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-ivall 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 



