52 PllOPLLSION' OF THE 55 A5', 



order to suit tlu^ constitutions oi tlifK;rcnt 

 tribes of plants destined for ditferent parts of 

 the p-lobo. It cannot but act as a stimulus 

 to the living principle, and is one of the most 

 powerful agents of Nature upon the vegetable 

 as well as animal constitution. Besides this, 

 however, various mechanical causes maj be 

 supposed to have their eflect; as the frequently 

 spiral or screw-like form of the vessels, in 

 some of which, ^^'\\vn separated from the plant, 

 jMalpighi tells us he once saw a very beauti- 

 ful undulating motion that appeared sponta- 

 neous. This indeed has not been seen by any 

 other person, nor c:m it be supposed that 

 parts so delicate can, in general, be removed 

 from their natural situation, without the de- 

 struction of that fine irritability on which such 

 a motion must depend. ^,Ve may also take 

 into consideration the agilation of the vege- 

 table body by 'svind'^, which is known by ex- 

 perii>nce lo b(^ so w liolesome to it*, and must 

 serve powerfully to propt^l the llnids of lofty 

 trees; the passage, and evolution perhaps, of 



* Sec Mr. Knight's experiments hi confirmation of this 

 in the Phil. Tnvis. for 1S03, p. 250. 



