84 Blackman and Paine.—-Studies in the 
a different shape. The experiments, however, seem clearly to indicate that 
in the case of the pulvinus of Mimosa , the temperature coefficient between 
35 0 and 50° C. for the rate of exosmosis of electrolytes into water is not 
markedly greater than unity. 
Summary. 
An excised pulvinus of Mimosa pudica when placed in warm water 
with its internal tissues freely exposed exhibits, on stimulation, repeated 
contractions during many hours. This indicates very clearly that the loss 
of turgor in the cells of the lower half of the pulvinus, which is associated 
with contraction, cannot be explained, as has been suggested, by a sudden 
increase of permeability of the tissues allowing of a rapid exosmosis of 
osmotic substances. 
By placing a pulvinus in a very small quantity of water in a specially 
constructed conductivity ‘cell’ the exosmosis of electrolytes can be studied 
by the change of the electrical conductivity of the fluid. By this means it 
has been shown that contraction is associated with an increase in the rate of 
exosmosis of electrolytes from the pulvinus. The exact nature of this 
increase is obscure, but the increase is far too small to account for the sudden 
loss of turgor of the cells. The loss of turgor is probably due to the dis¬ 
appearance or inactivation of a considerable part of the osmotic substances 
of the cells. 
The ‘ conductivity method * being direct is much superior to the 
indirect plasmolytic method used by previous workers for the study of the 
effect of light on permeability. Using the ‘ conductivity method ’ it can be 
shown that in the case of the pulvinus of Mimosa the permeability of the 
cells for electrolytes is markedly increased by exposure to light, but the 
effect rapidly falls off in time. A sudden change from light to darkness also 
increases permeability. 
A curious autonomous (autogenic) contraction of the pulvinus was 
observed. In some cases, after the pulvinus had been stimulated by cooling 
on being brought back into warm fluid, it exhibited a series of contractions 
(in one case sixteen in number) although the external conditions remained 
perfectly constant. The contractions occurred at gradually decreasing 
intervals, so that the time for recovery becoming less and less the contractions 
also decreased in extent, and the pulvinus at the end of the series remained 
in the contracted state. The pulvinus of Mimosa thus for a time showed 
movements as autonomous as those of Desmodium gyrans. 
Below 50° C. a slow rise of temperature appears to have little effect on 
the rate of exosmosis into water of electrolytes from the cells of the pulvinus 
of Mimosa . The increase of permeability with higher temperatures is 
apparently due to lethal, no doubt irreversible, changes. 
