Si PROPULSION OF THE SAP. 



prcler to suit the constitutions of different 

 tribes of plants destined for different parts of 

 the globe. 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 may be 

 supposed to have their effect ; as the frequently 

 spiral or screw-like form of the vessels, in 

 some of which, when separated from the plant, 

 Malpighi 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 can 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. We may also take 

 into consideration the agitation of the vege- 

 table body by winds, which is known by ex- 

 perience to be so wholesome to it*, and must 

 serve powerfully to propel the fluids of lofty 

 trees; the passage, and evolution perhaps, of 



* See Mr. Knight's experiments in confirmation of this 

 in the Phil. Trans, for 1803, p. 280. 



