CHAP. I. VITAL PROPERTIES AND STIMULANTS. l65 



or towards the extremity of the partial petiole on 

 which they are seated (6). Other pairs of leaflets, 

 nearest to the one first stimulated, will then close in 

 succession in a similar manner ; and at length the 

 partial petioles themselves fold together, by inclining 

 upwards and forwards. Last of all, the influence is 

 transmitted to the common petiole, which bends down- 

 wards with its extremity towards the ground (c) ; in 

 a direction the reverse of those which were taken in 

 the former cases. The effect is next continued to the 

 other leaves nearest to the one first stimulated, and 

 they fold their leaflets and depress their petioles in 

 a similar manner. When the plant is shaken, all the 

 leaflets close simultaneously, and the petioles droop 

 together ; but if the agitation be long continued, the 

 plant will at length become accustomed to the shock, 

 and after a lapse of some time, the leaflets expand 

 again. The mechanism by which these movements 

 are produced resides in the thickened or swollen joints, 

 seated at the bottom of each leaflet and petiole; for 

 if the upper part of these swellings are cut away, 

 the leaf remains erect ; but if the lower part is re- 

 moved it continues depressed. Hence it appears that 

 the elevation and depression of the leaf, is owing to 

 the elasticity of the tissue of which the swollen joint is 

 composed ; and that the stimulus employed to produce 

 motion, tends to weaken the upper parts of these joints 

 in the case of the leaflets and partial petioles, but the 

 lower part of those belonging to the main petioles 

 the contrary sides continuing elastic, as before. But 

 how the effect is produced, and what may be the law 

 which regulates its action, is not known. The causes 

 are active from the earliest stages of the plant's exist- 

 ence ; the cotyledons themselves exhibiting the property 

 so soon as they have expanded. The transmission of 

 the stimulus from one leaf to another along the stem 

 of the plant, has been shown by Dutrochet to take 

 place through the intervention of the ducts contained in 

 M 3 



