As regards response to other forms of stimuli, the root has been shown to behave like the shoot. We have now to inquire whether the reaction of the root to gravitational stimulus is specifically different to that of the shoot. The electric method of investigation described in the last chapter, holds out the possibility of discovering the character of the responsive reaction induced in the root by its displacement from vertical to horizontal position; we shall, moreover, be able to make an electrical exploration of the root-tip and the zone of growth, and thus determine the qualitative changes of response, induced in two regions of the root under the action of gravitational stimulus. For the detection of geotropic action in the shoot, electric contacts were made at two points diametrically opposite to each other. Displacement of the shoot from vertical to horizontal position induced excitatory change of galvanometric negativity at the upper side of the organ, demonstrating the effect of direct stimulation of that side; this excitatory reaction of the upper side finds independent mechanical expression in the induced contraction and concavity of that side of the organ. I employ a similar electric method for detection of geotropic excitation of the root, responses to geotropic stimulus being taken at the root-tip and also at the zone of growth in which geotropic curvature is effected. I shall now proceed to give a detailed description of the characteristic electric responses of the tip and of the growing region. The two diametrically opposite contacts at the tip will be distinguished as _a_ and _b_, the corresponding points higher up in the growing region being A and B. When the root is vertical the electric conditions of the two diametrically opposite points are practically the same. But when the root is rotated in a vertical plane through +90° a geo-electric response will be found to take place; the direction of the responsive current disappears when the root is brought back to the vertical. Rotation through -90° gives rise again to a responsive current, but its direction is found reversed. GEO-ELECTRIC RESPONSE OF THE ROOT-TIP. _Experiment 182._--I took the root of the bean plant and made two electric contacts with the diametrically opposite points, _a_ and _b_, of the root-tip at a distance of about 1·5 mm. from the extreme end. Owing to the very small size of the tip this is by no means an easy operation. Two platinum points tipped with kaolin paste are very carefully adjusted so as to make good electric contacts at the two opposite sides, without exerting undue pressure. For geotropic stimulation the root has to be laid horizontal, and as the root of the bean plant is somewhat long and limp, displacement from the vertical position is apt to cause a break of the electric contact. This is avoided by supporting the root from the top and also from the sides; for the latter purpose, I use paddings of cotton wool.