Experiment 47.—Stimulus of electric shock applied at a point on the long petiole of Averrhoa causes successive fall of pairs of leaflets. In the experiment to be described one 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 been able to devise a direct experimental test to settle the question.
EFFECT OF DISTANCE OF APPLICATION OF STIMULUS.
In the last experiment the stimulus was applied at the moderate distance of 50 mm. Let us now consider the respective effects, first, of an increase, and second, of a decrease of the intervening distance. In a tissue whose conducting power is not great, the excitatory impulse is weakened, even to extinction in transmission through a long distance. Thus the negative impulse may fail to reach the responding organ, when the stimulus is feeble or the intervening distance long or semi-conducting. Hence, under the above conditions, stimulus applied at a distance will give rise only to a positive response.
A reduction of the intervening distance will give rise to a different result. As the negative response is the more intense of the two, the feeble positive will be masked by the superposed negative. The separate exhibition of the two responses is only possible by a sufficient lag of the negative impulse behind the positive. This lag increases with increase of length of transmission and decreases with the diminution of the length. Hence the application of stimulus near the responding organ will give rise only to a negative response, in spite of the presence of the positive, which becomes masked by the predominant negative.[P]
These inferences have been fully borne out by results of experiments carried out with various specimens of plants under the action of diverse forms of stimuli. In all cases, application of stimulus at a distance causes a pure positive response; moderate reduction of the distance induces a diphasic response—a positive followed by a negative; further diminution of distance gives rise to a resultant negative response, the positive being masked by the predominant negative.
From what has been said it will be understood that the exhibition of positive response is favoured by the conditions, that the transmitting tissue should be semi-conducting, and the stimulus feeble. It is thus easier to exhibit the positive effect with the feebly conducting petiole of Averrhoa than with the better conducting petiole of Mimosa. It is, however, possible to obtain positive response in the Mimosa by application of indirect stimulus to the stem in which conduction is less rapid than in the petioles.
TABLE IX.—PERIODS OF TRANSMISSION OF POSITIVE AND NEGATIVE IMPULSES IN THE PETIOLE OF AVERRHOA AND STEM OF MIMOSA.
| No. | Specimen | Distance in mm. | Stimulus | Transmission period for positive impulse. | Transmission period for negative impulse. |
| 1 | Averrhoa | 70 | Thermal | 22.0secs | 65 secs. |
| 2 | " | 130 | " | 40.0 " | 95 " |
| 3 | " | 10 | Induction-shock | 6.0 " | 20 " |
| 4 | " | 20 | " | 14.0 " | 48 " |
| 5 | " | 35 | Chemical | 21.0 " | 50 " |
| 6 | Mimosa | 5 | Induction-shock | 0.5 " | 12 " |
| 7 | " | 10 | " | 0.6 " | 9.4 " |
| 8 | " | 20 | " | 1.1 " | 10 " |
| 9 | " | 60 | " | 2.0 " | 29 " |
| 10 | " | 35 | Chemical | 5.0 " | 17 " |