calculations are based on weight-normal solutions (gram molecules of 
solute per 1,000 grams of solvent), while in the plasmolytic method one 
generally uses volume-normal solutions (gram molecules of solute in 1,000 
grams of solution). Throughout the plasmolytic experiments weight- 
normal solutions were used. Therefore, the values obtained by each method 
should be more comparable. 
The osmotic-pressure values by the plasmolytic method were calculated, 
using 22.4 atmospheres as the osmotic pressure exerted by a weight-normal 
solution, and, for the cryoscopic method, osmotic pressure = A X 22.4/1.86. 
It might be more desirable to use the osmotic-pressure values obtained by 
Morse [see Findlay (3)] and his co-workers, but the values would be only 
slightly increased. In calculating osmotic pressures from the plasmolytic 
determinations made with calcium chloride, the formulas given by Living- 
ston (8) were used. Dissociation was calculated from conductivity tables in 
Landolt-Bornstein (7). 
All determinations were made of the leaves only. The plants were cut 
at 8 A.M. and placed with their cut ends in water for thirty minutes, so 
that the leaves would be in a turgid condition. In making the plasmolytic 
determinations, preliminary studies were previously made in order to obtain 
the approximate concentrations which would be isosmotic with the cell sap, 
and then, before each series of determinations, various dilutions of the two 
solutions were prepared, the concentrations of these ranging close to the 
anticipated threshold solution. In this way rapidity in determining the 
isosmotic concentration was obtained. 
In expressing the sap for the cryoscopic method, the leaves were frozen 
either in salt-ice mixture or in liquid air. For the liquid-air treatment the 
leaves were strung on a thread and immersed in a Dewar flask containing 
the liquid air. The leaves were removed when the liquid air stopped boiling, 
indicating that the leaves had assumed the temperature of liquid air. 
