THELEPHORACEAE 475 



and cymose tufts of short, broad basidia with four, less often six to eight, 

 sterigmata. The commonest parasitic species of this genus, P. filamentosa 

 (Pat.) Rogers {Corticium vagum var. solani Burt, Hypochnus solani 

 Prill. & Del., etc.) occurs as a parasite upon the stems and roots of potato 

 (Solarium tuberosum L.), bean (Phaseolus vulgaris L.), and very many 

 other plants of economic value. It produces cankers at or below the sur- 

 face of the soil which kill or seriously injure the parts affected. Small 

 sclerotia are formed which enable the fungus to overwinter. On the stem 

 of the host plant the mycelium creeps up as a thin gray or white hyphal 

 layer on which the oval basidia are produced in groups. The sclerotial 

 stage is known under the name of Rhizoctonia. (Fig. 153A, B.) 



The genus Tomentella {Hypochnus, as interpreted by Burt, 1916) 

 produces its basidia more or less scattered or in tufts on a loose cottony 

 mycelium. The basidiospores are nearly spherical and spiny. They are 

 mostly saprophytic. 



The species of Corticium in which gloeocystidia occur, are placed by 

 many authors in a separate genus, Gloeocystidium, but Burt (1926) and 

 Rogers and Jackson (1943) do not approve of this segregation. Peniophora 

 is practically a Corticium with true, fusiform, pointed cystidia (not 

 gloeocystidia). Asterostroma is similar but has stellately branched, thick- 

 walled bristles or cystidia in the hymenium. Epithele has the hymenium 

 interrupted here and there by sterile projections or pegs consisting of 

 bundles of hyphae. These differ from the teeth of the Hydnaceae which 

 are covered with basidia. Coniophora is practically a Corticium with 

 ferruginous spores. Some species have cystidia {Coniophorella Karst.) 

 and others not. Donk (1933) and Singer (1944) place the Family Merulia- 

 ceae close to these genera (see p. 484). 



In Aleurodiscus the spore fruit instead of remaining fiat against the 

 bark on which it develops curls up a little at the edge to form a fiat saucer 

 or shallow cup. The basidia and spores are rather large and there are 

 present various types of structures called by some cystidia, by others 

 paraphyses. Much similar is Vararia (Aslerostromella) but the modified 

 cells in the hymenium are many times dichotomously branched. (Fig. 

 153, C-E.) 



All of the foregoing genera have a rather thin and not much differ- 

 entiated spore fruit below the hymenium. The following genera have a 

 subhymenial structure much thicker and often in several distinct layers. 

 At the edges the spore fruit bends away from the substratum to form a 

 sort of shelf, with the hymenium on the smooth lower surface. Even a 

 sort of central stipe may be developed in some species, the spore fruit 

 being more or less funnel-shaped in that case. In Stereum conspicuous 

 cystidia are lacking, but in Hymenochaete the hymenial layer has numer- 

 ous long stiff, usually brown, pointed setae, whose probable function is 



