VEGETABLE IRRITABILITY. 319 



have a tendency to become parallel to the common petiole, at the ex- 

 tremity 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, 

 Decandolle found that its leaves expanded, and that they closed when 

 put into a dark room during the day, showing the influence which 

 light has on these phenomena. It is to be remarked, however, that if 

 the plant is kept for a long period of time in darkness, it will ultimately 

 expand its leaves, and the phenomenon of folding and opening will go 

 on, although at very irregular intervals. 



660. The ternate leaves of many species of Oxalis fold not merely dur- 

 ing darkness, but also when agitated or struck lightly and repeatedly. 

 Each of the leaflets folds iipon itself, and then bends downwards upon 

 the common petiole. The plant caUed Desmodium gyrans, the moving 

 plant of India, has compound leaves consisting of a large terminal 

 leaflet, and usually two smaller lateral ones. The latter are in con- 

 stant movement, being elevated by a succession of little jerks, until 

 they come into contact, and sometimes even slightly cross each other ; 

 after remaining in this position for a short time, they separate from 

 each other, and move downwards by rapid jerks on opposite sides of 

 the petiole. This process is constantly repeated, and goes on in a 

 greater or less degree, both during day and night, but is most vigorous 

 during warm moist weather. The large terminal leaflet undergoes 

 movements also, oscillating very gradually from one side to the other, 

 and becoming horizontal or depressed. By the lateral oscillatory 

 movement, the leaf becomes inclined in various ways, often assuming 

 a remarkable oblique direction. The upward and downward move- 

 ments seem to depend on the influence of light and darkness. During 

 the day the leaf becomes more or less horizontal, while during dark- 

 ness it hangs down. Similar movements are seen in other species of 

 Desmodium, as D. gyroides and vespertilionis. 



661. The movements in these cases have, by Martius and Meyen, 

 been referred to the presence of some structure analogous to the 

 nervous system in animals. There is, however, no evidence of such a 

 structure in plants, and these authors have not pretended to prove its 

 existence. It is to be remarked, that the movements differ in many 



