SUBCLASS EUBASIDIAE 467 



sent more primitive organisms phylogenetically, although as elsewhere 

 among the fungi there is ample evidence that retrogression from more 

 complex to simpler structures has occurred frequently. Assuming, as 

 appears to the author to be most likely, that in some cases at least the 

 simpler forms of the Hymenomycetes represent more primitive forms, the 

 original type of spore fruit may have been a thin structure, not many 

 layers of hyphae in thickness, adhering at all points to the outer surface 

 of the substratum within which the vegetative mycelium was actively 

 growing and accumulating food reserves. This young spore fruit spreads 

 from a central spot more or less radially and its size is limited by external 

 obstacles or by the exhaustion of the supply of food. Thus the sporocarp is 

 theoretically to be considered as of unlimited ability to extend itself. 

 From the external layer of hyphae there turn outward short branches 

 which produce the basidia as their terminal cells. These upright hyphae 

 may branch sympodially as the basidia approach maturity so that not 

 only are new basidia appearing on the new radial growth of the spore 

 fruit but also are arising among the older basidia. At the same time that 

 the basidia are developing, the paraphyses, cystidia, etc., make their 

 appearance but probably not to any great extent in the most primitive 

 forms. In the species in which the mycelial growth outside of the sub- 

 stratum is more rapid and widespread than the production of the basidia 

 the hymenium may consist of interrupted groups of sympodially produced 

 basidia, as occurs in some species of the genera PeUicularia {Botryo- 

 basidium) and Tomentella. The spore fruit may then be more or less cot- 

 tony with scattered clusters of basidia. Such fungi are indiscriminately 

 lumped in the older works, in the genus Hypochnus. 



From the simple type of fruit body illustrated by the foregoing, 

 evolutionary development progressed in various directions. The sub- 

 hymenial structure became more complicated, often with two or three 

 distinct layers, each several or many cells in thickness {Stereum, etc.). 

 The sporocarps more and more developed on the substratum in such a 

 position that the hymenium faces downward, especially in those forms in 

 which the edges pull away from the substratum to form a kind of shelf, 

 the effuse-refiexed forms. This tendency persists until we find that in 

 many genera the spore fruits are shelf-like, with little or no resupinate 

 portion. These shelf-like structures may be narrowed toward the point 

 of attachment and often a distinct stipe occurs. This stipe may be 

 attached at the edge of the pileus or show a tendency to be attached 

 excentrically or even centrally to the under side of the pileus (many 

 Polyporaceae, Boletaceae, most Agaricaceae, and other related families). 



In consistency the sporocarp may be cottony, papery, leathery, corky, 

 woody, or fleshy. It may be short lived or may persist for many years. 



