78 THE TISSUES OF PLANTS 



plant appears as if it were withered and dead ; but if it be 

 let alone, after awhile, the leaflets gradually separate from 

 each other, and also the partial petioles ; the main petiole 

 gently rises to its former angular position with reference 

 to the stem, and the plant resumes its usual appearance. 



Fig. 21. 



Branch and leaves of the sensitive plant (Mimosa pvdica), showing the 

 petiole in its erect state, a, and in its depressed state, & ; also, the- leaflets closed, 

 c, and the leaflets expanded, d. 



If the plant is sickly, the position of the leaves remains 

 permanently the same, and no sensible motion follows any 

 kind of excitement. When, however, it is in a healthy 

 state, it is difficult to approach without disturbing it. Even 

 the shaking of the ground caused by the tramp of a horse 

 will cause the mimosa to fold its leaves. 



These movements are manifested both in darkness and 

 in light, in water as well as air. They are also produced 

 by the light of the sun concentrated by a burning-glass, 

 or by a drop of any irritating fluid, such as sulphuric or 

 nitric acid. 



