74 TECHNICAL PKOrEliTIES UF WOOD. 



Decay spreads most easily along the grain of the wood, and 

 also centrifugally, but is often localized in a certain part of a 

 tree. According to the direction of the section made in the 

 wood during conversion, the decayed places will assume diflerent 

 forms. Thus a transverse section may show spots, or zones of 

 decay, termed Mdiulrinr/ in German, or liinurc in French. 

 Spruce and silver-fir left unbarked show bluish or black sap- 

 wood. When the stem is sawn into planks, the decaying parts 

 show like red, or white, lines and stripes. The wood may often 

 be penetrated by repeated decaying stripes, as in old oaks 

 attacked by Stcrcnm liirsutiim. i^Said to be white-piped.— Tr.] 



ii. External Decay. 



Whilst decay in the interior of the stem may often be com- 

 pletely concealed under an apparently healthy exterior, there are 

 other forms of decay which affect the bark, and can therefore be 

 readily recognized. Such decay is due to fungi of difierent 

 sorts, chiefly those which cause canker. Frost-cracks, or in- 

 juries of all kinds, and insects, facilitate the admission of the 

 spores of fungi. Thus silver-fir canker is caused by jEcidiuin 

 elatinum, larch-canker by Peziza Willcomniii, and species of 

 Nectria cause cankers in beech, ash, sycamore, and other broad- 

 leaved trees. 



Frost-cracks which remain open for some time are the 

 commonest causes of decay in trees. The fungi then penetrate 

 down into the heartwood, and cause the rotten radial cracks 

 which so often accompany frost-crack, and if the cracks should 

 close and no further progress of decay in the stem take place, 

 yet even then it will only be fit for fuel. 



All injuries done to the bark of trees which extend as far as 

 the sapwood may admit spores of fungi and cause decay. If the 

 injury is merely in the bark, corky tissues are formed which 

 protect the wood below from injury ; but once the cambium is 

 laid bare, the wound can be closed again only by the spread of a 

 callus from its sides, and, as long as occlusion*" has not taken 

 place, there is always danger from the spores of fungi. 



* For a full nccount of tlic process of occlusion of wounds, vide Hartig's Baum- 

 krankheiten. (Translation liy Sonierville, edited by Alarshall-Ward.) 



