THE STETTCTURE OF TIMBER. 277 



the pith. Vessels are long tubes, so long in fact that they may 

 extend from the leaves to the roots, and are usually sufficiently 

 large in the bore to be easily recognizable with the naked eye. 

 If any doubt exists as to their presence the point may usually 

 be decided by taking a thin transverse slice with a sharp knife 

 and holding it up to the light. Amongst the conifers the most 

 important genera are Abies or true firs, Picea the spruces, Pinus 

 the pines, and Larix the larches. Timber furnished by the first 

 two genera possesses many points of similarity. It is light in 

 colour, of uniform colour throughout — that is to say, from the 

 pith right oiit to the bark — and about the same specific gravity. 

 But there is always this distinction between specimens of wood 

 yielded by these genera of trees. If a thin cross section be taken 

 from a spruce [Picea) and held up to the light numerous small 

 brown spots will be noted, whereas no such marks are met with 

 in the wood of the firs {Abies) These spots are ducts in which 

 resin is manufactured, and are known as resin canals or resin 

 ducts. Not only does one fail to find them in the wood of Abies, 

 but they are also absent from the wood of the juniper, cypress, 

 yew, and some other conifers. We have next to distinguish be- 

 tween the wood of spruces, pines, and larches, and this is a 

 matter of no difficulty, because, as has already been said, the 

 wood of the spruce is of uniform colour throughout, whereas the 

 wood of pines and larches has a more or less red centre. The 

 red, brown, or black heart-wood that one meets w^ith in many 

 trees is technically known as the duramen. It differs in colour 

 from the lighter sap-wood or alburnum situated nearer the cir- 

 cumference, owing to some oxidation changes having occurred 

 in the resin or gum that it contains. These materials are de- 

 posited in the cells and in the cell-walls in a solid form, and so 

 plug up the organs that they are incapable of conducting sap. 

 The central wood of most trees is, in fact, to all intents and pur- 

 poses dead, and that it is not essential to the life of a tree is 

 proved by the fact that hollow trees live for years and, it may 

 be, centuries. The absence of duramen, then, separates the 

 wood of the spruces from that of the pine and larch, and these 

 two latter genera may be distinguished by the duramen of Pinus 



