DURABILITY OF WOOD. 71 



nual rings are most durable especially when grown on com- 

 paratively poor soils, in dense forests and at high altitudes. 

 On the contrary, the hardwoods with wide annual rings are 

 most durable and are grown on the low lands and in isolated 

 positions. The wood of most broad-leafed trees produced in 

 the open is more durable than that from the dense forest. 



Sound Mature Trees yield more Durable Timber than either young 

 or very old trees. A tree is considered mature when it ceases 

 growing vigorously, which condition is indicated by the flat- 

 tening out of the crown, by dead branches in the crown and 

 by changes in the color of the bark. It is not indicated by 

 size since this varies in the same species according to cir- 

 cumstances. A small tree poorly situated for growth may be 

 as old or older than a larger tree growing under better con- 

 ditions. 



Intense Coloration of the Heartwood is a measure of durability 

 in timber and faintly colored heartwood resembles sapwood 

 in its properties, only surpassing it in dryness. The tannin 

 or coloring matter of heartwood is antiseptic. Where heart- 

 wood does not change its color or is lighter than the sapwood 

 the protecting substances are generally absent and the wood is 

 therefore liable to decay. This is plainly shown in the hol- 

 low trunks of Willow and Basswood. 



Sapwood Contains More Ready Made Food in forms acceptable to 

 a great number of kinds of fungi than the heartwood. This 

 largely accounts for the fact that sapwood is much more li- 

 able to decay than heartwood. This is especially true in the 

 case of Cedar and Pine where the heartwood is protected by 

 resinous substances. But when the sapwood is well seasoned 

 and heavier than the heartwood it lasts as long. Wood that 

 has been once attacked by fungi becomes predisposed to 

 further decay. 



The Time of Cutting Timber affects its durability only as the 

 weather at the time of cutting affects the curing process. 

 Wood cut in summer is generally affected by decay-producing 

 fungi, rapid fermenting of sap and by bad checking owing to 

 very rapid curing on the outside. As the cracks thus made 

 go deep into the wood they may increase the danger from 

 fungi. Where summer-felled wood is worked up at once" and 



