[Illustration: 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.] _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 | Effect at the distal| | growing region. | side B. | +--------------------------------------+-----------------------+ |Galvanometric positivity, indicative | Negligible. | | of increase of turgor and expansion. | | |--------------------------------------------------------------| |The corresponding tropic curvature is negative, _i.e._, a | |movement away from stimulus. | +--------------------------------------------------------------+