STIMULATION IN THE LEAVES OF DIONAEA 
77 
In the case of Dionaea it also appears probable that the decreased 
activity observed with repeated stimulation may be due to depletion 
of previously accumulated materials, for considerable material must 
be expended in the excessive growth that follows stimulation. Such 
an enlargement might occur on account of either one of three possible 
causes, or on account of two or more acting together, (i) A sudden 
increase in the osmotic pressure of the cells that expand might stretch 
the cell walls and lead to permanent enlargement, without any pre- 
liminary change in the walls themselves. (2) The osmotic pressure 
of the cells in question might remain the same, but the extensibility 
of their cell walls might be increased (or their tendency to contract 
decreased) so that the same pressure as was previously effective only 
to hold them in equilibrium might now produce enlargement. This 
hypothesis seems very improbable. (3) Finally, the osmotic attrac- 
tion for water exerted by the cells near the upper surface might be 
decreased, so as to allow water to pass from these cells into those near 
the lower surface, thus allowing an expansion of the latter due to their 
original osmotic pressure. In any of these three cases the enlargement 
of the tissues on the lower side of the lobe must be concomitant with 
the passage of water into these cells due to their osmotic activity. 
Evidence will be adduced below showing that stimulation is 
followed by a decrease in the osmotic pressure of the cells of the upper 
layers, which would allow the passage of water out of these cells into 
those of the lower region, but it seems hardly probable that sufficient 
water to cause the observed changes may thus enter the lower cells 
unless one of the other possibilities just mentioned also becomes 
effective; it is to be supposed that these lower cells have attained 
equilibrium, as far as water is concerned, under the previously existing 
conditions of their osmotic pressure and of the elasticity of their walls. 
If the closing movement of Dionaea leaves occurred only at times of 
the day when incipient drying of the lower leaf tissues [Livingston and 
Brown (1912)] might be postulated, then the enlargement of these 
cells might occur because of the release of a large amount of water 
from the cells of the opposite side, but these leaves are capable of 
vigorous movement at all hours of the day, so that this supposition 
cannot be upheld. 
If the closing movement were due to a stretching of the cell walls 
of the lower region, later rendered permanent by growth, then leaves 
killed just after closure might perhaps be made to reopen by replacing 
