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? 
