SECT. II. ASCENT OF THE SAP. iOQ 



trunk of the Poplar-tree, there will be no notable 

 discharge till it penetrates almost to the centre.* 

 These facts afford a strong presumption that the sap 

 does not ascend by the bark ; but the following ex- 

 periments afford an indubitable proof. 



Du Hamel stript several trees of their bark en- Experi- 

 tirely, which continued, notwithstanding, to HveDu 

 for many years, protruding new leaves and new Anight, 

 branches as before.-}- Mr. Knight stript the trunk of 

 a number of young Crab-trees of a ring of bark 

 half an inch in breadth, but the leaves were pro- 

 truded, and the branches elongated, as if the 

 operation had not been performed.^ It is evi- 

 dent, therefore, that the sap does not ascend by the 

 bark. 



But it is equally evident that it does not ascend 

 by the pith, at least after the first year ; for then, 

 even upon Grew's own supposition, it becomes either 

 juiceless or wholly extinct : and even during the 

 first year it is not absolutely necessary, if at all 

 subservient to the ascent of the sap, as is proved 

 by an experiment of Mr. Knight's. Having contrived 

 to abstract from some annual shoots a portion of 

 their pith, so as to interrupt its continuity, but not 

 otherwise materially to injure the fabric of the 

 shoot, he found that the growth of the shoots 

 which had been made the subject of experiment 

 was not at all affected by it. 



* Exper. par Coulomb. f Phys. cles Arb. lir. v. chap, ii. 

 Phil. Trans. ISOi. 



