S8 Observations upon the Motion of 



leaflet is most delicately sensible ; a slight touch with the 

 pomt of a needle upon the upper surface of the intume- 

 scence of the subleaflet (c, Fig. 1,) causes the single sub- 

 leaflet so stimulated to rise ; and in this manner all the sub- 

 leaflets 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, (c^, Fig. 6,) either quite below or laterally, the petiole is 

 immediately depressed. The transition is abrupt from the sur- 

 face 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. 

 l^lt 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. 



We painted with a thick layer of lamp-black in oil the intu- 

 mescence of different petioles in different ways ; the upper sur- 

 face 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 intume- 

 scence of the petiole horizontally. The petiole, after this in- 

 jury, did not recover its usual direction ; the intumescence ap- 

 peared to have wholly lost its properties ; the leaf seemed to 

 depress the petiole by its weight alone, yet the leaflets ex- 

 panded, and exhibited their usual irritability, upon the depend- 

 ing stalk. The same effect, however, was observed, when the 



