46 TIMBER AND TIMBER TREES. [chap. 



The measurements in the foregoing table were taken 

 at consecutive distances of 3 inches from the pith of trees 

 having well-formed concentric rings, and by doubling 

 this, the diameters of 6, 12, 18, 24, 30, 36 inches, etc., 

 were obtained. There are only two or three of these 

 results that can be compared with the list furnished by 

 Decandolle, but severally they will be found useful for 

 reference ; and, later on, I shall have occasion to revert 

 to some of them, when treating of the characteristics 

 of the individual kinds to be noticed in these pages. 



The proportion which the width or thickness of each 

 layer at the upper bears to that at the lower extremity 

 of the tree, varies considerably in the several species, 

 the difference being the least marked where there is the 

 greatest length of clear stem. Indeed, as the material 

 which goes to form a branch may be regarded as so 

 much matter diverted from the trunk itself, it follows 

 that in a tree in which the branches occur low down, 

 the stem will taper more than in one which has 

 them only near the top. The diminution in the 

 thickness of the layers will be most apparent in the 

 trees which produce the largest branches, and will 

 be regular or irregular according as the branches are 

 thrown out at regular intervals or otherwise. 



It might be supposed that as every layer from the 

 pith, or medulla, to the bark is in a different stage of 

 perfection, the innermost or earliest, being the most 

 matured, would be the strongest ; but experience teaches 

 us that this is only true up to a certain period of growth, 

 and that in the majority of cases the maximum of 

 strength and toughness lies nearer the more recently- 

 formed heart-wood, or duramen. In some trees, indeed, 

 no true heart-wood is distinguishable ; but old stems 

 have a " false heart " of decaying and discoloured wood 



