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,
