of the leaflets of the plant was attached to the recorder. Stimulus was applied at a distance of 50 mm. The successive dots in the record are at intervals of a second. It will be noticed that two distinct impulses—a positive and a negative—were generated by the action of Indirect Stimulus. The positive impulse reached the responding organ after 1.5 second and caused an erectile movement. The velocity of the positive impulse in the present case is 33 mm. per second. The normal excitatory negative impulse reached the motile organ 44 seconds after the application of stimulus, and caused a very rapid fall of the leaflet, the fall being far more pronounced than the positive movement of erection (Fig. 52). In this and in all subsequent records, the positive and negative responses offer a great contrast. The movement in response to positive reaction is slow, whereas that due to negative reaction is very abrupt, almost 'explosive,' the successive dots being now very wide apart. As regards the velocity of impulse the relation is reversed, the positive being the quicker of the two. In the present case, the velocity of the excitatory negative impulse is 1.1 mm. per second, as against 33 mm. of the positive impulse.
The negative impulse is due to the comparatively slow propagation of the excitatory protoplasmic change, which brings about a diminution of turgor in the pulvinus and fall of the responding leaflet. The erectile movement of the leaflet by the positive impulse must be due to an increase of turgor, brought on evidently, by the forcing in of water. This presupposes a forcing out of water somewhere else, probably at the point of application of stimulus. It may be supposed that an active contraction occurred in plant cells under direct stimulus, in consequence of which water was forced out giving rise to a hydraulic wave. On this supposition the positive impulse is to be regarded as hydro-mechanical. I have, however, not yet
