In the primary petiole of Mimosa the highest velocity under electric stimulation I find to be about 30 mm. per second. This velocity is considerably lower than the velocity in the nerve of higher animals, but higher than in the lower animals. As an example of the latter, mention may be made of the velocity of 10 mm. per second in the nerve of Anodon and 1 mm. per second in the nerve of Eledone.
PREFERENTIAL DIRECTION OF CONDUCTION.
Experiment 33.—The conduction of excitatory impulse takes place in both directions. This can be demonstrated by taking a petiole of Biophytum sensitivum or of Averrhoa carambola. These petioles are provided with a series of motile leaflets. Stimulation at the middle point of the petiole gives rise to two waves of excitation, one of which travels towards the central axis of the plant, and the other away from it. The centrifugal velocity is greater than the centripetal as will be seen from the following results:
| Biophytum… | Velocity in centrifugal direction… | 2.9 mm. per second. |
| Velocity„ in centripetal direction„… | 2.92 mm. per„ second.„ | |
| BiophytumAverrhoa… | Velocity„ in centrifugal direction„… | 0.5 mm. per„ second.„ |
| Velocity„ in centripetal direction„… | 0.26 mm. per„ second.„ |
EFFECT OF TEMPERATURE.
Variation of temperature has a marked effect on the velocity of transmission of excitation. Lowering of temperature diminishes the velocity, culminating in an arrest. Rise of temperature, on the other hand, enhances the velocity. This enhancement is considerable in specimens in which the normal velocity is low, but in plants in optimum condition, the velocity being already high, cannot
