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5 The Mushrooms Abortiporus biennis (Bulliard) Singer Ecology: Saprobic on the wood of hardwoods; growing alone or gregariously around the bases of stumps and living trees; causing a white rot in deadwood and a white trunk rot in living wood; summer and fall; widely distributed. Cap: Often irregular and covered with the pore surface, but when definable up to 20 cm across; kidney-shaped to semicircular or irregular in outline; whitish to pale brown or reddish brown; sometimes with concentric zones; finely velvety or bald. Pore Surface: Whitish, bruising and discoloring reddish or pinkish brown; pores angular to maze-like or irregular, 1–4 per mm; tubes to 6 mm deep. Stem: Often absent or poorly defined, but when present up to 6 cm long; whitish; velvety. Flesh: White to pinkish or pale tan; exuding a pinkish juice when squeezed; 2-layered in mature specimens; tough. Chemical Reactions: KOH negative on flesh. Spore Print: White. Microscopic Features: Spores 5–8 × 3–5 µm; smooth; broadly ellipsoid; inamyloid. Chlamydospores often present; globose; 5–9 µm. Gloeocystidia infrequent to numerous; variously shaped. Comments: Some authors give a separate species name, Abortiporus distortus, to the gnarled form that lacks a clearly defined cap and stem. Both forms are illustrated. 81 the mushrooms Agaricus arvensis Schaeffer Ecology: Saprobic; growing alone, scattered, or gregariously in grassy places (lawns, fields, and so on); summer and fall; widely distributed. Cap: 7–20 cm; convex at first, often with a somewhat flattened center ; later broadly convex or flat; dry; white or pale yellowish when young; developing appressed fibers or fine scales (especially in dry weather); bruising yellow; the margin often with hanging partial veil remnants. Gills: Free from the stem; crowded; whitish at first, becoming brown without a pink stage. Stem: 5–15 cm long; 1–3 cm thick; equal, or slightly bulbous; bald, or with scales below the ring; white; sometimes bruising yellow; with a persistent, large ring that is often “cogwheeled” on the lower surface. Flesh: Thick and white throughout; not changing color when exposed , or yellowing slightly; not yellowing in stem base. Odor and Taste: Odor sweet (reminiscent of anise or almonds) when young and fresh, becoming less distinctive; taste pleasant. Chemical Reactions: Cap yellow with KOH. Spore Print: Brown. Microscopic Features: Spores 7–9 × 4.5–6µm; ellipsoid; smooth. Comments: Edible, but some people may be sensitive to it. Sometimes called the “horse mushroom.” Compare with Agaricus xanthodermus (p. 84), which has bright yellow flesh in the stem base and a strongly unpleasant odor when the stem base is crushed; also compare with the white form of Stropharia rugosoannulata (p. 364). Agaricus campestris Linnaeus Ecology: Saprobic; growing alone, gregariously, or sometimes in fairy rings in meadows, fields, lawns, and grassy areas; late summer to early winter; widely distributed and common. Cap: 3–11 cm; convex to broadly convex, occasionally nearly flat; whitish; bald and glossy to silky to nearly woolly or scaly. Gills: Free from the stem; deep pink becoming brown in maturity; crowded; covered with a thin white partial veil when in the button stage. Stem: 2–6 cm long; 1–2.5 cm thick; more or less equal; sometimes tapering slightly to base; with a quickly collapsing white ring; whitish; not bruising. Flesh: Thick and white throughout; unchanging or very rarely discoloring pinkish in wet weather. Odor and Taste: Not distinctive. Chemical Reactions: Cap surface negative with KOH. Spore Print: Brown. Microscopic Features: Spores: 5.5–10 × 4–7 µm; Fairy ring 82 the mushrooms ellipsoid; smooth. Comments: Edible and good. Sometimes called the “meadow mushroom.” The habitat in grass, whitish cap, squatty stature, pink immature gills, and lack of bruising reactions are good field characters. Agaricus porphyrocephalus (p. 83) is similar but features a brown, hairy to scaly cap. Agaricus placomyces Peck Ecology: Saprobic; growing scattered or gregariously under hardwoods and in mixed woods; summer and fall; widely distributed but more common in the northern Midwest. Cap: 5–12 cm, convex to broadly convex or nearly flat in age; dry; covered with brownish fibers and scales, especially over the center; whitish underneath the scales, or pinkish in wet weather. Gills: Free from the stem; crowded; pale grayish pink at first, becoming brown; at first covered by a partial veil that develops brownish to yellowish droplets. Stem: 6–15 cm long; 1–1.5 cm thick; more or less equal, or with a slightly enlarged (but not abruptly bulbous) base; bald; white; bruising yellow, especially at the base; with a persistent...


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