ror7] BROOKS—PERMEABILITY : 237 
carried far enough to cause plasmolysis, we have a means of avoid- 
ing this objection. We may consider first those methods in which 
plasmolysis occurs. 
METHODS INVOLVING PLASMOLYSIS 
1. Concentration Required to Produce Plasmolysis——DE VRIES 
(67) noticed that the concentration of a glycerine solution just 
concentrated enough to produce plasmolysis was higher than that 
expected from the calculated osmotic pressure of the solution. 
He attributed this to the penetration of glycerine into the cell. 
On the assumption that an increase in the concentration of a 
given substance required to produce plasmolysis indicates an 
increase of permeability, LEPESCHKIN (23, 24, 25, 26) and TRONDLE 
(62) claim to have demonstrated an increase of the permeability of 
the protoplasm due to increased illumination; and EcKERSON (6) 
seeks the cause of the thermotropic curvatures of roots in an 
increase in permeability due to rise in temperature. By the same 
method KREHAN (20, 21) has studied the effect of potassium 
‘cyanide on the permeability of cells of Tradescanta discolor, the 
_ experiments seeming to indicate that dilute sol 3 (0.001 M) of 
potassium —, cause a temporar y and eve ible increase in 
permeability, and hat t this is f jeg d rea: in p ea! if - 
which begins si 4, ly ee eo be esibility. oe 
"Oiseiaoee Cy aa t solutions of L cal os 
chlorides, either of which alone is unable to prade ! 
(of cells of Gites Se sp.), may cause rapid plasmolysis when mixed 
in such proportions that the ratio of sodium atoms to those cof se 
cal chum is about 2 20 tot 1  $inc } 
