chap, xi Sensitivity and Stimulation 499 



is removed before the latter is manifested. Another very 

 similar phenomenon is the prolonged maintenance of the 

 movement or response after the actual stimulation has 

 ceased. This is generally spoken of as the after cfject. 

 Attention was first called to it by Sachs in 1873, when 

 he found that a negatively geotropic stem placed hori- 

 zontally till a curvature began, and then placed vertically, 

 continued to curve against its apogeotropic tendency for 

 one to three hours. The observation was confirmed by 

 Wiesner and by Muller in 1876. Vines, two years later, 

 noticed an after effect of tonic stimulation of light. He 

 kept a leaf of Secale in the dark, but exposed it at in- 

 tervals to light for an hour at a time. Its growth was 

 retarded during the exposures, and when it was returned to 

 darkness the retardation persisted for at least another hour. 

 Darwin published many instances of after effect in 1880. 



An interesting case of the production of an induced 

 rhythmic movement as an after effect of prolonged rhyth- 

 mical geotropic stimulation was recorded by F. Darwin 

 and Pertz in 1892. 



An important contribution to our knowledge of after 

 effects was made by Rothert in 1896, when he showed that 

 they are not stopped by the removal of the sensory area. 

 Once started, the sensory impulses move along to the curving 

 zone and the subsequent injury does not arrest them. The 

 period of stimulation necessary to secure this after effect 

 varies with the nature of the stimulus. Rothert found the 

 interval in the case of heliotropism to be a very short 

 one. Similar observations on geotropic stimulation had 

 been made earlier by Ciesielski and by Darwin. 



Rothert's experiment shows that the maintenance of the 

 movement is not due to passive continuance of the irrita- 

 tion of the sensitive organ, but that its explanation must 

 be sought in the sluggish response of the protoplasm of 

 the curving region. Wortmann, in 1887, called attention 

 to the slow accumulation of protoplasm in the cells of the 



1 i 2 



