11^ SLEEP OF PLANTS, 



really be so hyperbolical as at first sight it seems, for the 

 cessation of the stimulus of light, and of the consequent 

 restrained position of the leaves, may be useful to the 

 vegetable constitution, as real sleep is -to the animaU 

 A iother purpose is answered by the nocturnal folding 

 of some le ives, that they shelter their flowers from the 

 dew, the advantage of which we shall explain hereafter. 

 Some pinnated leaves display a more extraordinary 

 sensibility, not merely to light, but to the touch of any 

 extrune(uis bodv, or to anv sudden concussion, as those 

 of Mimosa sfr?s'fiva, c\v.(\ pitdicn ; Oxal/s sensitiva, and< 

 Sinithia sensitiva. Ait Hori. Kew. v. 3,f. 13. An im- 

 pre.-,bion niacle even in the most gentle manner, upon one 

 of thtir leaflets, is communicated in succession to all of 

 them, evincing an exquisite initability, for it is in vain 

 to attempt any mechanica' solution of this phoenomenon. 

 One of this tribe, Hedysarum gyrcins, has a spontaneous 

 motion in its leaves, inde()endent of any external stimu- 

 ius, even of light, and only requiring a very warm still 

 atmosphere to l)e performed in perfection. Each leaf is 

 ternate, and the sn)all lateral leaflets are frequently mov- 

 ing up and down, either equably or by jerks, without 

 any unilornsity or cooperation among themselves. It is 

 difl[icult to guess at the purpose which this singular ac- 

 tion is designed to answer to the plant itself; its effect 

 on a tational bfchf>lder cannot be indiiferent. 



The chemical actions of light, heat, and the compo- 

 nent parts of the atmospheric air, upon leaves, and, 

 where the latter are wanting, on the green stems of 

 pi \nts, are now, as far as concerns all plants in common, 

 tokiably wcii uiiaersiood. The observations and ex- 



