CURVATURE IN THE PULVINI OF MIMOSA PUDICA. 
39 
there are several deep wrinkles running perpendicular to the 
longitudinal axis of the pulvinus. Any decrease in the pressure 
in the cells should cause a deepening of the wrinkles and conse- 
quently a curvature of the pulvinus. In living leaves the turgor 
of the cells of the concave half is probably not entirely removed 
when curvature is produced so that the pressure exerted on 
these by the turgid ones of the opposite side may at least increase 
the rapidity of movement. 
After taking observations on the leaves in xylene they were 
transferred back to alcohol and run down through several grades 
of water. Those pulvini which had been killed after curvature 
was complete remained as before while those with only a partial 
curvature, which had been completed in xylene, straightened 
out to the same extent as when first killed. It would seem that 
this could have been due only to the restoration of the osmotic 
pressure in the cells of the concave or reacting half of the pulvini 
by the redissolving in the water of the osmotically active sub- 
stances in the cells. In no case did the pulvini straighten out 
to a greater extent than when killed. The phenomena exhibited 
by these dead pulvini would seem to show that stimulation pro- 
duces a relatively permanent reduction in the number of osmot- 
ically active molecules in the cells and that the extent of the 
movement which can be produced in the dead pulvini stands in 
some inverse ratio to the extent of this reduction. The reduction 
in the number of osmotically active molecules in the cells might 
be brought about by a chemical change in the contents of the cells 
or by a change in the permeability of the plasma membranes 
which would allow an exosmosis of some of the osmotically active 
substances dissolved in the water of the cells. While, in the case 
of Mimosa, the last possibility is not excluded the first would 
seem more probable, for if the loss in turgor is due to a change 
in the permeability of the membranes which allows the passage 
into the intercellular spaces of dissolved substances, a reverse 
change in permeability would not restore these lost substances 
to the cells, and turgidity could be restored only after the manu- 
facture of additional osmotically active substances. If, how- 
ever, the fall in turgor is due to a chemical change in the con- 
tents of the cells, a reverse change would restore turgidity. The 
closure of the leaf of Dionaea, which will be discussed more fully 
in a later paper, also appears to be due to a change in osmotic 
pressure. When the leaves close there is a passage of water 
from the cells of the inner or concave surface to those of the 
outer or convex, which causes the latter to become greatly 
