32 
AMERICAN JOURNAL OF BOTANY 
[Vol. 8 
The third column is calculated from Jones' formula for osmotic con- 
centration of solutions expressed in atmospheres of pressure. The formula 
is as follows: 
In order to check the strength of the solutions used in the following 
experiments with those of Jones, the freezing-point depressions of three 
concentrations used were determined with the aid of the Beckman apparatus. 
These determinations checked very closely with those given by Jones for 
solutions of the same strength. Thus it was considered safe to use his 
results for all calculations. 
The experiments to be reported in this paper were made with a view to 
securing more data on the specific problem of the variations in osmotic 
concentration (i) of the guard cells during the opening and closing of the 
stomata, and (2) of the guard cells as compared with the other cells of the 
epidermis. 
The same general method was used throughout the various tests herein 
recorded. A series of concentrations of CaCW was made up varying in 
strength from 1. 00 molecular to 0.06 molecular by dilutions from a stock 
solution of gram-molecular concentration. The respective concentrations 
were: i.o; 0.8; 0.6; 0.5; 0.45; 0.40; 0.35; 0.30; 0.28; 0.26; 0.24; 0.22; 
0.20; 0.18; 0.16; 0.14; 0.12; o.io; 0.08; 0.06 molecular. This series was 
determined upon after some preliminary trials. In making tests for the 
threshold concentrations of the guard cells and also of the epidermal cells 
(both determinations being made at the same time) portions of the epidermis 
were removed from the lower surface of the leaf and placed on microscopic 
slides. Two drops of the solutions of varying concentrations were then 
placed on each section. These prepared slides were then placed under 
moist chambers for a period of five to ten minutes. At the end of this 
period the sections of epidermis were covered with cover glasses and ex- 
amined microscopically for plasmolysis both of the guard cells and of the 
cells of the epidermis. In this way a complete series could be tested and 
examined in a comparatively short time. Flat petri dishes moistened with 
the various solutions were used as moist chambers for the slides to prevent 
evaporation and a consequent change in the concentration of the solution. 
^ CaCU was used in these experiments in preference to KNO3 because of the tendency 
of the latter to make the protoplasm more permeable. 
Osmotic concentra- 
tion of a solution 
expressed in at- 
mospheres of 
pressure 
Freezing 
point 
depression of 
that solution 
X 
22.4 osmotic concentration of a 
molecular solution expressed 
in atmospheres of pressure 
1.86 freezing point depression of a 
molecular solution 
Methods 
