58 DISEASES OF DECIDUOUS FOREST TREES. 



The general appearance of wood decayed by this fungus is very 

 similar "to that destroyed by Polystictus versicolor. The microscopic 

 changes differ in certain respects. A cross section of oak wood cut 

 so as tp include both healthy and badly decayed wood shows areas 

 of the wood fibers, especially in the summer wood of the annual ring, 

 which are colorless, while the other parts of the section are more or 

 less yellow. The colorless parts are the rotted regions, and in very 

 badly affected wood they may include all of the wood fibers in an 

 annual ring. The vessels and the medullary ray cells, however, retain 

 their color and appear unaffected. When stained with cellulose 

 reagents like chloriodid of zinc the colorless areas stain deep blue. 

 From this it appears that the fungus removes the lignin elements 

 from the cell walls. In the worst affected areas the cells are broken 

 down still further and the walls appear broken and corroded, so that 

 little remains except small fragments of the cell walls with the 

 pieces of the middle lamella which are located at the corners of these 

 cells embedded within them, and the whole held together by masses 

 of fine fungous hyphse. The medullary ray cells resist the action of 

 the fungus longer than any other part, very much as is the case in the 

 form of decay caused by Polystictus versicolor. 



The rate of decay caused by this fungus is very much like that 

 described for Polystictus versicolor. Small pieces of oak wood inocu- 

 lated with spores of the fungus have been completely rotted in about 

 three months. The rate of decay varies with the amount of water 

 left in the wood, the air supply, and the temperature. 



The relative resistances of heartwood and sapwood to the attacks 

 of this fungus are very, similar to those already referred to for Poly- 

 stictus versicolor, and the same may be said of the relative resisting 

 power of different species of woods, so far as known at this time. 



The fruiting bodies of Polystictus pergamenus occur throughout any 

 deciduous forest tract. A large number usually grow together, one 

 above the other, and not infrequently they are joined laterally so as 

 to form long series of shelves. The body of the pileus is leathery and 

 rigid; the top is concentrically sulcate, generally white when young, 

 growing grayish when older; the upper surface is slightly hairy; the 

 lower surface is generally purplish in color; the pores are small and 

 the intervening walls become much torn and lacerated, so that in 

 older specimens they resemble teeth or spines. 



SAP-ROT CAUSED BY FOMES APPLANATUS. 



In a recent paper Heald (38) describes a disease of the cottonwood 

 due to Fomes applanatus (Pers.) Wallr., which he, however, calls 

 Elfvingia megaloma (Lev.) Murrill. He finds that cottonwood trees 

 are affected with a disease which attacks both heart and sap wood and 



149 



