CANKER AND DRY-ROT 61 



substances ; but they also destroy cellulose and Hgnin itself, at first 

 producing various diseolorations of the wood, and ultimately 

 reducing it to the condition of '' touchwood " or '' punk." It will 

 readily be understood that all these progressive changes are accom- 

 panied by a decrease in the specific gravity of the timber, for the 

 fungus decomposes the substance much in the same way as it is 

 decomposed by putrefaction or combustion, i.e. it causes the 

 burning off of the carbon, hydrogen, and nitrogen, in the 

 presence of oxygen, to carbon-dioxide, water, and ammonia, 

 retaining part in its own substance for the time being, and living 

 at its expense.^ 



Another true parasite, Trametes radiciperda, only attacks coni- 

 fers. Its spores, which can be readily conveyed in the fur of mice 

 or other burrowing animals, germinate in the moisture around the 

 roots : the fine threads of " spawn " penetrate the cortex and 

 spread through ^nd destroy the cambium, extending in thin, flat, 

 fan-like, white, silky bands, and, here and there, bursting through 

 the cortex in white oval cushions, on which the subterranean 

 fructifications are produced. Each of these is a yellowish-white 

 felt-like mass, with its outer surface covered with crowded minute 

 tubes or '' pores " in which the spores are produced. The wood 

 attacked by this fimgus first becomes rosy or purple, then turns 

 yellowish, and then exhibits minute black dots, which surround 

 themselves with extending soft white patches. 



The many pores in the fructification of Trametes indicate its 

 kinship with the genus Polyporus, many species of which are well 

 known as '' sheM-funguses," projecting like brackets from the 

 stems of trees, and having their pores on their under-surfaces. 

 Most of these are wound-parasites. One of the commonest, the 

 yellow cheese-like Polyporus sulphureus, occurs on Oak, Poplar, 

 Willow, Larch, and other standing timber, its spawn-threads 

 spreading from any exposed portion of cambium into the pith-rays 

 and between the annual rings, forming thick layers of yellowish- 

 white felt, and penetrating the vessels of the wood, which thereupon 

 becomes a deep brown colour and decays. 



The ravages of such wound-parasites are often the result of 

 neglect, broken branches being left untrimmed as a lodgment 

 for the spores of the fungus. We have known an Elm-tree to be 

 divided in this way by a broad zone of touchwood, originating 

 from the attack of a Polyporus on a snag, so that, though sound 

 timber both above and below, the tree snapped readily in half in a 

 slight gust of wind. 



^ Timber and some of its Diseases, by Prof. H. Marshall Ward, F.R.S., to which 

 work I am particularly indebted in the present chapter. 



