Now it is to be remarked that in the subleaflets the upper part of the little intumescence belonging to each corresponds, in one respect alluded to, with the lower portion of the intumescence of the petiole; _it is the portion shortened when the leaf is folded_. And we found, upon examination, that it likewise distinctly changes colour at the moment when the subleaflet rises, while the under surface of the intumescence of the subleaflet does not change its hue. In pursuing this inquiry, another point of correspondence between the mechanism which depresses the petiole, and that which raises the subleaflets, was stated, which has yet additional interest. [Illustration: _Fig._ 6.] [Illustration: _Fig._ 7.] [Illustration: _Fig._ 8.] When the plant is not in its most lively state, the under surface of the intumescence of the subleaflet (_b_, _Fig._ 2,) and the upper surface of the intumescence of the petiole (_a_, _Fig._ 6,) may be pricked with a needle, without producing action. But if the opposite surfaces, those namely, which change colour and are shortened when the petiole is depressed and the subleaflets folded, are touched with the point of the needle these actions are instantaneously produced. Here the [p082] subleaflet is most delicately sensible; _a slight touch_ with the point of a needle upon the upper surface of the intumescence of the subleaflet (_c_, _Fig._ 1,) causes the single subleaflet so stimulated to rise; and in this manner all the subleaflets upon one side of a leaflet may be raised, their fellows remaining expanded: if the touch be something sharper, the fellow subleaflet rises at the same time; if ruder still, the next pair of leaflets fold directly afterwards, and the irritation then proceeds entirely through the leaflet. But the most satisfactory and curious results are obtained on stimulating the extension surface of the intumescence of the petiole. The needle may be applied to every point upon the upper or solar half of the intumescence of the petiole (_a_, _Fig._ 6,) without producing any visible effect; but if the irritation be applied upon the under half, (_d_, _Fig._ 6,) either quite below or laterally, the petiole is immediately depressed. The transition is abrupt from the surface against which the needle may be made to prick, without exciting action, to one which, when the needle reaches it, causes the petiole to be instantaneously thrown down. It appears, therefore, that each intumescence has a surface especially adapted to receive mechanical impressions; which surface is placed on the side of the intumescence opposite to that, by which the consequent motion is produced. A curious but vague analogy may be traced between these surfaces of the sensitive plant and the organs of sense in animals.