110 PROCESS OF NUTRITION. CHAP. 111. 



Proving The sap then ascends neither by the bark nor 

 through" 1 pith, but by the wood only. , But the whole mass 

 ihe wood, Q t k e wooc | throughout is not equally well adapted 

 for the purpose of conveying it. The interior and 

 central part, or part that has acquired its last de- 

 gree of solidity, does not in general afford it a pas- 

 sage. This is proved by what is called the girdling 

 of trees, which consists in making a circular gap 

 or incision quite round the stem, and to the depth 

 of two or three inches, so as to cut through both 

 the bark and alburnum. The operation is very 

 generally performed by the American farmer on 

 trees whose further growth might be prejudicial to 

 his crop, but which he does not yet find it conve- 

 nient to cut down : and indeed there are but few 

 trees that will long survive the operation, particu- 

 larly if performed early in the spring.* An Oak-tree 

 on which Mr. Knight had performed the operation 

 with a view to the very object in question, namely, 

 that of ascertaining the channel of the sap's ascent, 

 exhibited not the slightest mark of vegetation in 

 the spring following.-^- The sap then does not 

 ascend through the channel of the matured wood. 

 Or rather But if the sap ascends neither through the chan- 

 "' nel of the bark, nor pith, nor matured wood ; 

 through what other channel does it actually ascend? 

 The only remaining channel through which it can 

 possibly ascend is that of the alburnum. But 

 though the object of our inquiry has been thus so 

 * Barton's Elcm. of But. p. 15,5. f Phil. Trans. 1805. 



