400 Hooker: 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) Cojn pari son of aiitotropic zvith 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. 



(b) 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- 



