PROPULSION OF THE SAP. 55 



the constitutlofts of different tribes of plants destined for 

 different parts of the globe. It cannot but act as a stim- 

 ulus Jo the^ living principle, and is one of the most pow- 

 erful 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, Mal- 

 pighi tells us he once saw a very beautiful undulating 

 motion that appeared spontaneous. This indeed has 

 not been seen by any other person, nor can it be sup- 

 posed that parts so delicate can, in general, be removed 

 from their natural situation, without the destruction of 

 that fine irritability on which such a motion must de- 

 pend. We may also take into consideration the agitation 

 of the vegetable body by winds, which is known by 

 experience to be so wholesome to it,* and must serve 

 powerfully to propel the fluids of lofty trees ; the pas- 

 sage, and evolution perhaps, of air in other parts or ves- 

 sels surrounding and compressing these ; and lastly the 

 action, so ingeniously supposed by Mr. Knight, of those 

 thin shining plates called the silver grain, visible in oak 

 wood, which passing upon the sap- vessels, and being 

 apparently susceptible of quick changes from varicitions 

 in heat or other causes, may h:ive a poweiful effect. 

 " Their restless temper," says M . Kaight, " after the 

 tree has ceased to live, inclines me to believe that they 

 are not made to be idle whilst it continues alive." Phil. 



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

 Phil. Trans, for 1803,/;. 208. 



