40 
BROWN. 
stretched. If the leaves are killed soon after closure and trans- 
ferred from water to xylene, so that the osmotic pressure in the 
cells of the convex side is removed, the leaves open. A change 
in the permeability of the membranes might cause the passage 
of water from the cells of the inner surface but could hardly 
explain its entrance into those of the opposite side. In both 
Mimosa and Dionaea there appears to be a reduction in the os- 
motic pressure in the cells of the side toward which bending 
takes place. In Mimosa the water which passes from the cells 
goes largely into the intercellular spaces, while in Dionaea it is 
taken up by the cells of the opposite surface. In Mimosa move- 
ment is due to a fall in the turgor of the cells of the concave 
half of the pulvinus while in Dionaea it is due to the stretching 
of the cells of the convex surface. 
The experiments with the dead pulvini of Mimosa show that 
changes in the osmotic pressure in the cells of the reacting half 
can cause movement without the aid of any vital phenomena 
and may therefore be taken as a confirmation of Pfeffer’a 
conclusion that ’the movement of the living pulvini is due to 
changes in the osmotic pressure of the cells of the reacting 
half. Since the experiments with the pulvini of the leaflets and 
Pfeifer’s with those of the petioles lead to similar conclusions 
it would seem that the reactions in the two cases are similar. 
