376 



VEGETABLE lEKITABILITY. 



petioles, as in tlie Sensitive plant, they may be bent inwards towards 

 each other, while the common petiole is bent downwards. 



Mimosa sensitiva and pudica (fig. 657), commonly called sensitive 

 plants, display these movements of their leaves in a remarkable degree, 



not only under the influence of light and 

 darkness, but also under mechanical and 

 other stimuli. They have bipinnate 

 leaves with four partial petioles pro- 

 ceeding from a common rachis, and each 

 of the petioles is furnished with nume- 

 rous pairs of leaflets (about twenty), 

 which are expanded horizontally during 

 the day. During darkness, or when 

 touched or irritated in any way, each 

 leaflet moves upwards towards its feUow 

 of the opposite side, which in its turn 

 I * rises up, so that their upper surfaces 

 Fi& 657. come into contact. When the movement 



commences at the apex of the leaf it usually proceeds downwards to 

 the base, and thence may be communicated to the leaflets of the next 

 partial petiole, and ultimately to the common petiole, which falls 

 down towards the stem. The partial petioles then converge towards 

 each other, and have a tendency to become parallel to the common 

 petiole, at the extremity of which they are suspended. When the 

 plant is shaken, as by the wind, all the leaflets close simultaneously, 

 and the petioles drop together. If, however, the agitation is long 

 continued, the plant seems as it were to become accustomed to the 

 shock, and the leaflets will expand again. The stem itself is not 

 concerned in the movements. It may be cut and wounded cautiously 

 without causing any change in the leaves, and a portion of it may be 

 removed with a leaf attached and still remaining expanded. If, 

 however, a mineral acid is applied to the stem, after some time the 

 petioles will fall and the leaflets collapse — the leaves perishing with 

 the stem which has been moistened. The chemical action of the acid 

 and absorption cause these phenomena. When a sensitive plant is 

 exposed to artificial light during the night, it is found that its 

 leaves expand, and that they close when put into a dark room 

 during the day, showing the influence which light has on these 



Fig. 657. Branch and leaves of Sensitive plant (Mvmosa pudica), showing the petiole in 

 its erect state, a, and in its depressed state, h ; also the leaflets closed, c, and the leaflets 

 expanded, d. At the base of the petiole a swelling or intumescence (pulvinus) is observed, 

 and smaUer swellings exist at the base of each partial petiole, and at the base of each 

 leaflet. During darkness the leafstalks hang down, and the leaflets are closed, while the 

 reverse is the case during light. The cellular swellings at the base of the petioles and leaves 

 are concerned in the movements. Protoplasmic contractions probably take place in the 

 cells. 



