Summary:
Collybia cookei is distinguished by small size, growth from roundish tan to yellow or yellow-orange sclerotium, and growth on old mushrooms or occasionally wood or humus.

Cap:
2-7 mm broad, convex with an incurved to inrolled margin when young, becoming flat-convex to flat or somewhat depressed on disc, with downcurved or straight margin when old; subhygrophanous [somewhat hygrophanous], whitish to pinkish buff or orangish gray at first, fading to a more or less uniform sordid whitish; dry or moist, minutely fibrillose to somewhat tomentose, nearly bald and canescent [hoary] when old, striate on the margin, (Halling), convex, becoming flat, slightly depressed or umbilicate when old, shallowly papillate [with a nipple] when young; subhygrophanous, white to pale pink or yellowish, uniformly colored, drying slightly pinker, (Lennox)

Flesh:
very thin; colored as cap surface, (Halling), very thin, membranous, rather tough; colored as cap surface, (Lennox)

Gills:
adnate to slightly subdecurrent, close to subdistant, moderately broad, thin; white to pinkish buff; edges even, (Halling), "rather broadly adnate to sinuate, arcuate, thin, subdistant", 20 gills reaching stem, narrow, less than 0.1cm broad; white, drying pale pink, edges the same color as faces; edges even, (Lennox)

Stem:
0.4-5cm in length, 0.1(0.2)cm wide, equal, flexuous [wavy], pliant-fibrous, becoming hollow, arising from a yellowish, ochraceous to orangish or ochraceous buff sclerotium, 0.5-1cm x 0.4-0.8cm, spherical or nearly spherical to amygdaliform [almond-shaped], sometimes highly irregular in outline with surface often rugose [wrinkled] and pitted; stem cinnamon buff above, somewhat darker below; "surface dry, pruinose above, thinly pubescent below, strigose from strands of rhizomorphs near the base", (Halling), 0.6-6cm x 0.1-0.2cm, often buried for half of its length, emerges from an orange-yellow sclerotium 0.1-0.6cm in diameter, "irregular in shape, often flattened and pumpkin-shaped" [sic]; stem "equal, outline often wavy, straight, then flexuous", solid, round in cross-section; colored as cap at top, generally slightly darker toward the base, pinkish; "pruinose at top, becoming pubescent, scantily tomentose at the base, with white coarse bristle-like rhizomorphs there as well", (Lennox)

Odor:
none (Halling)

Taste:
none (Halling)

Microscopic spores:
spores 4.5-5.6(6.4) x 2.8-3.5 microns, elliptic to oboval in face view, short-elliptic to lacrymoid [tear-shaped] in side view, smooth, inamyloid, acyanophilic; basidia 4-spored, 14-21 x 4.2-5.6 microns, subcylindric to clavate, not siderophilic; pleurocystidia and cheilocystidia absent; cap cuticle "a thin layer of generally radially oriented, repent, cylindric and subgelatinous hyphae", cells 3.5-7 microns in diameter, "occasionally with short, diverticulate knobs, with scattered end cells or intercalary branches forming pileocystidia"; clamp connections present in all tissues, (Halling), spores 4.5-5.5(6) x 2.5-3.5 microns, ovate, smooth, inamyloid, colorless, thin-walled, contents uniform, (Lennox)

Spore deposit:
[presumably pale]

Notes:
It has been found at least in BC, WA, AK, GA, NC, Belgium, Finland, Mexico, Russia, and the United Kingdom, (Hughes). It has been found in CA (Desjardin).

EDIBILITY

Synonyms and Alternate Names:
Microcollybia cookei (Bres.) Lennox