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IV. On the Formation of the Bark of Trees. In a Letter from 
T. A. Knight, Esq. F. R. S. to the Right Honourable Sir 
Joseph Banks, K. B. P. R. S. &c. 
Read February 19, 1807. 
My Dear. Sir, 
An extraordinary diversity of opinion appears to have pre- 
vailed among naturalists, respecting the production and sub- 
sequent state of the bark of trees. 
According to the theory of Malpighi, the cortical sub- 
stance, which is annually generated, derives its origin from 
the older bark ; 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 exterior 
covering, or cortex. 
The opinions of Grew do not appear to differ much from 
those of Malpighi ; but he conceives the interior bark to con- 
sist of two distinct substances, one of which becomes albur- 
num, whilst the other remains in the state of bark : he, how- 
ever, supposes the insertments in the wood, the “ utriculi” 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 subsequent transformation. 
The discoveries of Du Hamel have thrown much light 
