54 COURSE OF THE SAP. 



counted for by Dr. Darwin, on the principle 

 of their not collapsing when emptied of then'' 

 sap ; which is owing to their rigidity, and the 

 elastic nature of their coats. When a portion 

 of a stem or branch is cut off, the sap soon 

 exhales from it, or rather is pushed out by the 

 action of the vessels themselves : hence they 

 are found empty ; and for the same reason 

 the arteries of animals were formerly thought 

 to contain air only. When the sap-vessels 

 have parted with their natural contents, air 

 and even quicksilver will readily pass through 

 them, as is shown by various expermients. 

 Arguments in support of any theory must be 

 \ery cautiously deduced from such experi- 

 ments, or from any other observations not 

 made on vea'etables in their most natural state 

 and condition ; and, above all, that great 

 agent the vital principle must abvays be kept 

 in view^, in preference to mere mechanical coi> 

 siderations. 



These to which I give the common name 

 of sup- vessels, comprehending the common 

 tubes of the alburnum, and the central ves- 

 sels, of Mr. Knight, may be considered as 

 analogous to the arteries of animals ; or rather 



