to the Plant Cell. 
521 
I used were placed in a refrigerator at a temperature of 5 to io° C. during 
the night. The next morning they were removed and placed before 
a window in strong diffuse light at a temperature of 22° to 25 0 C. After 
two or three hours there was usually little difficulty in finding numerous 
cases of mitosis in the control cultures. Five hours after the material had 
been taken from the refrigerator, most of the mitotic divisions were completed, 
and the new cell- wall entirely formed. 
In carrying out these experiments I chose cultures of Spirogyra which 
had grown for thirty-five days in the absence of potassium salts, together 
with a number of control cultures which had grown for an equal length 
of time in complete nutrient solution. Both series of cultures were sub- 
jected to conditions which induced mitotic divisions in the control plants. 
The filaments in the potassium-free cultures appeared to be in a living 
condition, although by the absence of starch it was plainly to be seen that 
they felt the lack of potassium salts. The comparatively healthy condition 
of the filaments may be accounted for by the Tact that during much of the 
time that they had been deprived of potassium the temperature had been 
between 12 0 and i8°C. At that temperature there would be less activity 
in starch formation, consequently the absence of potassium was less 
severely felt. 
When the cells were exposed to conditions normally inducing mitotic 
division, the lack of potassium was very evident. A short time after they 
had been placed in optimum conditions it was found that the cells in the 
potassium-free solution had elongated to at least twice their normal length. 
The nuclei also had elongated in the direction of the longitudinal axis of 
the cell. A careful examination of both living and fixed material from 
these cultures failed to show any instances of cell or nuclear division. At 
the same time there were numerous instances of mitotic division in the 
control plants. In the species of Spii'ogyra which was employed the average 
length of the cells was between 0-4 and 0*5 mm. The average length of 
twenty cells which had been stimulated without being able to divide 
was 0-85 mm., while one cell was found which measured 1*85 mm. in 
length. 
A number of the filaments containing cells which had been unable to 
divide on account of the lack of potassium were carefully removed at 5 p.m., 
and placed in a watch-glass which contained some of their culture solution. 
During the night the temperature ranged from 15 0 to i8°C. When ex- 
amined on the following day at 9 a.m. they were found to retain their 
former undivided condition. 
In another experiment I tried the effect of transferring filaments from 
a culture in which sodium had been substituted for potassium, and in which 
the cells were consequently unable to divide, to a culture solution in which 
ammonium salts had been substituted for potassium. This transfer from 
Pp 
