80 ORGANOGRAPIIV. 



practical importance so far as growing trees for timber is 

 concerned. 



Duramen and Alburnum. — When tlic annual layers are first 

 formed, the wood-cells and vessels of which they are composed 

 are pervious to fluids. Their sides are then very thin and their 

 cavities are gorged with sap., which, as will be afterwards seen, 

 they arc the chief agents in transmitting upwards from the 

 root to the leaves. Their sides however, as they increase in age, 

 become thickened by various deposits from the contained sap, by 

 which their cavities are ultimately almost obliterated, and they 

 are thus rendered nearly impermeable to fluids. This change is 

 especially evident in the wood of those trees in which the in- 

 crusting matters arc of a coloured nature, as in Ebony {Dios- 

 pyrus Ebemis), Mahogany (Suuetenia Ma/iagoni), Kosc-wood 

 (^Triptolemea species), Lignum Vit;c {Guaiacum officinale'), &c. 

 Such coloured deposits arc generally more evident in tropical 

 trees, although they occur more or less in most of the trees 

 of cold and temperate regions. In some, however, as in 

 the Poplar and the Willow, the whole of the wood is nearly 

 colourless, and exhil)its no ditferencc in the appearance of the 

 internal and external layers. The value of wood as timber 

 depends upon the nature of this incrusting matter, and is com- 

 monly in projjortion to its colour ; those woods, such as Ebony, 

 Iron-wood, Mahogany, &c., which arc dccjjly coloured, being 

 far harder and more durable than the white tvoods, such as the 

 Poplar, &c. 



From the above characters which wood presents according to 

 its age, we distinguish in it two parts : namely, an internal or 

 central one, in which the wood- ccUs have thickened sides, are 

 impermeable to fluids, hard in texture, coloured, and of a dry 

 nature, which is called the Duramen or Heart-tvood ; and an 

 outer i)art, in which the wood-cells have thin sides, are pervious 

 to and full of sa]», i)ale or colourless, and soft, to such the name 

 of Alburnum or Sap-tvood is given. 



When the internal part of the wood has become of the nature 

 of heart-wood it ceases to perform any active functions in the 

 plant, its oflUcc being then chiefly to act as a support to it. All 

 the vital and essential functions of the stem arc tlien carried on 

 by the sap- wood, llcnce we sec the reason why a tree in which 

 the crntral j)!irt is conii)lctely destroyed, with the outer ]iart or 

 alhurnum remaining, continues to live, put forth new branches, 

 and add to its substance. 



A(je of Trees. — As each zone of wood in an exogenous 

 stem is produced annually, it should follow that by counting 

 the inimber of zones in a transverse section of any tree wc 

 ought to l)e able to ascertain its age, and this is true with a few 

 exce])ti(Mis, when such trees are natives of cold clinnites, be- 

 cause in these as we have seen, the annual zones arc distinctly 



