484 
S. C. BROOKS 
A great many conclusions as to the nature of protoplasm or its 
surface layer (the so-called "plasma membrane") have been drawn 
from experiments on plasmolysis. But since the alterations in turgidity 
or in degree of plasmolysis may not only be increased by the entry 
("endosmosis") of osmotically active substances from the solution 
bathing the cell, but may also be decreased by outward diffusion of 
similar substances ("exosmosis"), the rate of which may be altered 
by the plasmolyzing agent, it seemed highly desirable to observe 
the effect on exosmosis immediately following the application of 
solutions isotonic with the cells of the material used. 
A series of such experiments was conducted in which the exosmosis 
of electrolytes into distilled water from strips of peduncles of the 
dandelion {Taraxacum officinale Weber)^ was determined immediately 
following a previous treatment with distilled water or with sodium, 
calcium, or cerium chlorides. 
Method and Precautions 
In these experiments the best grade of water distilled from glass 
was used; the salts were Baker's "analyzed" sodium chloride, Kahl- 
baum's calcium chloride, and Merck's "Reagent," cerium chloride. 
The solutions were made up with a maximum error of 0.5 percent. 
That this accuracy was sufficient will be seen from the fact that a 
change in the concentration of the CaCU solution from 3 percent 
below to 3 percent above that isotonic with the sodium chloride solution 
used produced no appreciable difference in the results of the experi- 
ment. Solutions were considered to be isotonic with the cells when 
there was for a few seconds a barely perceptible decrease in the 
curvature, as observed by the use of a microscope, of freshly cut strips 
of peduncle on immersion in the solution. A detailed discussion of 
the method of determination, and of the accuracy and significance of 
this criterion will be presented in a subsequent paper. 
A peduncle was cut into pieces 5 cm. in length, and these were cut 
longitudinally into as mmy strips as there were solutions to be in- 
vestigated; one strip from each piece was placed in each solution. 
About fifteen or twenty pieces of peduncle were so psed. Since 
relative results only were sought, the number of strips used was not 
important; it was only necessary to divide each piece accurately, so 
^ The dandelions were grown in the greenhouse from wild plants dug up in the 
autumn. The plants were in the height of flowering when the material was used. 
