G Comparison let ween A7iimal and Fefretalle Life. 91 



and one which increases every season at every joint, whose juices 

 are independent of itself, and taken from another, and may na- 

 turally he &up])osed, tkerejore, to draw up ovly that hqii'id which 

 is necessary ;or thai increase P But if there were a circulating 

 law established by Nature for the sap, it would most probably 

 be a itniversat late, general to all plants : What then wovdd 

 become of those trees and shruhs which lose their bark in the 

 winter ? whose vessels are supposed by some botanists to con- 

 vey the sap? If thev remain, they would be doubly conspicuous, 

 when the rest had d' parted ; and they are certainly not to be 

 found. A trifling piece of the green bark is discovered in the 

 winter, and the rest of the space zs empiij, aad the inner bark 

 vessels are generally deprived of their juiCcs. How is then the 

 sap to convey its circulation ? If we are to assert that the sap 

 descends again after its ascent, we must skviv in udial vcs.els. 

 If in the wood -vessels? the bark could not hold a tenth part, as 

 I shall show. If in the bark-vessels ? then must we discover 

 the anastomoses or joinings of !)oth series of vessels, that is, the 

 communication between the cortuat and Uaneotts cylinders, the 

 meeting of tlie wood and bark-vessels. I injected a v/eak quan- 

 tity of .'ugar of lead into a plant, and sent the German test after 

 it; but no joinings of vessels were to be perceived : they gave 

 the result indeed, but traced ii to the terminations of che wood- 

 vessels in each fresh forming twig. This is always the method 

 I pursue v.'hen puzzled respecting tiie terminating vessels, and 

 it rarely fails me. It is often given as a proof, that the sap circu- 

 lates in trees, instead of flowing up, as, I think, only to form the 

 different productions it has to comi)lete: " tliat when the fellers 

 and peelers set about barking, they continually find the young 

 shoot so full of juice as to be rea<!y to pour over, while in the 

 larger shoot, or lower part of the tree, they are almost dry, at 

 least there is a much smaller quantity of liquid ; so that the meu 

 often exclaim that the sap enters at the ends of the twigs: but 

 this is the very circumstance that proves the truth of what I ad- 

 vance. It must be recollected tliat they do not b^-gin to bark 

 till after a quantity of licjuid has passed up the tree J'i<,m the 

 root, loosening the bark all the way from tlie wood: tlierefore it 

 is almost at the end of its Jloiuiiiif time when they begin to 

 peel, or "it is very difiicult to tear off tlie bark. The liquid is 

 therefore already arrested in the young shoots. Now its airesting 

 is always the time it takes to become jelly, and soon to receive 

 the wood-vessels and conqilete the shoot. This, therefore, is the 

 cause of the (juanlity of sap being always found in tlie young 

 twigs at this time; three weeks sooner tlie sap would ha\e been 

 discovered in the middle root, 1 have often drawn a pint from 

 them ut (his tirrii:, 



Jha 



