DURAMEN AND ALBURNUM. 85 
and it begins to get old. The age in which trees are in full 
vigour varies according to the species ; thus the Oak, it is said, 
will form most timber from the age of twenty to thirty, and that 
after sixty years of age the amount formed will be much less 
considerable. Again, in the Larch, the vigour of growth appears 
to diminish after it is forty years of age ; in the Elm after fifty 
years ; in the Beech after thirty years ; in the Spruce Fir after 
forty ; and in the Yew after sixty years. 
Duramen and. Alburnwm.—When the annual rings are first 
formed, the walls of their component wood-cells and vessels 
are pervious to fluids, and very thin, and their cavities gorged 
with sap, which they transmit upwards from the root to the 
leaves. As they increase in age, however, their walls become 
so thickened by various deposits from the contained sap, that 
their cavities are ultimately almost or entirely obliterated, and 
they are thus rendered nearly or entirely impervious to fluids. 
This change is especially evident in the wood of those trees in 
which the thickening layers are coloured, as in the Ebony, 
Mahogany, Rosewood, and Guaiacum. Such coloured deposits 
are generally most evident in tropical trees, although they 
also occur more or less in most of the trees of cold and tem- 
perate regions. In some of the latter, however, as the Poplar 
and the Willow, the whole of the wood is nearly colourless, and 
exhibits no difference in this respect in the appearance of the 
internal and external rings. The value of wood as timber 
depends chiefly upon the nature of this incrusting matter, and 
is commonly in proportion to its colour ; hence those woods, 
as Ebony, Ironwood, and Mahogany, which are deeply coloured, 
are far harder and more durable than white woods, such as the 
Poplar and the Willow. 
From the above characters presented by the wood according 
to its age, we distinguish in it two parts: namely, an internal 
portion, in which the wood-cells and vessels have thickened 
walls, are impervious to fluids, hard in texture, of a dry nature, 
and commonly more or less coloured, which is called the Dura- 
men or Heart-wood ; and an outer portion, in which the wood- 
cells and vessels have thin sides, are pervious to, and full of 
sap, soft in texture, and pale or colourless, to which the name 
of Alburnwm or Sap- wood is given. 
Age of Dicotyledonous Trees.—As each ring of wood in an 
Exogenous stem is produced annually, it should follow that by 
counting the number of rings in a transverse section of a tree 
presenting this structure, we é6ught to be able to ascertain its 
age, and this is true with a few exceptions, when such trees are 
natives of cold climates, because in these, as we have seen, the 
annual rings are usually distinctly marked. In Dicotyledonous 
trees, however, of warm climates it is generally difficult, and 
frequently impossible, to ascertain their age in this manner, in 
consequence of several disturbing causes: thus, in the first 
