Mukrill: Polyporaceae of North America 425 



It is chiefly distinguished from nearly related genera by its separ- 

 able hymenium, which, upon investigation, is found to be cut off 

 from the context proper by a thin flesh-colored layer of mycelium 

 connected very loosely with the adjacent layers and allowing them 

 to separate easily. In age this layer disintegrates and the tubes 

 fall away in irregular masses, leaving the smooth white surface of 

 the context conspicuously exposed. 



Piptoporus SU15ER0SUS (L.) Murrill 



Boletus siibcrosus L. Sp. PI. 1176. 1753. 



Boletus bctulinus Bull. Herb. France, //. ^12. 1786. 



Polyporus betidinus Fr. Syst. Myc. i : 358. 1821. 



Piptoporus betnlimis Karst. Rev. Myc. 3: 17. 1881. 



Piptoponis siiberosus Murrill, Jour. Myc. 9: 94. 1 903. 



Pileus fleshy to corky, compressed ungulate, convex above, 

 plane below, attached by a short umbo behind, varying to bell- 

 shaped when hanging from horizontal trunks, 5-30 X 5-20 x 2-5 

 cm. ; surface smoky, covered with a thin separating pellicle, gla- 

 brous, devoid of markings, cracking with age ; margin velvety, 

 concolorous, obtuse, projecting beyond the hymenium nearly a 

 centimeter : context fleshy-tough, elastic, homogeneous, 3 cm. 

 thick, milk white ; tubes separated from context by a thin pink 

 layer, 0.5 cm. long, 2-3 to a mm., sodden white; mouths very 

 irregular, dissepiments thicker than pores, obtuse, entire, crumb- 

 ling away in age, leaving the smooth white context : spores white, 

 cylindrical, curved, 4-5 n in length. 



This plant is confined to species of birch, upon the dead or decay- 

 ing trunks of which it is quite abundant in the northern United 

 States and Canada. Its common occurence in Sweden explains why 

 it was noticed and described by Linnaeus in Flora Suecica as early 

 as 1745. From the following selected exsiccati it will be seen 

 that this species is as cosmopolitan as its hosts. France, Roiune- 

 gtiere 2102; Germany, Sydow 62 j ; Saxony, Krieger po/; 

 Sweden, Roniell 10, Murrill ; Russia, Jaczewski j6 ; Wisconsin, 

 Baker ; New Jersey, Murrill ; New York, Underwood, Murrill; 

 New Hampshire, Miss Minns, Grout. 



Scutiger Paul. Icon. Champ, pi. ji.f. i-j. 1793 

 Seven species were included in this genus by Paulet, one of 

 them an agaric, four hydnums and two polypores. The first species 



