438 THE UNIVERSE. 



them shut up ; then in a few seconds the branches droop 

 successively towards the earth, and the plant displays signs 

 of the most profound disturbance, appearing as if struck by 

 lightning. 



In vain have certain botanists tried to explain this ex- 

 traordinary phenomenon through the intervention of chem- 

 ico-physical forces ; it is evident that we have only to deal 

 here with a vital manifestation. 



If we preserve a sensitive plant from being shaken, and 

 place upon one of its leaves a drop of acid, the contact of 

 the irritant suffices to make the whole plant shrink up; and 

 if we merely heat one of its little leaflets by placing it in 

 the focus of a burning-glass, the injury seems to be felt 

 through every part of the fragile Mimosa ; its boughs and 

 leafage sink down as though it were struck by stupor. 



This charming leguminous plant, the subject of so many 

 ingenious comparisons, possesses a delicacy of sensation 

 which we should never think of meeting with in the veg- 

 etable kingdom. When Von Martius was traversing the 

 savannas of tropical America, where it abounds, he ob- 

 served that the sound of his horse's hoofs at a distance 

 made all the sensitive plants contract, as if they had been 

 frightened. A ray of sunlight, or the shadow of a cloud 

 even, is enough to produce a manifest change in the midst 

 of them. 



Such very singular phenomena ought to suffice to make 

 us suppose that the vegetable fibre conceals in its hidden 

 folds some traces of the structure which everywhere presides 

 over animal life. Dutrochet even thought he had found in 

 it the regulator of so many mysterious actions, a nervous 



