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iV. On the Formation of the Bark of Trees, hi a Letter 

 from T. A. Knight, Esq. F. R. S. to the Right Honour* 

 ahh Sir Joseph Banks, A'. B. P. R. S. &c* 



AMY DEAR SIR, 

 N extraordinary diversity of opinion appears to have pre- 

 vailed among naturallsls, respecting the production and sub- 

 sequent state of the bark ot" trei.s. 



According to the theory of Malpighi, the cortical sub- 

 stance, which is annually generated, derives its origin from 

 the older bark j and the interior part of this new substance 

 is annually transmuted into alburnum, or sap wood ; whilst 

 the exterior part, becoming dry and lifeless, forms the ex- 

 terior covering, or cortex. 



The opinions of Grew do not appear to differ much from 

 those- of Malpighi ; but he conceives the interior bark to 

 consist of two distinct substances, one of which becomes 

 alburnum, whilst the other remains in the state of bark : he, 

 however, supposes the insertments in the wood, the " utri- 

 culi" of Malpighi, and the " tissu cellulaire" of Du Hamel, 

 to have originally existed in the bark. 



Hales on the contrary contends, that the bark derives its 

 existence from the alburnum, and that it does not undergo 

 any subsecpient transformation. 



The discoveries of Du Hamel have thrown much light 

 on the subject ; but his experiments do not afford any con- 

 clusive result, and some of them may be adduced in support 

 of either of the preceding hypotheses : and a modern writer 

 (Mirbelf) has endeavoured to combine and reconcile, in 

 some degree, the apparently discordant theories of Maliiighi 

 and Hales. He contends with Hales, that the alburnum gives 

 existence to the new layer of bark ; but that this bark sub- 

 sequently changes into alburnum ; though not precisely in 

 the manner described by Malpighi. 



So much diffitrtnce of opinion, amongst men so capable 

 of observing, sufficiently evinces the difficulty of the subject 



• From Transactions of the Royal Soriily af London, part i. for l607. 

 t TraUi U'Anatomie et de Physiolu^ie vtgttalct, 



y ^ C 2 they 



