Hooker: Movement in Drosera rotundifolia 395 



passive stretching of the walls by turgidity. This conclusion is 

 corroborated by direct observation. During bending the cells on 

 the convex side in the curved region as seen under the microscope 

 are noticeably distended, the outer walls appearing in optical 

 cross-section as arcs extending between the end walls. A simple 

 way of demonstrating this is afforded by mounting a straight 

 tentacle in a one per cent, solution of tartaric acid, covering it 

 with a supported cover glass and examining under the microscope. 

 Tartaric acid as well as dilute solutions of many other organic 

 and mineral acids induce rapid and violent bending. The acid 

 enters the tentacle through the broken end of the pedicel at the 

 base, where it was formerly attached to the leaf-blade, and affects 

 the bending region directly, for inflexion proceeds even when the 

 gland has not been stimulated in any way either by being touched 

 or by coming in contact with the acid solution.* Gardiner ('85) 

 observed that in well-inflected tentacles of Drosera dichotoma the 

 cells on the convex side at the bending point are very turgid. It 

 must be borne in mind that turgidity is the external manifestation 

 of a balance between two antagonistic factors; one factor due to 

 the presence of osmotically active substances in solution tends to 

 increase the volume of the cell and to stretch the wall ; the other 

 factor due to the elasticity of the cell-wall tends to compress the 

 cell contents. An increase in the size of the cell under such cir- 

 cumstances follows either from an increase in the amount of os- 

 motically active material or from a diminished elasticity of the 

 cell-wall, unless simultaneous changes in permeability interfere. 



(a) Osmotic concentration 



The experimental data show that during bending the osmotic 



concentration in the cells on the abaxial side of the pedicel in the 



growing region falls from eight or nine to six or eight atmospheres. 



It is at once evident that the elongation of the cells is not the 



result of an increase in the amount of osmotically active material. 



Exact data concerning the increase in length of the abaxial side 



* The bending produced in detached tentacles by this means is frequently aston- 

 ishing. The tentacle may bend around completely on itself until it forms a spiral. 

 It is known that stimulation of the glands of Drosera tentacles is followed by the 

 secretion of an acid. Is it possible that the impulse which is conducted from the 

 gland down the pedicel to the base of the tentacle and which there starts movement 

 is the acid secreted by the gland diffusing from cell to cell? 



