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Astraeus hygrometricus

[ Basidiomycota > Boletales > Diplocystidiaceae > Astraeus . . . ]

by Michael Kuo

This little thing is in just about every North American field guide ever published (except mine) and it is collected so frequently that its records take up 18 screens-worth of collections in our nation's major herbaria, databased at MyCoPortal. But I have seen it only once, in more than 20 years of collecting. Perhaps I keep missing it, or maybe I don't collect very often in sandy, disturbed-ground areas (which is where Astraeus hygrometricus is usually found). Or maybe I'm just flipping incompetent. At any rate, I have only collected it with the help of the New Jersey Mycological Association, on a foray in the Jersey pine barrens—and, more recently, I have studied a collection sent to me from North Carolina.

Astraeus hygrometricus looks like "earth stars" in the genus Geastrum, but its rays are "hygroscopic"; they cover the round spore case in dry weather but peel away from it in wet conditions. The inner (or "upper") surfaces of the rays become finely cracked, and the surface of the spore case is matted-fibrillose. The spore powder, at maturity, is chocolate brown—and, under the microscope, the spores are much larger than the spores of Geastrum species.

Geastrum and Astraeus are not, in fact, very closely related; Geastrum is related to the stinkhorns, while Astraeus is related to the boletes. Thus the ball-on-a-star strategy for dispersing spores is an example of convergent evolution, not phylogenetic similarity.

Thanks to Greg Allikas for documenting, collecting, and preserving Astraeus hygrometricus for study; his collection is deposited in The Herbarium of Michael Kuo.


Ecology: Saprobic; growing alone, scattered, or gregariously in sandy soil, especially in disturbed-ground areas; summer and fall (over winter in warmer climates); widely distributed in North America; also widely distributed in Europe and Asia. The illustrated and described collections are from New Jersey and North Carolina.

Fruiting Body: A more or less spherical spore case sitting atop pointed rays that fold over the spore case in dry conditions.

Spore Case: 1–1.5 cm across; more or less spherical; dry; matted-fibrillose; papery; rupturing at the top with maturity; whitish becoming brownish.

Interior: White and fleshy when young; becoming chocolate brown and powdery.

Rays: Numbering 6–10; more or less triangular; about 1 mm thick; inner/upper surface dark brown to black, becoming finely cracked overall; outer/lower surface brown, matted-fibrillose, usually covered with sand.

Odor: Not distinctive.

Microscopic Features: Spores 7.5–10 µm; globose; echinate; spines densely crowded, about 1 µm long; brownish golden in KOH. Capillitial threads 2.5–7 µm; wide; yellowish to brownish in KOH; roughened; thick-walled.

REFERENCES: (Persoon, 1801) Morgan, 1889. (Saccardo, 1891; Coker & Couch, 1928; Smith, 1951; Smith, 1975; Smith, Smith & Weber, 1981; Arora, 1986; States, 1990; Phillips, 1991/2005; Lincoff, 1992; Metzler & Metzler, 1992; Barron, 1999; Roody, 2003; Bates, 2004; Phosri et al., 2004; McNeil, 2006; Miller & Miller, 2006; Buczacki, 2012; Desjardin, Wood & Stevens, 2015; Evenson, 2015; Siegel & Schwarz, 2016; Jeppson in Knudsen & Vesterholdt, 2018; Sturgeon, 2018; Læssøe & Petersen, 2019.) Herb. Kuo 10101509, 01152002.

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Astraeus hygrometricus

Astraeus hygrometricus

Astraeus hygrometricus

Astraeus hygrometricus

Astraeus hygrometricus

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Kuo, M. (2020, February). Astraeus hygrometricus. Retrieved from the MushroomExpert.Com Web site: