INFLUENCE OF INTENSITY OF STIMULUS 123 



experimenting with sub-petioles of Mimosa bearing numerous 

 pairs of sensitive leaflets. The two electrical connections 

 are made on the middle points of two neighbouring sub- 

 petioles. If a single induction-shock be passed, then it 

 will be found that the point by which the current leaves 

 the petiole — the kathode — becomes the seat of excitation, 

 which is transmitted serially to the neighbouring leaflets. 

 The characteristics of this phenomenon will be dealt with 

 in detail in a subsequent chapter. If instead of a single 

 shock a few alternating-shocks of moderately strong intensity 

 be next passed in rapid succession through the sub-petiole, 

 it will be found that the excitation has become diffuse, 

 the leaflets in the intrapolar tract exhibiting excitation 

 simultaneosly. 



This fact of simultaneous excitation in an interposed 

 tract may be demonstrated by the following experiment 

 giving quantitative results : Two successive records are 

 taken of the response of the pulvinus of Mimosa, the 

 exciting electrode being first placed with the interposed 

 pulvinus 10 mm. apart and again 80 mm. apart. The 

 distance of the pulvinus, in the first case, would be 5 mm. 

 from either electrode, and in the second case, 40 mm. The 

 average velocity of transmission of excitation, as will be seen 

 in the next chapter, may be taken as approximately 16 mm. 

 per second in summer. If the excitation in the interposed 

 tract is not simultaneous, but locally initiated at the points 

 of application of the electrodes, we may expect to find 

 that the periods intervening between the beginning of 

 stimulation and the initiation of response will differ greatly 

 from each other in the two cases. In the first case, where 

 either electrode is distant from the pulvinus by 5 mm., the 

 delay in the response may be expected to be of the order 

 of "3 second. In the second case, where either electrode is 

 distant from the pulvinus by 40 mm., we may expect 

 the delay caused by transmission to be about 2*5 

 seconds. 



If the excitation were to prove simultaneous, however, 



