MECHANICAL RESPONSE TO STIMULUS 23 



This necessitates some method of record more expeditious 

 than that of photography. 



Fortunately, the responsive movements of these sensitive 

 organs are relatively slow, usually requiring several minutes 

 for completion. And it is quite easy to follow the excursion 

 of the responding spot of light, with the recording pen, on a 

 horizontal drum. There are some few special investigations, 

 such as those on exceedingly short latent periods, in which 

 automatic records by photography are a necessity, but for the 

 majority of the records the second method is all that is 

 required. By the latter means, moreover, we overcome the 

 serious difficulty occasioned by the variation of sensibility 

 which the plant undergoes when kept long in a dark chamber. 

 When the second method is employed, the specimen may be 

 placed in a well-lighted and well-ventilated room, and under 

 these conditions it is found to maintain its sensitiveness 

 unchanged for a considerable length of time. The fact that 

 the spot of light reflected on the drum becomes inconspicuous 

 in the surrounding daylight is overcome by placing in front 

 of the recording drum a special hood with a long horizontal 

 slit. The back of this hood carves over the head of the 

 observer, and the spot of light then appears very bright. 



Response of Biophytum. — I shall next deal with the 

 responses obtained from Biophytum. In fig. 17 are given two 

 successive responses to two successive thermal stimuli. It 

 will be noticed how uniform these responses are. The up- 

 curve represents the fall of the leaflets, and the subsequent 

 down-curve of the response exhibits its gradual return to 

 the normal outspread horizontal position. An abnormal 

 erectile twitch will be noticed at the beginning of each of 

 these responses. This effect is usually present when a 

 stimulus of whatever nature is applied at a distance from 

 the responding leaflet. Its cause will be explained later. 

 It should be stated here that stimulus was in this case 

 applied at a distance of 35 mm. from the responsive leaflet, 

 and that the true excitatory reaction, by the depression of 

 the leaflet, took place fifteen seconds after the application. In 



