Methods for Determining Osmotic Pressures, i 75 
high ground of the marsh was found to be much greater than that 
of a plant from a depression on the marsh. The same values had 
previously been obtained by plasmolysis of the epidermal cells; 
when some of the plants examined from the higher parts of the 
marsh were found to have a cell sap isotonic with that of a 1*45 
gram-molecular solution of sodium chloride while many of those 
from the lower parts had cell sap isotonic with that of a 1*15 
solution. These values are high but they lie between the limits 
subsequently observed by Fitting 1 who found that the cell sap of 
plants of Salicornia herbacea from a very damp and salt habitat had 
cell sap of an osmotic pressure greater than that of a 1-gram-mole¬ 
cular solution and less than that of a 2-gram-molecular solution. 
The strength of the cell sap of Ceveus sp. found by Barger’s 
method is in accordance with that found by Livingston 2 for a species 
of Ceveus. He determined the osmotic pressure of the sap in two 
ways :—(i) by finding the depression of the freezing point which 
gave a pressure equal to that of 5’5 atmospheres, and (ii) by 
observing the change in curvature of “ strips ” of tissue placed in 
salt solutions of various strengths, when the pressure was found to 
vary from 3'9 to 7 atmospheres in different plants. 
The values obtained for the other plants are somewhat lower 
than those generally expected for plant cells, but they are not so 
low as some recorded for other succulents. Cavara 3 estimates the 
osmotic pressure of the sap of Aloe arborescens to be equal to a 
pressure of F5 atmospheres. 
The advantages which Barger’s method has over other physical 
methods that have been tried for finding the osmotic pressure of 
the sap in plant cells, are twofold. In the first place a very small 
quantity of sap is sufficient to make a determination, so that the 
osmotic pressure of the cell sap of any small part of a plant, e.g ., a 
single leaf, can be estimated. This is necessary when it is desired to 
correlate the osmotic pressure of the cell sap of leaves, with age of 
leaf as done by Pringsheim 4 for certain succulent plants, or with 
any special factor in the life of the plant. The juice of the organ 
can be squeezed out by pressure of the fingers and the drops 
obtained can be used directly to fill the tubes. In the second place 
1 “ Die Wasserversorgung und die osmotischen Druckverhaltnisse der 
Wustenpflanzen.” Zeitschr. f. Bot., Band 3, 1911, p. 250. 
* “The Relation of Desert Plants to Soil Moisture and to Evaporation.” 
Carnegie Institute, Publication No. 50, 1906. 
* Loc. cit. 
* “ Wasserbewegung und Turgorregulation in welkenden Pflanzen.” 
Jahrb. f. wiss. Bot., Bd. 43, 1896, pp. 89-144. 
