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 frequently 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, whenever 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 uniformity, if somewhat less rapidly, than in more exposed situations.
This peculiarity of growth is more strikingly exemplified in the Firs and Pines, and occurs with greater frequency in trees of this kind than in any others. Accidental 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 excessive 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
