We painted with a thick layer of lamp-black in oil the intumescence of different petioles in different ways; the upper surface of one, the under surface of another, the side of a third. The experiment was followed by no sensible effect. After a few minutes the petioles, which had been thrown down by the operation, rose again in each case, and fell again as readily as before upon being stimulated afresh. We tried what result would ensue upon slitting the intumescence of the petiole horizontally. The petiole, after this injury, did not recover its usual direction; the intumescence appeared to have wholly lost its properties; the leaf seemed to depress the petiole by its weight alone, yet the leaflets expanded, and exhibited their usual irritability, upon the depending stalk. The same effect, however, was observed, when the [p083] intumescence was divided by a longitudinal incision, made vertically instead of horizontally. I have already mentioned that Dutrochet discovered that the ligneous fibre is the channel, along which an impression is conveyed from one part to another. Mr. Burnett and myself had made one or two experiments upon the course which the irritation follows when spreading from leaflet to leaflet, where several are placed upon the same petiole. If the upper third of a petiole bearing four leaflets be divided longitudinally, the irritability of the leaflets remains for many days unimpaired; upon cutting with scissors one subleaflet after the plant has recovered itself, the irritation is observed to descend the wounded leaflet, and then to pass to that adjoining upon the same side of the petiole: afterwards the petiole falls, but there the effect stops; it does not extend to the two other leaflets; the direct route is cut through, and the irritation seems to find no circuitous way, as might have been expected, perhaps through the intumescence of the petiole back again to the leaflets, on its summit. If on a petiole, bearing four leaflets, a lateral incision be made, cutting the petiole half through it at a point between the two leaflets which are situated on one side, upon irritating either of the leaflets, between which the incision has been made, it folds its subleaflets; then the two opposite leaflets fold _their_ subleaflets; and _last of all_, the leaflet next adjoining that first irritated, but isolated from it by the incision, becomes folded. In the few remarks which I have thus put together, I have quoted Lindsay and Dutrochet only as far as their researches anticipated my own: I leave unnoticed many experiments, in several of which these authors are again found to have accidentally coincided. The experiments to which I allude do not, however, serve to illustrate the nature of the motion exhibited by the sensitive plant, to the examination of which subject alone my attention was, in the present instance, directed, in the expectation that it might throw light upon the obscure and interesting subject of muscular action.