NVrTITKOPir MOVEMENTS 679 



motile impulses generated ? and (2) how is it that the move- 

 ment will still persist in the absence of a periodically exciting 

 cause ? 



Diurnal movement of plagiotropic stem.— In order to 

 understand clearly the periodic action of light, in induc- 

 ing diurnal movements, it is best to take as our starting- 

 point the simplest type of anisotropic organ, in which this 

 effect may be observed, and for this we may select the 

 plagiotropic stem of, say, Cncurhita. This creeping stem is 

 acted on, under natural conditions, by vertical light from 

 above, the excitatory effect of which, by long-continued 

 action, reaches the lower and more excitable side. The 

 stimulus thus becomes internally diffused, and there is also a 

 certain amount of externally diffused light from the environ- 

 ment. Hence it happens that, by the contraction of the more 

 excitable lower half, the free end of the stem becomes progres- 

 sively depressed, under the continued action of light, during 

 the course of the day (p. 627). We have seen that in conse- 

 quence of this the greatest depression occurs about the time 

 of evening, and that on the cessation of the stimulus of light 

 there is a recovery, the stem erecting itself more and more as 

 the night advances (fig. 252). Thus this nyctitropic depression 

 at nightfall is not dne to the on-coming of darkness, but repre- 

 sents the cumulative responsive effect oj the day's illumina- 

 tion. In fact we may regard the responsive movement 

 down, and recovery up, which are executed in the course of 

 the diurnal period, as parallel to that phenomenon with which 

 we are already familiar, 'of a^single response to a single 

 transient stimulus. The only difference lies in the fact that, 

 while in this latter case the whole process is completed in a 

 few minutes, in the former the stimulus acts continuously 

 during something like twelve hours, and the recovery is 

 allowed to take place during the course of the night. Thus, 

 in the ordinary case of records of uniform responses to 

 uniform stimuli, the successive stimuli, each lasting for a {^\^ 

 seconds, are given at intervals of some minutes, but in the 

 case of the diurnal responses we have a series of stimuli. 



