DURABILITY. 10 1 



product from a colourless chromogen. [Deodar-wood is also 

 pale yellowish-browD but extremely durable. Tr.] 



The colouring-matter in heartwood is derived from tannin ; 

 as the water disappears from the tissues and oxygen is 

 admitted, it is formed by oxidation in the border zone 

 separating sapwood from heartwood. This requires complete 

 illumination of the tree's foliage ; the colour is deepest in the 

 heartwood of branches, it is paler in the stem-wood and palest 

 in the roots. The colour of the heartwood is also deeper in 

 the wood of trees that, have grown in full sunlight than in 

 those grown in a crowded crop or under the shade of other 

 trees; therefore the wood of trees exposed to full light is 

 more durable than that of trees grown in restricted light. 

 The influence of thinnings and of setting trees free from 

 their neighbours and retaining standards over underwood 

 on the increased colour and durability of their wood is 

 therefore evident. 



The deepest colours and the greatest durability occur in 

 the heartwood of certain tropical trees ; in cooler climatic 

 zones the colours are less deep and the woods less durable. 

 In the coolest climate, indeed, perishable sprucewood and 

 very durable larchwood are produced, but as a general rule 

 it may be predicated for broad leaved trees within their native 

 habitat, that great heat implies durability. 



Turpentine becomes oxidised into rosin, which possesses 

 extraordinary durability: the more gradual is this process, 

 the greater the quantity of liquid and volatile oil that is con- 

 verted into rosin; such a transformation is secured by keeping 

 coniferous wood as long as possible in the form of logs or balks. 

 The effects of rosin on the durability of wood are not, how- 

 ever, sufficient for it to replace other factors, <v/., colouring- 

 matter. \Veymouth pine contains more turpentine than any 

 Kiiropean conifer except maritime pine, yet its wood is not 

 nearly so durable as that of larch, which contains little turpen- 

 tine, but much colouring-matter. The greater durability of 

 sprucewood than the wood of silver-fir is, however, due to its 

 being the more resinous of the two. 



Wet wood is always less durable than dry wood, for wet 

 wood in the form of logs or balks requires two or three years 



