!•] 
SAP-WOOD. 
ii 
Such indurations of portions of the layers occur more 
frequently in the Firs and Pines than in the wood of 
trees of harder and more compact texture. In Dantzic 
Fir, for example, I have noticed parts of twenty or 
more concentric rings changed from alburnum into 
duramen, or heart-wood, while the remaining portions of 
the circles retained their sap-like or alburnum character, 
and greater or less deviations in this respect are fre¬ 
quently met with in other species. It may be that these 
can only be accounted for by the exceptional influences 
before mentioned, for it seems quite possible that, when¬ 
ever a tree is suddenly thrown open and exposed by the 
clearing away of others from its vicinity, the hardening 
process will go on with unusual rapidity. 
In such Firs and Pines as have been sheltered in the 
depths of a forest, we do not find the wood of this 
variable character, as the perfecting of the duramen 
takes place then with much greater regularity and uni¬ 
formity, if somewhat less rapidly, than in more exposed 
situations. 
This peculiarity of growth is more strikingly exem¬ 
plified in the Firs and Pines, and occurs with greater 
frequency in trees of this kind than in any others. Acci¬ 
dental circumstances no doubt affect the sap-wood of 
many other kinds to a greater or less degree; but in 
trees of a close texture the want of vigour in the sap is 
generally found to affect the whole circumference of a 
layer rather than several distinct portions of it. 
The proportion of sap-wood, or alburnum, to heart- 
wood, or duramen, in trees in which it occurs, is exces¬ 
sive in the young, but decreases rapidly as they advance 
in age, the difference being in some measure attributable 
to the fact that, as the circumference of the tree increases, 
the tissues of each successive layer, or annual ring, are 
