56 RESEARCHES ON IRRITABILITY OF PLANTS 



specimen, but using the reduced stimulus-intensity of '2. 

 The result given in fig. 25 shows that the stimulus had to be 

 repeated five times to become effective. We see once more 

 in this experiment that the additive effect is strictly quanti- 

 tative, and that the effective stimulation is constant under 

 varying intensity of stimulus, being equal to the individual 

 intensity multiplied by the number of repetition. In the 



Fig. 24. — Stimulus of intensity *5 Fig. 25. — Stimulus of intensity '2 

 became effective after two became effective after five 



repetitions. repetitions. 



two cases here given we have a strict reaffirmation of this 

 quantitative relation — namely, '5 X 2 = *2 X 5 = constant. 



Influence of Load 



In the response of muscle it is found that the muscle- 

 curve is modified by the effect of the load which it has 

 to raise during contraction. With an increasing load the 

 height of response undergoes a progressive diminution, 

 but the period of recovery is at the same time correspond- 

 ingly shortened. In the contractile response of Mimosa 

 a similar phenomenon is observed. In carrying out this 

 experiment a load was placed on the arm of the horizontal 

 lever opposite to that of the leaf-attachment, and at an 

 equal distance from the fulcrum. The leaf, during its 

 contractile movement, has to lift this weight. In the first 

 experiment of the series a load of 100 milligrammes was 

 employed. In the second this was increased to 500 mgrms., 



