Agaricus columellatus (Long) R.M. Chapm., V.S. Evenson & S.T. Bates
Notes: STB 12 Sept 2014
Source: MycoBank
Family: Agaricaceae
[Araneosa columellata Long]
Agaricus columellatus image
Basidiocarp 20–35 mm in diameter × 20–30 mm in height, secotioid, subglobose, depressed-globose to broadly ovoid or pyriform, furnished with a rudimentary stipe at the base that extends into the glebum as one unit to form a distinct columella, the entire unit often detaching completely and leaving a circular opening at the base of the peridial body; stipe-columella unit concolorous with the exoperidium, up to 30 mm in length, with roughly half of the unit (~15 mm) protruding from the base of the basidiocarp and the upper half (~15 mm) inserted into the glebum, slightly bulbous at the base, up to 5 mm in diameter and even along the stipe portion, somewhat swollen at the point of insertion, and tapering toward the apex within the glebum as the columella, with desiccation developing longitudinal striations and becoming more firm, surrounded at the point of insertion by a thin layer of peridial tissue, this with striations radiating out from the central point of the stipe and resembling velar tissue; peridia slowly breaking apart to expose the glebum, often starting from the basal opening formed as the stipe-columella unit detaches. Exoperidium whitish to pale yellow (3A1–3), dull, coarsely to irregularly rugose overall, though glabrous at the surface, thin, firm and persistent, cracking apart or flaking off with age to expose portions of the endoperidium. Endoperidium yellowish gray (3B2–4B2), dull glabrous, thin, firm and persistent, slowly breaking apart in conjunction with the exoperidium. Glebum brown to grayish brown (6E3–5), composed of labyrinthiform, sinuous tramal tissue, forming numerous locules, the interior of which is a hymenial surface densely covered with brownish spore mass, and an exterior composed of grayish, loosely woven hyphal tissue, breaking apart into irregular shaped glebal fragments with age.

Basidiospores subglobose to ovoid or occasionally irregular ovoid, 4.0–5.6 × 4.8–7.2 μm [X¯ = 4.8 ± 0.5 × 6.1 ± 0.7 μm, Qm = 1.3, n = 20], smooth, brownish in water mounts; pedicel rudimentary or up to 4 μm in length; sterigmal remnants mostly absent from mounts. Capillitium absent. Peridia composed of thin-walled hyphal elements.