73 
Permeability of the Pulvinus of Mimosa pudica. 
Various methods were used for stimulating the pulvinus, such as 
mechanical shock by touching or cutting the top of the petiole, electrical 
stimulation with the help of an induction coil, and stimulation by sudden 
alteration of temperature. The last method was found to be most convenient, 
for the stimulus could be applied by simply raising the pulvinus out of the 
warm fluid. The sudden transition to the cooler air above the fluid acts as 
a stimulus and brings about contraction. 
An experiment was conducted in the following way: 
The condition of the cell was tested by filling it with conductivity water 
which was kept continually stirred by means of a slow stream of bubbles. 
The conductivity was then noted periodically, and the cell was considered to 
be satisfactory when the rise of conductivity was not above a certain rate ; 
owing to the cell being made of ordinary glass and to very slight evaporation 
from the fluid there was always a steady, slight rise of conductivity. This 
‘ washing factor ’ could be allowed for, if necessary, in the later readings. 
The cut pulvinus, previously washed, was then inserted. The rate of 
exosmosis from the cells indicated by the rate of increase of conductivity 
was then determined periodically, the fluid being changed, if necessary, after 
the first few minutes. Stimulation was brought about by raising, by means 
of a rack-and-pinion adjustment, the cork through which the petiole was 
threaded, care being taken that the pulvinus did not come in contact with 
the sides of the cell. If the petiole was raised only sufficiently to bring the 
pulvinus just above the fluid no stimulation resulted, but if the pulvinus was 
raised 25 to 30 mm. above, so that the fall in temperature was considerable, 
stimulation was quickly effected in favourable cases, and the end of the 
pulvinus showed the characteristic curvature after the lapse of a few seconds. 
After a period of time (often 30 sec.) which was constant for each experi¬ 
ment, the pulvinus was returned to the fluid. Readings were taken at 
regular intervals of -J, 1, 2, 3, or 5 min. The bubbling was continued 
throughout the experiment, being stopped only for a definite period to 
allow of a reading being taken. 
As the permeability of the pulvinus was found to be affected by light 
changes (see later) the experiments were usually conducted with the 
pulvinus brightly illuminated by means of a Nernst burner (micro-Nernst 
lamp of Zeiss). 
Table I. 
Effect of Contraction on the Rate of Exosmosis from the Pulvinus 
of Mimosa pudica. 
August 3, 1915. Pulvinus with sides sliced. Temperature 39°9° C. 
Illuminated by Nernst lamp. Stimulated at intervals of fifteen minutes by 
raising the pulvinus into cold air for thirty seconds while taking the reading 
of the conductivity. At other times the readings were taken with the 
