CAUSE OF MOTION IN PLANTS. J2| 



leaf; in all others, it is found even to the smallest spire. 

 The spiral wire is made to stretch, and so does its case. 

 They both contract and dilate in the solar microscope. The 

 spiral wire is found in all the sensitive plants in great quan- 

 tities, and in every part except the seminal leaves; and it is 

 the seminal leaves alone that have no motion. I may add, 

 that, when the spiral wires were divided, the leaf would not 

 turn. I think it is hardly possible, where positive evidence 

 is not to be had, to prove a fact in a more direct manner; 

 and that I may say, that plants have spiral wires, which, con- 

 tracted and dilated by light and moisture, are the cause of 

 all motion in plants. 



The sleep of plants is nothing more than the dilating and Sleepofpknfc. 

 lengthening of the spiral wire from the evening moisture; 

 and I think the very appearance of it proves it to be so. 

 I?oes not every violent rain, long continued, produce the 

 same effect? Does not the common acacia, when wetted 

 by continued rain, drop her leaves as at night ? So does the 

 gieditsia triacanthos also with even less moisture. The 

 a,'sculus hippocastanum droops as soon as the leaves are 

 old, and begin to decay. Every plant drops its leaves before 

 the leaves go off. It appears to me, that theie is no expres- 

 sion in the human countenance more easy to be understood, strength and 

 than the expression of strength and debility in the appear- debility ia 

 ance of plants. IS'or did I ever see a plant close its leaves P a " " 

 without showing even an excess of debility in every other 

 part. 



In one of my two former letters I mentioned two in- pi ants j, aTe n# 

 stances of apparent volition in plants, to show how many volition, 

 things of that sort happen, to mislead the judgment: but I 

 have now too often pursued them, till undeceived, through 

 such a course of experiments, perpetually renewed to gain 

 nothing but disappointment, that I am now most absolutely 

 convinced that all plants are merely machines, governed by 

 light and moisture; and that every idea of their sensibility, 

 or of their volition, is only a proof, that we too often let 

 imagination run away with our judgment. Mechanical 

 power is sometimes so delicately managed, that it is difficult 

 to trace it even with the solar microscope; especially as that 

 is of no use till the specimen is most delicately dissected, and 



placed 



