Malpighi and Grew thought it was formed by the bark, and the best observations have confirmed their opinion. Hales supposed the wood added a new layer to itself externally every year. Linnæus had a peculiar notion, that a new layer of wood was secreted annually from the pith, and added internally to the former ones. Truth obliges us to confess that the latter theory is most devoid of any kind of proof or probability.
Du Hamel, by many experiments, proved the wood to be secreted or deposited from the innermost part of the bark or liber. He introduced plates of tinfoil under the barks of growing trees, carefully binding up their wounds, and, after some years, on cutting them across, he found the layers of new wood on the outside of the tin. His original specimens I have examined in the public museum at Paris.
Dr. Hope, the late worthy Professor of Botany at Edinburgh, instituted an experiment, if possible more decisive, upon a branch of Willow three or four years old. The bark was carefully cut through longitudinally on one side for the length of several inches, so
