COMPARED WITH THOSE OF ANIMALS. 77 



are continuous by nigit and by day, even when the ter- 

 minal leaflet is asleep. Th«y move, like the second-hand 

 of a time keeper, by a succession of little jerks, each leaflet 

 describing the arc of a circle with its point. Whilst one 

 leaflet is rising, the other is sinking, but in such a manner 

 that the axes of both leaflets always remain in the same 

 straight line. These movements, although independent of 

 light, are rendered more active by heat, and by a more 

 vigorous and healthy condition of the plant. The point of 

 the leaflets describe the arc in about thirty or forty seconds; 

 the movement then stops for about a minute, and is again 

 resumed in the contrary direction. No satisfactory ex- 

 planation of these movements has yet been given. 



This plant belongs to the natural order Leguminosae, of 

 which the pea and bean are familiar examples. 



The Mimosa pudica or sensitive plant. Everybody has 

 heard about this plant, and we should think that many 

 of our readers have seen it. Its movements, when touched, 

 would almost seem to imply in it an obscure degree of 

 consciousness ; but if the phenomena be more carefully 

 examined, however closely they may correspond with the 

 effects of sensation and instinct, it appears very certain 

 that they flow from simpler principles. 



The leaf of the sensitive plant is a compound bipinnate 

 one, having four partial leaf-stalks proceeding from a com- 

 mon petiole. The small pinnules or leaflets are expanded 

 horizontally when the plant is in the light and unmo- 

 lested ; but when it is in darkness, as well as when the 

 leaves are touched or irritated, the pinnules fold upwards, 

 so as to bring their upper surfaces into contact; and at 

 length the impulse reaches the base of the leaf-stalk, which 

 immediately drops downward. When the pinnules are 

 thus folded together, and the leaf-stalks depressed, the 



