1918] 



BURT THELEPHORACEAE OF NORTH AMERICA. X 303 



pinate base as the fungus occurs higher on the side of the log ; 

 beyond one-fourth of a circumference upward from the bottom 

 of the log, umbonate-sessile, dimidiate, and flabelliform speci- 

 mens are frequently collected. I have a fine campanulate 

 specimen of Stereum fasciatum which I found on the top of 

 a log surrounded by this species. For these reasons the form 

 of fungi growing on prostrate logs is not as reliable a char- 

 acter as it is in case of species growing on the ground or in 

 the case of a flowering plant, and a student having in hand 

 only a resupinate or only a flabelliform fructification from 

 some herbarium must not be too confident that the respective 

 species are always resupinate or always flabelliform. 



The degree of differentiation in structure of the fructifica- 

 tion of Hymenochaete has not been used in systematic work 

 heretofore, so far as I am aware. Such structure affords, 

 however, constant, positive, fundamental characters of easy 

 determination. In the simplest condition of the fructification 

 in this genus, only a setigerous layer is present, in the next 

 degree higher of development, a hyphal layer connects the 

 setigerous layer with the substratum or may be extended from 

 the substratum as the upper surface of the pileus ; in a still 

 more highly developed condition, the hyphal layer is differ- 

 entiated into an intermediate layer and a denser and dark 

 zone, and usually into a second hyphal layer adnate to the sub- 

 stratum or forming the surface of the pileus. 



Key to the Species 



Fructifications in preparations of sections show at least both a setiger- 

 ous layer and a hyphal layer destitute of setae 1 



Fructifications lack a hyphal layer, i. e., have the setigerous layer seated 



directly on the substratum 16 



Fructifications dimidiate or flabellate, of unknown structure .... 37. H. pallida 

 1, Not stratose, i. e., consisting of but one setigerous layer of more or less 



thickness and of a hyphal layer 2 



1. Stratose, composed of two or more strata, of which each consists of a 



setigerous layer and a hyphal layer 



2. Hyphal layer simple and homogeneous throughout, i. e., not with 

 a portion differentiated as an internal or bordering, conspicuously 



denser and darker zone 3 



2. Hyphal layer not simple but differentiated into an intermediate 

 layer and at least a bordering, denser and darker zone on the 

 side towards the substratum or upper surface of the reflexed 



part 9 



3. Fructification stipitate, erect; stem with two or more short branches 



at or near its apex and each bearing an expanded pileus. .1. H. damaecomis 



