402 HooKER: MOVEMENT IN DROSERA ROTUNDIFOLIA 
on the concave side. Without doubt the same factors found to 
determine bending are involved in the unbending, but the process 
is so slow that it resembles ordinary plant growth. It does not 
seem probable that this deferred reaction can be a direct response 
to changes produced by a difference of osmotic concentration which 
existed during bending, but is now completely effaced. In these 
cases unbending is probably a response to changes taking place 
as the result of a cessation of absorption and the end of the aggre- 
gated condition. Nevertheless the effects of the difference in 
osmotic concentration are conditioning factors of the unbending 
reaction, for an accelerated rate of growth on the adaxial side of 
the tentacle occurs only after previous bending. 
IV. SUMMARY 
The osmotic concentration in cells of Drosera rotundifolia ten- 
tacles was measured by plasmolysis in potassium nitrate and 
glucose solutions. Measurements were made on straight, bending, 
bent and unbending tentacles. The osmotic concentration in the 
cells on the abaxial side of the stalk, in the growing region, was 
found to diminish during bending; no change was observed on the 
adaxial side. The decrease in osmotic concentration is accounted 
for by the increase in volume of the cells, and is therefore considered 
an effect and not a cause of their elongation. There is no indica- 
tion that changes in permeability occur. 
The elongation is produced by a decrease in the elasticity of the 
cell-walls, and is later fixed by growth. The movement of ten- 
tacles is therefore brought about by the same mechanism found in 
geotropically reacting organs, where a decrease has been observed 
in the osmotic concentration in the cells whose growth causes 
bending. 
Similarities between hydrotropic reactions and autotropic 
unbending of tentacles and of geotropically bent roots indicate 
that the growth on the concave side which brings about the un- 
bending is a response to changes resulting from the difference in 
osmotic concentration present during bending. As in hydrotropic 
reactions, growth takes place on the side with the higher osmotic 
concentration. 
