Summary:
Section Celluloderma. Pluteus thomsonii is characterized by 1) a reticulate-veined, blackish brown cap that becomes gray on drying, 2) free, close, white to grayish young gills, 3) a pruinose silvery gray stem, 4) growth on hardwood, 5) a pinkish spore deposit, 6) broadly ovate to nearly round spores, and 7) a needle-like apical projection on the cheilocystidia and caulocystidia. Minnis(1) say "No other species with a pileipellis having cystidioid elements has the rostrate cystidia and gray to brown pileus coloration".

Gills:
free, close to subdistant, broad; white to grayish becoming pale flesh color from spores, edge colored as face; edge entire to fimbriate [fringed], (Homola), free, 60 reaching stem, broad, 0.2 subgills between pairs of neighboring gills; white becoming dingy pink; edges smooth, (Breitenbach), subgills in 1 to 2 tiers (Minnis)

Stem:
(1)2-4.5cm x 0.1-0.6cm, equal or widening slightly toward base, hollow, fragile; vertically silvery streaked, whitish to silvery gray to darker gray at base, not markedly discoloring; densely pruinose at first, appearing silky when old, (Homola), 2-4(6)cm x 0.2-0.4cm, cylindric, slightly widened and bent toward base, solid becoming hollow, fragile; gray-white in upper half, increasingly gray in lower part, white-powdered especially toward apex, (Breitenbach), stuffed becoming hollow; grayish white; "innately fibrillose with a few scattered brown fibrils, twisted", (Minnis)

Odor:
not distinctive, mild, (Breitenbach)

Taste:
not distinctive, mild, (Breitenbach)

Microscopic spores:
spores 6-8 x 5.5-6 microns, ovate to nearly round, smooth, with slightly thickened wall, pinkish in KOH; basidia 4-spored, 25-35 x 8-9 microns, "somewhat fusoid-ventricose with a pedicel", colorless in KOH; pleurocystidia scattered to rare, 36-63 x 8-9 microns, "fusoid to fusoid-ventricose with obtuse apex, some with a short papilla, thin-walled", colorless in KOH, cheilocystidia numerous, 39-61 x 10-18 microns, "clavate to fusoid-ventricose, with short neck and broad rounded to merely obtuse apex with a large number having a short to elongate apical projection" (up to 45 microns long), thin-walled, colorless in KOH, cap cuticle a hymeniform layer, elements 58-90 x 10-28 microns, "short to elongate-clavate to fusoid to fusoid-ventricose, with a few having a short apical papilla, thin-walled", colorless or some with a pale brown vacuolar pigment in KOH, clamp connections absent; caulocystidia similar to cheilocystidia, clustered, "clavate to fusoid-ventricose, with short neck and broad obtuse apex, some with short to elongate apical projection, thin-walled", colorless in KOH, (Homola), spores 5.7-9 x 5.4-7 microns, nearly round to round, smooth, gray pink; basidia 4-spored, 25-38 x 8-10 microns, clavate to ventricose, without basal clamp connection; pleurocystidia sparse, 33-55 x 10-17 microns, fusiform to clavate, cheilocystidia 25-60 x 9-21 microns, "clavate to vesicular, but usually fusiform, with a prolonged apex", (Breitenbach), spores appearing straw-colored under the microscope; pleurocystidia abundant, 41.5-76 x 12-23 microns, fusiform, with long, acute apex, cheilocystidia similar in shape to pleurocystidia but smaller; clamp connections not observed, (Wartchow), spores 5.3-9.2 x 4.8-7.0 microns, nearly round, broadly elliptic, elliptic, ovate to rarely round to oblong to inequilateral in face and side views, circular in end view; pleurocystidia scattered, none to few to common, 39-68 x 10-22 including any apical projection, projection 1-21 x 1-3 microns, "lageniform with pedicels and short necks having broad, obtuse apices to clavate to pyriform to utriform and occasionally with a short apical mucro to long rostrum, walls thin and hyaline in KOH, no apparent contents, apices at times covered with amorphous, mucilaginous material"; cheilocystidia crowded, 30-86 x 7-25 microns including any apical projection, projection 1-33 x 1-4 microns, "lageniform with pedicels and short necks having broad, obtuse apices to clavate to utriform and often with a short apical mucro to long rostrum, walls thin and hyaline in KOH, lacking discernable [sic] intracellular contents, not uncommonly covered with an apical cap of mucilaginous material", (Minnis)

Spore deposit:
reddish ocher (Breitenbach, illustration looks pinkish brown)

Notes:
Homola examined collections from WA and MI. Minnis(1) examined collections from CA, IL, KY, MI, MO, NC, and the United Kingdom. There are collections from BC at the University of British Columbia. Breitenbach(4) gave the distribution as North America, Europe, Asia, and North Africa. It has also been reported from Brazil (Wartchow).

EDIBILITY

Synonyms and Alternate Names:
Pluteus cinereus Quel.