It has been shown that under geotropic stimulus the upper side of the tip, a, becomes galvanometrically negative, while the point A, higher up in the growing region, becomes galvanometrically positive. If now we make the two galvanometric connections with a and A, the induced electric difference is increased, and the galvanometric response becomes enhanced.
Experiment 184.—The root was at first held vertical, and two electric contacts made with a and A. In this neutral position there is little or no current. But as soon as the root was laid horizontal, an electro-motive response was obtained which showed that a was galvanometrically negative, and A galvanometrically positive (Fig. 173d). The induced electric response disappeared on restoration of the root to the vertical position. I give below the results of typical experiments with a vigorous specimen which gave strong electric response. It was possible to repeat the geotropic stimulation six times in succession, the results being perfectly consistent. The responses taken in succession exhibited slight fatigue, the first deflection being 140 divisions, and the sixth 115 divisions of the galvanometer scale.
TABLE XXXIX.—INDUCED E. M. F. VARIATION BETWEEN THE TIP AND THE GROWING REGION (a NEGATIVE AND A POSITIVE).
| Geotropic stimulation. | Resulting electric response. |
|---|---|
| First stimulation | 140 divisions. |
| Second " | 130 " |
| Third " | 130 " |
| Fourth " | 123 " |
| Fifth " | 127 " |
| Sixth " | 115 " |
The results of experiments 182 and 183 are summarised as follows:—
(1) the induced galvanometric negativity at root tip indicates direct stimulation of the tip, and
(2) the induced galvanometric positivity of the growing region shows that it is the effect of indirect stimulus that reaches it.
From these facts it will be seen that the tip perceives the stimulus and thus undergoes excitation, and that owing to the intervening tissue being a semi-conductor of excitation, it is the positive impulse that reaches the growing region and induces there an expansion and a convex curvature.
GEO-PERCEPTION AT THE ROOT TIP.
The results given above fully confirm Charles Darwin's discovery that it is the root tip that perceives the stimulus of gravity[37]; he found that removal of the tip abolished the geotropic response of the root. Objection has been raised about the shock-effect of operation itself being the cause of abolition of response. But subsequent observations have shown that Darwin's conclusions are in the main correct.