11] 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 
