5:6 ACTION OF THE SILVER GRAIN. 



Trans, for 1801,/?. 344. These plates are presumed 

 by the author just quoted to be peculiarly useful in as- 

 sisting the ascent of the sap through the alburnum of 

 the trunk or chief branches, where indeed the spiral 

 coats of the vessels are either wanting, or less elastic 

 than in the leaf-stalks and summits of the more tender 

 shoots. 



However its conveyance may be accomplished, it is 

 certain that the sap does reach the parts above mention- 

 ed, and there can surely be now as little doubt of the 

 vessels in which it runs. That these vessels have been 

 thought to contain air only, is well accounted for bv 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 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 experiments. Arguments in support of any 

 theorv must be very cautiously deduced from such ex- 

 periments, or from any other observations not made on 

 vegetables in their most natural state and condition ; andj 

 above all, that great agent the vital principle must always 

 be kept in view, in preference to mere mechanical con- 

 siderations. 



These to which I give the common name of sap- 

 vessels, comprehending the common tubes of the albur- 



