7*4 • ORGANOGRAPHY, 



poles have very narrow annual zones. The influence of soil and 

 other circumstances will also materially affect the thickness of the 

 annual zones in the same tree. We find also that the same zone 

 will vary in diameter at different parts, so that the pith, instead 

 of being in the centre of the wood, is more or less eccentric, 

 owing to the zones being thicker on one side than on the other. 

 This irregular thickness of the different parts of the annual 

 zones is owing to several causes, but the greater growth on one 

 side is chiefly due to the fact of its being more exposed to light 

 and air than the other. 



The annual zones also vary in thickness in the same tree, ac- 

 cording to the age of that tree. Thus when a tree is in full 

 vigour it will form larger zones than when that period is past, 

 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 consider- 

 able. 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. Further observations 

 are required however upon these points, which are of great prac- 

 tical importance so far as growing trees for timber is concerned. 



Duramen and Alburmcm. — When the annual layers are first 

 formed, their walls are pervious to fluids ; their component 

 wood-cells and vessels are then also very thin and their canities 

 gorged with saj), which, as will be afterwards seen, they are the 

 chief agents in transmitting upwards from the root to the leaves. 

 Their walls, 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 impervious to fluids. This change is especially 

 evident in the wood of those trees in which the incrusting 

 TDatters are of a coloured nature, as in the Ebony {Diospyros 

 Ebenus), Mahogany (Swietenia Mahagoni), Eose-wood {Tripfo- 

 lonea species), Lignum Vitae (Chcaiacum officinale), &c. Such 

 coloured deposits are generally more evident in tropical trees, 

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

 and temperate regions. In some of the latter, however, as the 

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

 less, and exhibits no difference in the appearance of the internal 

 and external layers. The A-alue of wood as timber depends 

 upon the nature of this incrusting matter, and is commonl}' in 

 proportion to its colour ; hence those woods, as Ebony, Iron- 

 wood, Mahogany, &c., which are deeply coloured, are far harder 

 and more durable than the white woods, such as the Poplar, &c. 



From the above characters presented by the wood according 

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



