82 RESEARCHES ON IRRITABILITY OF PLANTS 



to be borne in mind that the effect of continuous stimulation 

 is, after all, the effect of successive stimuli with the resting 

 interval shortened. On referring back to fig. 38 we notice 

 two phases in the response-series : in the first phase the ex- 

 citability is increasing ; in the second phase, it is decreasing. 

 In the first phase again, we notice that there is a residual 

 contraction, the recovery being incomplete. Owing to this, 

 the base-line is gradually shifting upwards. This, coupled 

 with the enhancing excitability and consequent staircase 

 increase in the individual responses, brings about a maximum 

 additive contraction, as will be understood, by joining the 

 tops of these contractile responses. The additive effect of 

 such contractions would be a responsive fall much greater 

 than could have taken place under any single stimulation. 



If we were now to repeat this experiment, shortening 

 the intervals between the successive stimuli, we should 

 obtain a somewhat similar result, with the sole difference 

 that the successive component responses would appear 

 nearer each other and with their recoveries still further 

 reduced. The result of this would be slight notches in an 

 ascending curve. Carrying this process to a limit — that is 

 to say, when the successive stimuli follow each other 

 quickly, as in continuous tetanisation — the notches them- 

 selves will disappear and we shall have merely an ascending 

 curve. 



Turning to the second phase in the response-series, where 

 the excitability has reached a maximum, we find these 

 phenomena reversed. The leaf having attained its maxi- 

 mum limit of fall, its capacity for further contraction is now 

 reduced. In sharp contrast to the first phase of the series, 

 however, successive contractions now grow smaller and 

 smaller, under growing fatigue, while the relaxations tend 

 to become increasingly large. In the extreme case of con- 

 tinuous tetanisation the resulting record in this phase would 

 be one of relaxation, appearing as a down-curve. Thus 

 under tetanisation we should have a response-curve, showing 

 first the normal contraction, followed in the second place by 



