ORDER POLYPORALES (aPHYLLOPHORALES) 471 



there occur species that are quite similar, some of which possess clamp 

 connections and some lacking them. 



In the sequence of famihes followed below it must be considered that a 

 logical arrangement should be based upon the supposed phylogenetic 

 relationships of the group. In the absence of decisive fossil remains we 

 have to fall back upon a comparison of species and genera now existent. 

 The surmised origins of the different families are quite various. Thus 

 Singer (1936) and others have been led to believe that the Agaricales 

 have descended from the Gasteromyceteae while others (e.g., Heim, 

 1937) believe the reverse to be the case. In the following discussion the 

 writer follows somewhat the latter's interpretation although admitting 

 that the evidence for Singer's view is quite strong. 



Order Polyporales (Aphyllophorales) . Hymenium always gymno- 

 carpic, i.e., from its inception exposed to the air and not enclosed by a 

 veil, with the possible exception of Cryptoporus volvatus (Pk.) Hulb. 

 Growth of the hymenium is more often without definite morphological 

 limits. In the supposedly more primitive forms (at least forms of simpler 

 structure) no highly developed morphological or anatomical differentia- 

 tion occurs beyond the resupinate layers of hyphae upon which arise the 

 more or less separate clusters of basidia or a continuous hymenial layer 

 which may continue to enlarge in all directions at the margin. In the 

 higher forms the subhymenial portion of the spore fruit becomes dis- 

 tinguished into several layers differing more or less morphologically and 

 anatomically and the tendency to develop upright or reflexed (shelf-like) 

 portions becomes more strongly marked. With evolution progressing in 

 various directions there appear clavate or dendroid structures or pileate 

 forms which are sessile or laterally, excentrically or even centrally 

 stipitate. Along with this external differentiation the incipient hymenium 

 may be increased in surface area by being thrown into folds or by the 

 production of emergent tubercles or teeth or by reticulate outward growth 

 leaving shallow or deep pits (pores) which are lined by hymenium. With 

 some of the simpler types of structure is correlated the stichobasidial 

 type of basidia, but the more complex structures mostly have the chiasto- 

 basidial type. The chief distinction in the following order, the Agaricales, 

 is the fact that in the latter the poroid or more often lamelloid hymenial 

 portion, though in its younger stages sometimes gymnocarpic, often 

 becomes secondarily enclosed (pseudoangiocarpic) or may arise from the 

 beginning as an internal development (angiocarpic). In both the latter 

 cases prior to the maturation of the basidia the spore fruits become 

 opened in a regular manner to permit the distribution of the spores. 



In both of these orders the basidiospores are perched obliquely at the 

 tips of the sterigmata and are shot off violently so that spore distribution 

 is effected by air currents. This distinguishes the Hymenomycetes from 



