_Wound-reaction._--I have shown that a prick acts as a mechanical stimulus, and in normal excitable tissues induces an excitatory change of galvanometric negativity. This wound-reaction increases with the extent of the wound, and the suddenness with which it is inflicted. On account of the fineness of the probe, it insinuates itself into the tissue rather than make any marked rupture; the probe again is introduced very gradually; with these precautions the wound-reaction is found to be greatly reduced. The immediate effect of the prick is a negative deflection of the galvanometer, which declines and attains a steady value in the course of about 5 minutes. _Effect of wound on excitability._--I have shewn (p. 81) that severe wound caused by transverse section induced a temporary abolition of irritability in _Mimosa_, but that the normal excitability was restored in the course of an hour. A prick from a thick pin was shown to depress temporarily the rate of growth, the normal rate being restored after an interval of 15 minutes (p. 202). In the case of geo-electric excitability, the depressing effect of the passage of the probe, I find, to disappear in the course of about 10 minutes. For a choice of experimental material we have to find specimens which are not merely geotropically sensitive, but also exhibit large electric response under stimulus. In both these respects the shoot of _Bryophyllum_ and the flower stalk of _Nymphæa_ give good results. ELECTRIC EXPLORATION FOR GEO-PERCEPTIVE LAYER BY MEANS OF THE PROBE. _Experiment 185._--I shall now proceed to give a detailed account of the experiments. The first specimen employed was the shoot of _Bryophyllum_, one contact being made with the side of the stem, and the other with an indifferent point on the leaf which was always held vertical. In a particular experiment, the probe was introduced into the stem through 0·4 mm. and a feeble galvanometric negativity was induced as the wound-effect. After an interval of 5 minutes, this attained a steady value of -15 divisions. On the rotation of stem through +90°, the point A was above and a very much larger deflection of -82 divisions was obtained, being the result of summation of wound and geo-electric effects. On restoration of the plant to vertical position the geo-electric reaction disappeared, leaving the persistent wound reaction of -15 divisions unchanged. The true geo-electric reaction at a point 0·4 mm. inside the stem was thus -67 divisions which is the difference between -82 and -15 divisions. I obtained in this manner the excitatory reactions at different layers of the organ. The following table gives true values of geo-electric reaction at different layers of the stem as the probe entered it by steps of 0·4 mm. TABLE XL.--SHOWING THE GEO-ELECTRIC REACTION AT DIFFERENT DEPTHS OF THE ORGAN (_Bryophyllum_).