1 8 Macfarlane. — Contributions to the History of 



cannot attempt to decide what meaning attaches to these 

 phenomena, unless on t'he lines of Sanderson's work. 



The above observations, coupled with Sanderson's state- 

 ment, that electrical disturbance is first "appreciable by 

 the electrometer at the external surface at about one-sixth 

 of a second after mechanical excitation," and that "the 

 excursion attains its maximum in one second, and that its 

 return occupies about the same time," having satisfied us that 

 propagation of a mechanical stimulus is comparatively slow, we 

 determined to try whether two stimuli, very rapidly applied, 

 with a time interval of about one-fourth second or less, might 

 not be propagated through the protoplasm as a practically con- 

 tinuous impulse. Two rapid touches, with a time interval of 

 about the fourth of a second, were given to a hair without 

 causing visible result other than rhythmic movement of the 

 halves. A third touch then effected closure. The above 

 has been frequently verified with uniform results. Similarly 

 two forceps-snips, applied very rapidly in succession, required 

 to be followed by a third to effect closure. If, however, 

 the interval between the first and second shocks be expanded 

 to a third of a second or more, the two shocks are separately 

 propagated, so that the leaf closes. The above phenomena 

 in contraction of Dioncea tissues agree with those of animal 

 muscle where a definite strength of stimulus must act for a 

 certain time. There has not been opportunity as yet for using 

 an apparatus which would give three, four, or more stimuli 

 within the fourth of a second, but there is strong probability 

 that such could be applied previous to another stimulus that 

 would cause closure. It is evident, however, that three dis- 

 tinct stimuli can be given previous to leaf movement. It 

 has not as yet been determined whether the double stimulus, 

 propagated as a single wave, can thereby shorten the duration 

 of a summation series when wide time intervals are allowed to 

 intervene. 



We can now refer to what may have misled observers hitherto, 

 and given rise to the statement that one touch is sufficient 

 to cause closure. If a hair is touched in a rather slow and 

 deliberate manner, the leaf closes ; but it must be remembered 



