STIMULATION IN THE LEAVES OF DIONAEA 8 1 
the lobes were usually slightly reflexed. While in this condition, 
which did not appear to be connected with their age, they did not 
respond to mechanical stimulation but they became sensitive later. 
During the apparently insensitive phase all of the cells contained 
considerable starch. Leaves of the second class were exceptionally 
thin, never contained starch either before or after stimulation and 
never showed any response. Such leaves were characteristic of certain 
plants, and were not observed to alter in the respects mentioned as 
they became older. The large amount of starch found in the first 
class of insensitive leaves, suggesting high osmotic pressure of their 
cells, and the entire absence of starch in leaves of the other class may 
perhaps be connected with the failure of these leaves to respond to 
mechanical stimulation. 
The curvature of the primary pulvini of Mimosa, according to 
Pfeffer (1906), while aided by the force of gravitation acting upon the 
leaf segments, is mainly produced by an active contraction of the cell 
walls of the lower region and by compression exerted by the cells of 
the upper region, the latter remaining turgid after stimulation while 
the lower cells become flaccid. This flaccidity is brought about by 
loss of turgor due to the passage of water out of the cells into the 
intercellular spaces of the shrinking region of the pulvinus. Brown 
(1912) has shown that curvature ma}^ be produced in killed pulvini 
of Mimosa leaflets, as in the leaves of Dionaea, by transferring them 
from water through alcohol to xylene. In Mimosa, however, no 
movement is produced if the curvature has been completed before the 
pulvini are killed. If curvature has not been completed at the time 
of killing, transfer to xylene results in the completion of the curvature, 
and a return from xylene to water causes these leaflets to resume the 
position which they had when killed. Transfer from water to xylene 
must reduce the osmotic pressure in all the cells of the pulvini, and 
the movement of the dead organs seems, therefore, to have been due 
to shrinkage of the cells of the concave region while those of the 
convex region remained more rigid. 
These experiments appear to be in agreement with Pfeffer's results, 
and they emphasize an apparent difi^erence between Mimosa and 
Dionaea. In Dionaea, only leaves killed just after closing show opening 
movement when transferred from water to xylene; such leaves open by 
reason of decreased osmotic pressure in the cells of the convex region, 
this pressure having previously served to keep the tissue of this region 
