20 INCONVERTIBILITY OF BARK INTO ALBURNUM. 



and the obvious fact, that the adhesion of the cortical ves- 

 sels and fibres to each other is much more strong than the 

 adhesion of the bark to the alburnum, affords another cir- 

 Objectionto cumstauce almost as inconsistent with the theory of Malpi- 

 Mulpiglu. ,ghi, as with that of Mirbel. 



Dvihamers ex- Many of the experiments of Duhaniel are, however, ap- 

 p.eriment of parently favourable to the theory of Malpighi, respecting 

 burnurn "^ ^ ' *^^^ conversion of bark into alburnum; and Mirbel has cited 

 two, which lie 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 al- 

 buraum. But Duhawiel himself has shown, with his usual 

 acuteness and candour, that the evidence aUbrded by this 

 •experiment is extretnely defective; and he declares himself 

 to be uncertain, that the pieces of wire did not, at their first 

 insertion, pass between the bark and the alburnum ; in 

 which case they would necessarily have been covered by 

 every successive layer of alburnum, without any transmuta- 

 tion of burk into that substaucef . 

 HI- experiment ^^ ^^^ second experiment cited by Mfrbel, Duhamel has 

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

 ser'iedina '^^•'^ attached to it, is inserted in a plum stock, a layer of 

 plum stock, wood pei-fc'ctly similar to that of the peach tree will be 

 found, in the succeediug winter, beneath the inserted bark. 

 The statement of Duhamel is perfectly correct: but the 

 experiment does not by any means prove the conversion of 

 "^ bark into wood ; for if it be difficult to conceive (as he re- 



marks) that an inserted piece of bark can deposit a layer of 

 alburnum, it is at least au difficult to conceive, how the 

 same piece of baik can be converted into a layer of albur- 

 num of more than twice its own thickness (and the thick- 

 ness of the alburnum deposited frequently exceeds that of 

 the bark in this proportion), v/ithout any perceptible dimi- 

 mition of its own proper substance. The probable opera- 

 tion of the in>^erted bud, which is a well orgai>»:ed plant, 

 at the peiiod when it becomes capable of being transposed 



* Chap. Ill, Article 5. 



i Phijsiqus dcs Jrhres, Lib IV, Ch. III. 



with 



