Fig. 171.—Diagrammatic representation of mechanical and electric response of root to indirect stimulus applied at the tip a. Figure to the left shows responsive movement away from stimulus. The electric response to indirect stimulus is indicated in the figure to the right; the point on the same side exhibiting galvanometric positivity. The shaded part indicates the responsive region of growth at some distance from the tip.
The next investigation was for the determination of the electrical change induced in the growing region by application of unilateral stimulus at the root-tip.
Experiment 181.—One of the two electrical connections with the galvanometer is made at one side of the growing region A, the other connection being made with the diametrically opposite point B. Unilateral stimulus was applied at the root tip a, of the bean plant and on the same side as A. I subjected the tip to various modes of unilateral stimulation. Mechanical stimulation was effected by emery-paper friction or by pin-prick; chemical stimulation was produced by application of dilute hydrochloric acid. Thermal stimulation was caused by the proximity of electrically heated platinum wire. In every case the response was by induced galvanometric positivity at A (Fig. 171). This electrical variation took place within about ten seconds of the application of stimulus; the interval would obviously depend on the length of path to be traversed by the transmitted effect of indirect stimulation.
The galvanometric positivity at A indicated that there was induced at that point an increase of turgor and expansion, in consequence of which the organ would move away from stimulus. Thus both by the mechanical and electrical methods of investigation we arrive at an identical conclusion that the effects of unilateral stimulus at the tip of the root gives rise to a movement, by which the organ is moved away from the source of stimulus; since tropic movement towards stimulus is termed positive, this opposite response must be regarded as negative.
TABLE XXXVI.—EFFECT OF INDIRECT STIMULUS UNILATERALLY APPLIED AT THE ROOT-TIP.
| Effect at the proximal side A in the growing region. | Effect at the distal side B. |
| Galvanometric positivity, indicative of increase of turgor and expansion. | Negligible. |
| The corresponding tropic curvature is negative, i.e., a movement away from stimulus. | |
The root-tip when burrowing its way underground comes in contact with hard substances and moves away from the source of irritation. The irritability of the root-tip is generally regarded as being specially evolved for the advantage of the plant. But reference to experiments that, have been described shows that this reaction is not unique but exhibited by all plant organs, growing and non-growing. Indirect stimulus has been shown to give rise, in both shoot and root, to a negative tropic curvature in contrast to the positive curvature brought about by direct stimulation; the response of the root is therefore in no way different from that of vegetable tissues in general.
It will also be seen that an identical stimulus induces two opposite effects, according as the stimulus is applied at the tip or at the growing region itself. In the former case, the stimulus is indirect, and in the latter case it is direct. The results are in strict conformity with the laws of effects of direct and indirect stimulations that have been established regarding plant response in general (p. 231).
SUMMARY.
In the root, the responsive region is in the zone of growth. The tip of the root is separated from the region of response by a semi-conducting or non-conducting tissue.