10 THE POLYPORACEAE OF WISCONSIN. 



rot produced by Polystictus pergamenus. The latter, however, as 

 noted, is found cnly on deciduous trees. Both fungi produce rot in 

 the sapwood. Neither seems to penetrate more than an inch or two 

 into the wood. When branches are infected, the rot may soften them 

 through their entire diameter, if they are not too thick. 



My observations support the generally accepted view that the break- 

 ing down of the wood cells is due to an enzyme produced by the fun- 

 gus. The lignified walls seem first to be changed to cellulose and the 

 latter is then dissolved. 



There is hardly a change in the wood visible to the unaided eye, when 

 the fungus first begins to spread, after infection. "Wood of a red pine 

 that had been infected for less than a year appeared somewhat grey- 

 ish in color in and about the regions where young pilei were growing 

 out. Otherwise nothing could be seen of rot or decomposition with 

 the unaided eye. Sections under the microscope, however, showed 

 unmistakable signs of decomposition. Here and there the inner la- 

 mellae of the large tracheids as well as of the smaller wood fibers showed 

 traces of reduction to cellulose when tested with zinc chloriodide. 

 Staining with ruthenium red showed the middle lamella unaffected. 

 The ray-cells also showed the action of the fungus, the walls of many 

 having been broken down more or less completely. The inner layers 

 of the cell wall seem to be attacked first and slowly changed to cellu- 

 lose, as is shown by their more or less deep blue color when treated 

 with zinc chloriodide. The inner surface of the lamellae becomes ir- 

 regular as though it were dissolved unequally. 



The wood, when it begins to decay, may become a trifle paler in 

 color. Sometimes the very earliest stages are characterized by a dark or 

 grayish color, as stated above, but this is not a constant characteristic. 

 Again there may be irregular black lines in the wood next to the bark, 

 but this is also not constant. The cells in these black lines seem filled 

 with a dark brownish substance insoluble in alcohol. Mycelium is 

 usually abundant in cells near these lines. 



A little later there are numerous white streaks passing through the 

 summer wood of the rings of growth, both tangentiaUy and longitud- 

 inally, thus marking off the wood into more or less rectangular areas. 

 Near these white lines the microscope reveals numerous hyphae running 

 generally in the direction of the streaks. Later, the wood fibres 

 break down along these lines, making little pits and holes which in- 

 crease in size as the decay advances. 



Some tracheids break down quite completely, while others are in- 

 tact and form anastomosing strands which intersect at right angles 

 and are full of air, giving them a white appearance. The wood 



