LATENT PERIOD AND REFRACTORY PERIOD 



^75 



effects of superposition of stimuli. In the right-hand record 

 in fig. 113 the stimuli were applied at intervals of thirty 

 seconds. The successive responses, except the last, show 

 a regular decrease. In the left-hand record, successive 

 stimuli were applied at intervals of a minute, and appear 

 much more equal. 



But even in these additive effects we find one peculiarity 

 which is also characteristic of cardiac response. In the latter 

 case, though on repetition of stimulus there is no summation 

 of height of response, yet the apex-time of the second 

 response is shorter than that of the first. We see this in 

 the case of Biophytum, in the right-hand curve. The first 

 response has a slightly rounded top, but this is reduced to an 

 acute angle in the second. The record, having been reduced 

 to one-eighth for reproduction, does not show this so plainly 

 as does the original. 



Though each single response of Biophytum is maximal, 

 yet from fig. 113 we have seen that this maximal response 

 does not represent the utmost movement of which the leaflet 

 is capable. In the particular plant here used for experiment, 

 the maximal response to individual stimulus was always 

 about thirty-eight divisions, but four superpositions produced 

 a total movement of ninety-four divisions. It must be 

 remembered that such effects can only be possible when 

 the second stimulus is timed to fall at the expiration of 

 the preceding refractory period. 



If the absolute value of each individual minimally effective 

 stimulus be represented by S, and if the whole be added 

 together, or, in other words, if a stimulus of 4s be given 

 at once, we may regard such a stimulus as made up of one 

 minimally effective, plus three others which fall within the 

 refractory period, and are thereby rendered totally ineffective. 

 In other words, we may regard a very strong stimulus as 

 made up of so many minimally effective stimuli. It is as 

 if the first effective fraction alone acted, the succeeding 

 portions, which arrive within the refractory period, being 

 inoperative. 



