Durability 45 



darker, and less water is contained in it ; the waste products 

 produced in the life of the tree are stored up in it, so that the 

 woody tissue of the cells is thickened, and the timber becomes 

 denser in consequence. 



Gum resin and other substances are deposited in this portion of 

 the trunk, and it contains little or no starch or other substances 

 which would nourish rot-causing organisms. The result is that 

 the heart is always far more durable than the outer, lighter-coloured, 

 and younger rings. Heart-wood, on account of the waste products 

 stored therein, cannot be successfully injected with a preservative. 



Sap-wood (alburnum) comprises the outer, lighter-coloured 

 rings ; and by its means the material obtained by the roots is con- 

 veyed to the crown. This portion of the trunk is of small durability, 

 owing to its small density and to the presence of sap. 



The proportions of heart-wood and sap-wood vary in different 

 species. In the Turkey oak, the sap-wood occupies many rings 

 which are of small durability ; in the acacia, the sap-wood is limited 

 to comparatively few rings. In the case of the silver fir, beech, 

 spruce, aspen, lime, hornbeam, birch, and ash, little or no deepening 

 in colour of the older rings is observable. 



When a species of tree shows a distinct heart-wood, the timber 

 is more durable when the tree is mature than when it is young. 

 This is especially the case as regards conifers, as, for example, the 

 resinous heart-wood of old Scotch pine. 



Medullary rays is the term applied to the lines which are seen 

 on a cross section of such a tree as the oak. They are bands of tissue 

 running radially from the pith, or from some of the younger rings, 

 towards the bark ; they extend vertically to varying depths. They 

 are of importance in determining the beauty of furniture woods, and 

 therefore of deciding their market value. 



Durability. 



The decrease in the supply of the more durable kinds of 

 hard wood timber in this country, has compelled the substitution 

 of inferior species for purposes for which they were formerly 



