WOODY LAYERS. 



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, 



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, there- 

 fore, to the opinion that it must be the case in every 



