_Experiment 179._--The terminals of the galvanometer were suitably connected with the two diametrically opposite points A and B in the growing region of the root. Stimulus was now applied very near the point A, the various stimuli employed in different experiments being: (1) mechanical, (2) chemical, and (3) thermal. In every instance the excited point A becomes galvanometrically negative. This shows that the response of the root is in no way different from that of the shoot. MECHANICAL RESPONSE TO INDIRECT STIMULUS. Before describing the effect of indirect stimulus on the root, I shall recapitulate its effects on ordinary tissues. I have shown that the effect of indirect unilateral stimulus is to induce a movement away from stimulus. This was shown to be the case with the bud of _Crinum_ (p. 275) and the tendril of _Passiflora_ (p. 291). The mechanical and electric response to indirect stimulation in the shoot is shown in the diagrammatic representation (Fig. 170). I shall now proceed to describe the mechanical response induced by unilateral stimulation of the root tip. As the responding region of growth is at some distance from the tip, the stimulation is therefore indirect. [Illustration: FIG. 170.--Mechanical and electrical response to indirect stimulation at dotted arrow. In figure to the left, the point A, on the same side undergoes expansion, with responsive mechanical movement away from stimulus indicated by continuous arrow. In figure to the right, indirect stimulus at dotted arrow induces electric response of galvanometric positivity at A, indicative of increase of turgor and expansion.] _Experiment 180._--I employed at first mechanical stimulus of moderate intensity by rubbing one side of the tip of the root of _Bindweed_; this induced a movement away from stimulus. Unilateral application of dilute acid gave rise to a similar response. Thermal stimulus of moderate intensity also induced responsive movement away from the stimulus (Fig. 171). Darwin in his _Movements of Plants_ described experiments on the responsive behaviour of the tip of the radicle. He produced unilateral stimulation in three different ways, first by attaching minute fragments of cardboard to one side of the root-tip; this moderate and constant irritation was found to induce a convexity on the same side of the growing region, with the resulting negative movement, _i.e._, away from stimulus. His second method was chemical, one side of the tip being touched with silver nitrate; the third method of stimulation was a slanting cut. All these methods induced a movement away from stimulus. ELECTRICAL RESPONSE TO INDIRECT STIMULATION. 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.