42 COURSE OF THE SAP. 



the vessels in which it runs. That these vessels have 

 been thought to contain air only, is well accounted for 

 by Dr. Darwin, on the principle of their not collapsing 

 when emptied of their 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 w'ere 

 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 experiments. Arguments in support 

 of any theory must be very cautiously deduced from 

 such experiments, or from any other observations not 

 made on vegetables in their most natural state and 

 condition ; and, above all, that great agent the vital 

 principle must always be kept in view, in preference 

 to mere mechanical considerations. 



These to which I give the common name of sap- 

 vessels, comprehending the common tubes of the al- 

 burnum, and the central vessels, of Mr. Knight, may 

 be considered as analogous to the arteries of animals ; 

 or rather they are the stomach, lacteals and arteries 

 all in one, for I conceive it to be a great error in 

 Dr. Darwin to call by this name the vessels which 

 contain the peculiar secretions of the plant*. These 

 sap-vessels, no doubt, absorb the nutritious fluids af- 



* Phj/fo/ogia, sect. '2. 



