STIMULATION IN THE LEAVES OF DIONAEA 
79 
It appears that the only way by which transfer from water to 
xylene may produce opening of the leaf, as above described, must be 
through the removal of the osmotic pressure in all of the cells, and 
these experiments seem to show that the stimulation closure that had 
occurred just before the experiment began was due to a stretching, 
by osmotic pressure from within, of the cell walls of the ventral portion 
of the leaf. It thus appears that the movement of closure is due to 
stretching of these cell walls rather than by ordinary growth. The 
enlargement thus effected is soon fixed by permanent growth changes, 
however, as is shown by the fact that the transfer from water to 
xylene does not affect the form of the leaves if these have been closed 
for a half-hour before being killed. Since the tests involving measure- 
ments of the changes in the surface dimensions of normal leaves as 
these close and reopen agree with the experiment just described, it 
seems clear that stimulation closure is not due to an active contraction 
of the tissues near the dorsal surface, but is due to osmotic expansion 
of the cells in the ventral region. 
The supposition that the closing response in Dionaea leaves may 
be due to alterations in the permeability of the protoplasmic mem- 
branes seems to be excluded by the experiments described above. 
Enlargement of the cells in the ventral region must be preceded and 
accompanied by entrance of water into these cells, probably from those 
of the dorsal region. Changes in permeability might account for the 
passage of water out of the latter cells although as Pfeffer (1906) 
points out this would necessitate the movement of dissolved substances 
along with the water. Changes in permeability can not, however, 
explain the passage of water into the cells of the ventral region of the 
leaf, and it is this movement of water which appears to be the first 
condition necessary for stimulation closure. 
A study of the cell contents in stimulated and unstimulated leaves 
of Dionaea brought out some interesting and apparently important 
facts. Forty-eight apparently normal and sensitive leaves were killed 
in boiling water, some open, others just after closing and still others 
fifteen minutes or more after closing, and all were then examined for 
starch. Those killed while open showed very little or no starch in 
any of their cells, but when starch occurred most of it was in the upper 
epidermis and all of it was in the upper region of the leaf, between 
the veins and the dorsal surface. Leaves killed just after closing gave 
similar results. Those that had been closed for fifteen minutes or 
