The Blacl- Zones Formed hy Wood-Destroying Fungi 25 



with certain decays of dicotyledonous woods. Similar for- 

 mations are characteristic of the decay of spruce and larch 

 caused by Porodaedalea pini (Thore) Murrill, but they are 

 inconspicuous when compared to the broad zones of decom- 

 position products w'hich commonly occur in the decay of 

 dicotyledonous woods. Such decomposition products arising 

 through the decay of coniferous woods by wood-destroying 

 fimgi apj)arently are small in quantity compared with those 

 arising through the decay of dicotyledonous woods. Why we 

 have such a condition is not known. 



If the decomposition products in question were dependent 

 upon the hemicellulose, xylan (wood gum), for their source 

 we might have a plausible explanation of this peculiar cir- 

 cumstance. According to Storer (1898) the evidence thus 

 far accumulated tends to show that the wood gum in the 

 trunks of trees is, comparatively speaking, difficult of diges- 

 tion and less active, physiologically speaking, than the time 

 cellulose with which it is accompanied and combined. In 

 proof of the small quantity of wood gum obtainable from 

 coniferous wood by the direct action of alkaline solutions, 

 Thomsen (1879, p. 159) says that, unlike the wood of 

 dicotyledonous trees which give up to dilute soda lye from 

 8 to 2G per cent of wood gum, fir wood is hardly at all acted 

 upon by soda lye. He obtained less than 0.8 per cent of 

 wood gum from spruce wood and less than 0.5 per cent- from 

 fir wood. In these coniferous woods he states that wood gum 

 appears to be present only in minute quantities. Other 

 authors have come to similar conclusions in regard to the 

 wood gum content of other coniferous woods. 



The great differences in structure between coniferous and 

 dicotyledonous woods suggests another plausible reason for 

 the discrepancy between the relative amounts of decomposi- 

 tion products arising through the decay of these respective 

 classes of w^oods. In the dicotyledonous w^oods, owing to the 

 presence of much larger elements — the vessels — we should 



