75 
Permeability of the Pulvinus of Mimosa pudica 
The fall in the early period is possibly due to the washing away of substances 
derived from the cut cells, but it may be that there is an actual decrease of 
permeability as a result of the washing out of salts from the living cells as 
indicated by some of Loeb’s experiments on eggs. The fall in the rate of 
exosmosis cannot be due to a fall in the diffusion gradient between the 
living cells and the surrounding medium. For a dead pulvinus may raise 
the conductivity of the fluid to 400 gemhos (which corresponds to about 
N/10 NaCl), and as the volume of the pulvinus is only o-i, or less, of that 
of the surrounding fluid the concentration of electrolytes in the cells would 
seem to be of an order corresponding to several thousand gemhos. 
Turning now to the actual rate of exosmosis in relation to contraction 
we find that the response to stimulation is associated with an increased loss 
by exosmosis; the increased loss being of the order of 2-3 times, as seen in 
the readings of the second hour. In the earlier period of the experiment the 
greater rapidity of the fall in what one may call the normal rate of exosmosis 
tends to reduce the apparent effect of contraction in increasing the rate of loss. 
As, however, the rate of normal loss of electrolytes under the conditions 
of the experiment is so small that the pulvinus may be subjected to it for 
many hours without loss of power of response, it is clear that an increase 
of that loss during five minutes to two or three times the normal can in 
no way account for the loss of turgor of the cells. It is clear then that 
contraction of the pulvinus is not associated with an increase in the per¬ 
meability of the cells to electrolytes in any way sufficient to produce such 
a loss of turgor of the cells as would explain the contraction. 
These experimental results give no direct information as to the 
behaviour of non-electrolytes such as sugars, and it might be suggested that 
a rapid loss of these substances from the cell would explain the fall in turgor 
of the cells of the lower half of the pulvinus. It is, however, improbable 
that there should be a very marked increase of permeability in relation to 
non-electrolytes unaccompanied by any corresponding increase in the rate 
of exosmosis of electrolytes. Apart from this, however, the repeated con¬ 
tractions and recoveries of the cut pulvinus when immersed in warm water 
seem to exclude, as already pointed out, any explanation of the contraction 
which is based on the passage of osmotic substances out of the cells, whether 
those substances be electrolytes or non-electrolytes. 
The increased loss of electrolytes associated with the contraction of the 
pulvinus may be due to a change of permeability associated with stimulation 
or it may be a mere secondary mechanical effect, *the contraction of the 
pulvinus squeezing a certain amount of material out of the intercellular 
spaces or cell walls. An experiment with the immotile pulvinus of Sapindus 
emarginatus suggested that here also cooling affected the rate of exosmosis 
in spite of the absence of contraction. An experiment with a pulvinus 
of Mimosa which was not responsive to stimuli also gave the same result. 
