J 38 On the Inconvertibility of Bark into Allurnum. 

 burnum must sufely be m-uch more intricate and interwoven 

 than it is, and its lubes would lie less accurately parallel with 

 each other than they do : and were the fibrous substance of 

 the bark progressively changing into alburnum, the bark 

 roust of necessity be firmly attached to the alburnum during 

 the spring and summer by the continuity, and indeed iden- 

 tity of the vessels and fibres of both the* substances. This, 

 however, is not in any degree the case, and the bark is in 

 those seasons very easily separated from the alburnum ; to 

 which it appears to be attached by a substance that is appa- 

 rently rather gelatinous than fibrous or vascular : and the 

 obvious fact, that the adhesion of the cortical vessels and 

 fibres to each other is mnch more strong than the adhesion 

 of the bark to the alburnum, affords another circumstance 

 almost as inconsistent with the theory of Malpighi, as with 

 that of Mirbel. 



Many of the experiments of Duhame! are, however, ap- 

 pirently favourable to the theory of Malpighi, respecting 

 the conversion of bark 'into alburnum ; and Mirbel has cited 

 two, which he appears to think conclusive*. In the first 

 of these, Duhamel shows that pieces of silver wire, inserted 

 in the bark of trees, were subsequently found in their albur- 

 num; but Duhamel himself has shown, ' with his usual 

 acuteness and candour, that the evidence afforded by this ex- 

 periment is extremely defective; and he declares himself to 

 bs uncertain that the pieces of wire did not, at their first in- 

 sertion, pass between the bark and the alburnum ; in which 

 case they would necessarily have been covered by every suc- 

 cessive layer of alburnum, without any transmutation of 

 bark into that substance f. 



In the second experiment cited by Mirbel, Duhamel has 

 shown that when a bud of a peach tree, with a piece of bark 

 attached to it, is inserted in a plum stock, a layer of wood 

 perfectly similar to that of the peach tree will be found, in 

 the succeeding winter, beneath the inserted bark. The state- 

 ment of Duhamel is perfectly correct; but the experiment 

 docs not by any means prove the conversion of bark into 

 wood ; for if it be difficult to conceive (as he * emarks) that 



* Chap. iii. article 5, Traite d 'Anatomic et it Phi/siologie J't'gHate. 

 •J- Physique ties Arhes, liv. iv. ch. 3. 



an 



