72 BOTANICAL GAZETTE [JANUARY 
used were made up with distilled water so treated. In all cases solu- 
tions were made up on the basis of the desired number of gram mole- 
cules of dry substance dissolved in water sufficient to make a liter of 
solution, that is, on the volume-normal basis. 
In order to calibrate the indicator plants, solutions of cane sugar 
and of NaCl in a graduated series of concentrations were carefully 
prepared from high grade chemicals. These dilutions when in use 
were kept in covered beakers of 250 cc. capacity. The algae were 
quickly freed of surplus solution by the use of fresh filter paper before 
transfer, and freed from remaining traces of the solution by a quick 
rinsing in a duplicate portion of the solution into which they were to 
go. After the transfer, filaments were removed at definite intervals 
for microscopic examination, either on a watch glass or on a slide. 
In determining the osmotic equivalent, some difficulty was 
experienced owing to the fact that all cells of the same filament did 
not show the same plasmolytic response to a given concentration. 
This difference was especially marked as the critical concentration 
was approached. As a rule the tip cells of a filament showed incipi- 
ent plasmolysis in a weaker solution than did the other cells. ‘Those 
that had lately undergone division seemed to plasmolyze more 
promptly as a rule. Since plasmolysis begins to take place only 
after the concentration of the outer medium is in excess of the con- 
centration of the cell sap, in this study the osmotic end reaction was 
regarded as reached when the first traces of withdrawal of the proto- 
plast were seen in the tip cells. Since the problem of absorption was 
not under investigation, the persistence of signs of plasmolysis was 
not studied. In order to avoid as far as possible complications due 
to the penetration of the materials from the solution under test, 
results seen within an hour after the application of the solutions in 
question were accepted. At times slight plasmolysis seen within 
this time would soon disappear. Obviously, therefore, the prompt- 
est possible registration of osmotic conditions would be expected to 
give the best evidence of conditions normally existing in the cell. 
As a result of a series of tests made with cane sugar, it appeared 
that for a major part of normal Spirogyra and Oedogonium cells a 
concentration of 0.25 gm. molecules per liter of solution was just 
short of producing plasmolysis at 2220 C. Only in the tip cells was 
