Summary:
The distinguishing feature is growth from a small, black, easily overlooked tuber, but even without that, it is easily recognized in the field by a broadly convex cap that soon becomes flattened, long, slender, minutely pubescent stem, lack of a veil, bitter taste, and (microscopically) remarkable digitate pleurocystidia. The description from Hansen is for A. arvalis (Fr.) Heim & Romagn. A. arvalis as interpreted by Singer in the 1936-1950 period is different and now referred to as Agrocybe subpediades (Murrill) Watling.

Cap:
1-2cm across, convex becoming flat, sometimes depressed at center or with shallow umbo; pale ochraceous or buff with or without distinct cinnamon tinge, drying deep ochraceous, expallent [? becoming pale on drying]; smooth to slightly rugulose [wrinkled], but strongly wrinkled when dry, "greasy when moist, matt when dry", margin membranous, hardly or not striate, (Watling), 1.5-2.5cm across, convex; yellow-brownish to ocher-brownish often pale, (Moser), up to 2.5cm across, "ochraceous brownish with yellowish cast, paler when dry", (Courtecuisse), 1-3.5cm across, deep yellow to ochraceous, (Hansen)

Flesh:
buff in cap, paler in stem, (Watling)

Gills:
slightly adnate, fairly broad; "buff when young, then tinged cinnamon and finally brick", (Watling), narrow; brown, (Moser), broadly adnate, not very close; brownish to warm tobacco brown, (Courtecuisse)

Stem:
5-12.5cm x 0.2-0.4cm, equal or widening slightly downward, tough to elastic, stuffed, narrowing below soil surface into a rooting base or mycelial root, "or attached to a blackish brown sclerotium up to the size of a large pea"; ivory to pale buff at top, darker below; pruinose throughout, (Watling), with +/- well developed mycelial root originating in black sclerotium, (Moser), up to 10cm long and up to 0.5cm wide, rooting, often arising from black sclerotium (which can be as large as a chickpea), (Courtecuisse), 5-10(15)cm x 0.2-0.5cm, rooting, usually (always?) springing from a black 2-3(5)cm large sclerotium; stem pale yellowish; pulverulent [powdery], (Hansen)

Odor:
not distinctive (Watling)

Taste:
bitter (Watling), somewhat bitter (Courtecuisse)

Microscopic spores:
spores 9-10 x 5-6 microns, elliptic in face view, only slightly flattened on one side in side view, dull fulvous in water and alkali, germ pore small; basidia 4-spored, 25-30 x 5-7.5 microns, cylindric-clavate; pleurocystidia club-shaped or with 2-5 finger-shaped, upright, apical appendages, 45-60 x 10-15 microns (including appendages), neck 4-5 microns broad, cheilocystidia 35-50 x 7-10 microns, cylindric or subulate to lageniform, colorless; cap cuticle a hymeniform layer of vesiculose cells, 20-35 x 8-10 microns, "intermixed with lageniform or subfusiform cystidia"; stem cuticle of parallel hyphae bearing caulocystidia 50-100 x 7-10 microns, lageniform to fusiform; clamp connections present, (Watling), spores 9-12 x 5-6 microns, pleurocystidia with finger-like extensions, (Moser), spores 9-12 x 4.5-6 microns; pleurocystidia digitate, cheilocystidia lageniform to subfusoid, (Hansen), spores 8.6-11 x 4.6-5.8 microns, elliptic, smooth, pale honey-brown, thick-walled, with small germ pore up to about 1 micron across; basidia 4-spored, 25-30 x 6-7 microns, cylindric, some with basal clamp connection; cheilocystidia 35-55 x 6-15 microns, lageniform to subulate, pleurocystidia ventricose, with 3-5 digitiform outgrowths apically, 60-80 x 15-20 microns including the outgrowths; no clamp connections seen on cap cuticle, (Breitenbach)

Spore deposit:
snuff brown (Watling), tobacco brown (Breitenbach)

Notes:
Agrocybe arvalis is rare. It was reported from BC by Redhead(16). Breitenbach(4) give the distribution as Europe and Asia.

EDIBILITY

Synonyms and Alternate Names:
Agrocybe tuberosa (Henn.) Singer
Naucoria tuberosa Henn.