Illustrations: See Microfiche No. 10

Pileus 6-17 cm latus, siccus, tomentosus demum fibrillosus vel fibrilloso-squamosus, brunneus. Contextus albus, immutabilis. Sapor et odor mitis. Tubuli 1.5-2.5 cm longi, pallide flavi, aetate obscure flavi, immutabiles; pori 0.5-1 mm lati, concolores, immutabiles. Stipes 10-16 cm longus, 2-3 cm crassus, siccus, reticulatus, subbrunneus. Sporae 13-17.5 X 3.5-5 Ám, subfusoideae vel ventricosae. Cystidia 38-46 X 6-8 Ám, subcylindrica vel fusoideo-ventricosa. Cuticula intertexta. Holotypus (no. 8907) a H. D. Thiers lectus prope Jackson State Forest, Mendocino County, November 18, 1961; in Herbarium San Francisco State University conservatus.

Pileus 6-17 cm broad at maturity, convex when young, becoming plano-convex to broadly convex to plane with a flared margin with age; surface dry, even to somewhat wrinkled or rugose, typically tomentose to velutinous, becoming fibrillose, sometimes appearing fibrillose-scaly on the disc; color brown to dark brown ("cinnamon-brown" to "chestnut-brown" to "Mars brown" to "warm sepia" to "burnt umber"), occasionally with paler, buff-colored spots, with age unchanging or becoming somewhat paler on the margin; margin incurved to decurved, becoming plane to flared, entire. Context 1.5-3 cm thick, soft, floccose, white to pale buff, darkening slightly near the cuticle, unchanging when exposed. Taste and odor mild.

Tubes 1.5-2.5 cm long, narrowly and shallowly depressed around the stipe, light yellow ("seafoam yellow" to "seafoam green") when young, darkening to dark yellow ("oil yellow" to "olive lake" to "old gold") with age, unchanging or darkening slightly when bruised; pores 0.5-1 mm broad, angular, concolorous with tubes, unchanging when bruised.

Stipe 10-16 cm long, 2-3 cm thick at the apex, equal to clavate, rarely tapering, often pinched at the base, solid, white mycelium at the base; surface dry to moist, reticulate over most portions; color pale yellow at apex, otherwise light brown to brown ("cinnamon-buff" to "amber brown" to "hazel"). Context white, unchanging when exposed.

Spore print dark olive brown. Spores 13-17.5 X 3.5-5.5 Ám, pale ochraceous in Melzer's, pale yellow to hyaline in KOH, subfusoid to ventricose in face view, inequilateral in side view, smooth, moderately thickwalled.

Basidia 35-40 X 8-1 2 Ám, hyaline in KOH, clavate, four-spored. Hymenial cystidia 38-46 X 6-8 Ám, scattered, inconspicuous, hyaline, thin-walled, subcylindric to fusoid-ventricose with elongated apices.

Tube trama hyaline, gelatinous in KOH, divergent. Pileus trama interwoven, homogeneous. Pileus cuticle differentiated as a trichodermium composed of hyphae having ▒ equal cells. Stipe cuticle differentiated as a layer of fertile basidia and caulocystidia, which are hyaline, subcylindric to clavate, abundant on ridges of reticulum.

Chemical reactions KOH-cuticle red to pink; NH4OH-cuticle red to pink, HCl-context yellow, cuticle pink to red; HNO3-context pale yellow, cuticle pink to red; sulfoformalin-cuticle pink to red.

Habit, habitat, and distribution Solitary to scattered in soil in dense mixed coastal forests. It has been most frequently collected in the vicinity of Mendocino, but isolated collections have been made as far north as Crescent City. There are published reports of its presence throughout the Pacific Coast, but it never occurs abundantly. Basidiocarps generally appear early in the fall collecting season, but fruiting continues for only a short time.

Material studied Del Norte County: Thiers 21545. Mendocino County: Breckon 281; Malloch 10-27-62; Motta 9; Sigal 11-14-70; Thiers 8873, 8907 type, 9298, 10581, 10640, 11012, 14608, 18102, 21316, 21526, 24158, 26899, 26936.

Observations This fungus has been commonly referred to as Boletus olivaceobrunneus, a species described by Zeller from Oregon. However, Zeller clearly indicated in the type description that B. olivaceobrunneus is almost black, has almost black pores, dark-colored spores, and a smooth rather than reticulate stipe. Boletus fibrillosus shows none of these characters. Through the courtesy of the New York Botanical Garden, the type of B. olivaceobrunneus was examined. It is the opinion of this author that Zeller's type is a collection of Tylopilus pseudoscaber. This taxonomic problem is discussed more fully in the discussion of T. pseudoscaber.

Boletus fibrillosus is characterized by the dry, dark-brown, fibrillose pileus, pores that are yellow during all stages, a brown, reticulate stipe and a tangled trichodermial type of cuticle. These characters make it obviously distinct from T. pseudoscaber and also serve to separate it from B. edulis, with which it has been confused in the past. It belongs to the section Boletus but does not appear to be very closely related to any known species in that taxon.

Because of the confusion regarding its identity, no definitive statement can be made regarding its edibility. In all probability, however, it is quite safe.

The Boletes of California
Copyright © 1975 by Dr. Harry D. Thiers
Additional content for the online edition © 1998 by Michael Wood, Fred Stevens, & Michael Boom
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