HooKER: MOVEMENT IN DROSERA ROTUNDIFOLIA 397 
The external walls appear in optical cross-section as straight lines 
extending between the end walls. 
(b) Permeability 
It is of course possible that the elongation of the cells on the 
convex side of inflected tentacles might be caused by increased 
osmotic pressure, if the detection of the increased osmotic concen- 
tration were rendered impossible by a simultaneous increase in 
the permeability of the cells to the plasmolyzing solutions. The 
possibility that such is the case here is ruled out by two consider- 
ations. Firstly, the determinations of osmotic concentration 
made with an electrolyte, potassium nitrate, were confirmed by 
determinations made with a non-electrolyte, glucose. It is highly 
improbable that there should occur a differential change in per- 
meability of such a nature as to permit increased diffusion of both 
potassium nitrate and glucose, which at the same time would not 
permit increased diffusion of the osmotically active material within 
the cell. Secondly, the correspondence between the increased 
volume of the elongating cells and the decreased osmotic concen- 
tration indicated by experiments cannot be disregarded, since it 
offers a direct and simple interpretation of the experimental data. 
(c) Cell-wall elasticity 
Since there is no evidence that the permeability is altered, and 
since the osmotic concentration decreases during bending, the 
increased turgidity of the elongating cells must be due to a decrease 
in the elasticity of their cell-walls. It is evident that irreversible 
changes take place in the cell-wall, for the increased size of the cell- 
wall is soon rendered permanent, probably by the deposition of 
new cell-wall material. This is shown by the fact that after bend- 
ing is completed, the cells on the convex side lose their excess 
turgidity. The distended outer walls become flat, yet the tentacle 
remains bent. At this stage plasmolysis no longer causes un- 
bending. Gardiner (’85) states that in Drosera dichotoma the 
cells on the concave side not only lose their turgidity after bending 
is finished, but become flaccid. These irreversible changes in 
the cell-wall apparently begin soon after bending starts, for when 
a bending tentacle is forcibly straightened, the distended outer 
