IL] RATE OF GROWTH. 19 



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, &c., 

 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 sap which 

 goes to form a branch must 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 ac- 

 cording 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. For this reason it 

 becomes a matter of great importance, in selecting timber 

 for use especially if it be intended for works of any 



