SUILLUS RIPARIUS Thiers, Madrono 19:156. 1967

Illustrations: See Microfiche No. 48

Pileus 5-12 cm broad at maturity, convex to obtusely conic when young, becoming plano-convex to plane to broadly convex to shallowly depressed with age, often highly irregular and undulating; surface viscid, when young smooth to glabrous to subglabrous, occasionally somewhat streaked to appressed fibrillose, typically becoming somewhat rimose to areolate to fibrillose-scaly, occasionally in very old pilei appearing somewhat squarrose scaly; color when young brown ("buckthorn brown" to "Dresden brown") to occasionally dark brown ("Prout's brown"), margin yellow ("antimony yellow" to "yellow ocher"), scales brown ("Prout's brown" to "cinnamon-brown") to occasionally rust brown ("ochraceous-tawny"), with age pileus dark yellow ("yellow ocher" to "old gold" to sometimes "Naples yellow" to "straw yellow") to occasionally reddish brown ("ochraceous-tawny" to "ochraceous-buff") when very old, scales typically unchanging or sometimes becoming pale vinaceous when bruised; margin when young with a pronounced, large roll of veil tissue that fades to whitish ("pale olive-buff") with age, either appendiculate or glabrous to eroded with age. Context up to 1 cm thick, soft, yellow ("light ochraceous-buff" to "massicot yellow" to "Naples yellow"), changing to pinkish ("avellaneous" to "drab") after exposure. Taste acid; odor not distinctive.

Tubes up to 1.5 cm in length, bluntly adnate when young, becoming arcuate-decurrent to decurrent with age; when young deep yellow ("mustard yellow" to "amber yellow"), changing to dark yellow ("antimony yellow" to "yellow ocher") with age, sometimes becoming brownish with age, unchanging upon exposure; pores up to 4 mm in length, 3 mm in width, distinctly compound, concolorous with tubes, unchanging when bruised.

Stipe 3-8 cm long, 0.5-1.5 cm thick at the apex, more or less equal, stuffed to hollow; surface moist to subviscid, glabrous except for strongly developed, elongated, irregularly shaped, brown ("Sudan brown" to "amber brown") glandulae; color yellow, often darkening slightly toward the base, frequently with a slight vinaceous tint at the base; no annulus, but fibrillose zone sometimes apparent. Context concolorous with flesh of pileus, unchanging when exposed.

Spore print brown. Spores 8-11.5 X 3.2-5 µm, subcylindric to subfusoid, hyaline to pale ochraceous in KOH, pale ochraceous in Melzer's, smooth, thin-walled.

Basidia 21-29 X 8-10 µm, clavate to subcylindric, two- and four-spored, hyaline in KOH. Hymenial cystidia 35-75 X 5-10 µm, clustered, scattered to numerous, more abundant on the pores, staining dark brown in KOH and Melzer's, sometimes discolored only in base, incrusted, incrustations often floating free in mounting medium, cylindric to subclavate to subfusoid, thin-walled, occasionally hyaline in KOH.

Tube trama hyaline in KOH, divergent from an indistinct mediostratum, subgelatinous in KOH, hyphae 3-5 µm wide. Pileus trama interwoven, homogeneous, hyaline in KOH. Pileus cuticle differentiated as an ixotrichodermium, staining brown in KOH and ± reddish brown ("ochraceous-tawny") in Melzer's, incrusted, hyphae 3-4 µm wide; hypodermium compactly interwoven, well differentiated, brown in KOH, reddish brown ("ochraceous-tawny") in Melzer's. Stipe cuticle hymeniform with large clusters of fascicled caulocystidia, dark reddish brown in KOH. Clamp connections not seen.

Chemical reactions unknown.

Habit, habitat, and distribution Gregarious to cespitose in the vicinity of dead logs and stumps near the edges of streams in the vicinity of ponderosa and sugar pine. It is usually found in large numbers during September and October in the Sierra Nevada at elevations of 5,000 to 7,000 feet.

Material studied Calaveras County: Thiers 21253, 21257, 21261. Fresno County: Thiers 13350, 13351, 13356. Tuolumne County: Thiers 13283, 13284, 13883, 13885, 13889, 21294, 21311.

Observations The reddish brown scales on the pileus and the general stature of the basidiocarps of S. riparius make it somewhat suggestive of S. americanus. The sturdier, generally broader stipe of S. riparius in addition to its yellow-brown pores, darker-brown pileus and dark-brown glandulae on the stipe readily distinguish the two species. Also, so far as is definitely known, S. americanus. The sturdier, generally broader stipe of S. riparius in addition to its presence in the western United States is open to question. The earlier report by Thiers of the presence of S. americanus in California is now believed to be in error, and the basidiocarps so identified belong to S. riparius. Because of its large, rather labyrinthine pores, S. riparius might also be confused with S. megaporinus, which, however, has a much paler colored pileus, a poorly developed, often eccentric stipe, narrower spores, and a different type of habitat.

Edibility not determined.