130 HEART-ROT CAUSED BY OTHER FUNGI 



more confined than in the larch (compare fig. 49 with 

 fig. 52). This suggests that the fungus is purely sapro- 

 phytic. At the same time dead trees are not infrequently 

 found which have become rotted through to the bark, and 

 it is only then that fructifications are borne on the trunk, 

 except when large branches have fallen away so as to expose 

 the heart-wood. For the present it must remain an open 

 question whether such dead trees are killed by other causes, 

 or whether P. Schweinitzii can kill the roots, taking advan- 

 tage of the more moist conditions in the soil, and thus, 

 secondarily, produce suitable conditions for growth in the 

 sap-wood of the stem. 



In the earliest stages of rot, the heart- wood has a rather 

 deeper red colour than normally, but it soon loses its reddish 

 tinge and takes on more the colour of cork (fig. 50). It 

 also becomes very much lighter in weight. Further decom- 

 position is generally attended by deepening of the colour 

 (fig. 49), in which case the wood becomes dark walnut 

 brown, and has a strong smell of turpentine. It cracks 

 along transverse, radial, and tangential planes, producing 

 wedge-shaped or cubical blocks which can be picked out 

 with the fingers. The cracks are often filled with mycelium 

 of the fungus, bound together with resin and of cheesy con- 

 sistency. If a section of a trunk cut in this stage is kept 

 in a dry laboratory the rotted wood contracts on drying to 

 such an extent that small pieces fall away by their own 

 weight, and the section is eventually left with a hollow 

 centre. The wood is then so far reduced in weight that, 

 according to Hartig, its specific gravity is only 0-19, as 

 compared with 0-57 for normal wjood. 



With the help of a microscope mycelium can be found in 

 the wood from the earliest stages of rot. But dense agglome- 

 rations of hyphae, such as occupy the regions of most 

 intense wood destruction in Fomes annosus and Armillaria 

 mellea, are nowhere present in wood rotted by this fungus, 

 except in the cracks as mentioned above. Consequently, 

 decomposition is less localized and more evenly distributed. 

 The hyphae present in the wood are mostly colourless and 



