Protoplasm to some Reagents. 499 
in a few minutes , while in saponin-treated material nearly every cell in the 
leaf will be prominently plasmolysed. 
The change in critical concentration of salt just described for saponin 
material also takes place in smilacin and senegin treated cells, with the 
difference that the reduction in osmotic value of the cells is more rapid 
in senegin than in saponin. 1 
Throughout this work a decrease in the critical concentration of the 
plasmolysing salt is interpreted to mean that there has been a reduction 
in osmotic pressure of the cell contents due to an increase in permea¬ 
bility ; that is, that during treatment there has been an increase in 
permeability of the plasma membrane which has permitted an exosmosis 
of dissolved substances from the cell resulting in a lower osmotic value ; 
consequently, a salt solution which is isosmotic with the contents of the 
treated cell will be of lower concentration than the salt solution which is 
isosmotic with the untreated cell. 
There are two criticisms which can be directed against the above 
point of view. First, a change in osmotic value of the cell contents is not 
in itself alone conclusive evidence of a change in permeability. Variations 
in the osmotic pressure of a cell might well occur as a result of chemical 
change within the cell without any alteration in the permeability of the 
plasma membrane. The synthesis of sugar from the organic acids, or of 
cane sugar from two molecules of glucose, or the formation of insoluble 
starch from cane sugar, would greatly lower the osmotic value of the cell 
without any change in permeability. The toxic substance itself (alcohol or 
saponin) might, and probably does, break down large indiffusible molecules 
into smaller diffusible ones, permitting exosmosis of them without any change 
in permeability. Second, a reduction in critical plasmolytic concentration 
may mean a decrease in permeability as a result of treatment. This opinion 
is held by some workers, the assumption being that a reduction in critical 
plasmolytic concentration implies that, because of decreased permeability, 
the plasmolysing salt can less readily enter the cell, and will, therefore, 
very slowly raise the concentration (osmotic value) of the cell interior up 
to that of the external salt: consequently, a lower concentration of salt will 
plasmolyse the treated cell than will plasmolyse the normal cell. Con¬ 
versely, an increase in critical plasmolytic concentration may mean that 
there has been an increase in permeability, since then the external salt 
solution can more readily enter the cell and raise its osmotic value, there¬ 
fore a higher concentration of salt is needed to plasmolyse the treated 
cell. 
If the above interpretation is correct it would explain the apparent 
discrepancies between the results of Osterhout ( 17 , p. 318) and those of the 
1 The critical concentration of smilacin-treated cells is determined with difficulty owing to the 
rapid collapse of the slightly plasmolysed cells. (This phenomenon will be considered later.) 
