they are not nearly so distinct; indeed, in some it is impossible to trace them.
The woody layers, when first formed, are full of sap, but they change and gradually become solidified by the compression of each subsequent layer; and it seems obvious, that as each zone is moulded upon the one of the previous year's growth, it would, by cohesion, become amalgamated with it. The perfecting of the concentric layer is, however, a very gradual process,
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| FIG. 4. | FIG. 5. |
and the time necessary to convert a new layer of sap-wood into heart-wood (which alone represents the serviceable timber in most trees) varies from about one year, as in Hornbeam, Ash, Beech, and in some other species, to thirty years or even more, as in Oak, &c. &c. It seems, as a rule, from evidence to be shown later on in Table II., that Oak trees which form their wood most rapidly under ordinary conditions of growth are the best in quality. In the Firs it is the same, and we see it also in the Pines (Kauri excepted). I incline, therefore, to the opinion that it must be the case in every
