400 HooxerR: MOVEMENT IN DROSERA ROTUNDIFOLIA 
phase are present, in which case an increase in the proportion of 
the former to the latter would decrease the elasticity and the reverse 
change would restore the original properties. Whatever changes 
in molecular structure actually occur, they must be controlled and 
regulated by the activity of the protoplasm. 
4. AUTOTROPIC NATURE OF UNBENDING 
(a) Comparison of autotropic with hydrotropic stimulus 
When roots are exposed to a hydrotropic stimulus, greater 
evaporation takes place from the cells of the drier side and this 
tends to increase their osmotic concentration. The resulting 
inequality of osmotic concentration or changes which this induces 
on opposite sides of the root constitute the stimulus that released 
the hydrotropic reaction, which consists in bending toward the 
source of moisture by faster growth on the side with the higher 
osmotic concentration (Hooker, ’15). It is characteristic of hydro- 
tropic reactions that small differences in the relative moisture on 
opposite side of the exposed root, and consequently small differ- 
ences in osmotic concentration are sufficient to produce changes 
that release a reaction. On the other hand the exposure must be 
prolonged, for the reaction does not commence until the roots have 
been subjected to a hydrotropic stimulus for at least six hours. 
Bending proceeds much more slowly than in geotropic reactions. 
The autotropic unbending of Drosera tentacles as well as of 
geotropically bent roots and shoots resembles hydrotropic reactions 
in several respects. The unbending is produced by growth on that 
side of the organ that tends to have the higher osmotic concen- 
tration during bending. The reaction does not begin until some 
time after the bending, and it proceeds at a very much slower rate. 
During this process no difference of osmotic concentration was 
ever observed in opposite sides of the tentacle, which indicates that 
the formation of osmotically active material keeps pace with the 
increase in volume of the growing cells. 
(6) Internal changes that follow bending 
In a discussion of the autotropic unbending of tendrils, Fitting 
(03, p. 612) expresses the opinion that the unbending may be a 
response to a new stimulus produced by ‘the inequality of con- 
