Astraeus hygrometricus: The Ultimate Mushroom Guide
About The Astraeus hygrometricus Mushroom
Astraeus hygrometricus is a species of fungus in the family Diplocystaceae. Young specimens resemble a puffball when unopened. In maturity, the mushroom displays the characteristic earthstar shape that is a result of the outer layer of fruit body tissue splitting open in a star-like manner.
This mushroom grows in association with various trees, especially in sandy soils. A. hygrometricus was previously thought to have a cosmopolitan distribution, though it is now thought to be restricted to Southern Europe, and Astraeus are common in temperate and tropical regions.
The rays of this intriguing gasteromycete mushroom open and close, but unlike flower petals which respond to sunlight the rays formed from the outer peridium of this earthstar react to the moisture content of the air and the soil on which they grow.
Other names: Hygroscopic Earthstar, The False Earthstar, Barometer Earthstar, Water measurer.
Astraeus hygrometricus Identification
Saprobic; growing alone, scattered, or gregariously in sandy soil, especially in disturbed-ground areas; summer and fall (overwinter in warmer climates); widely distributed in North America; also widely distributed in Europe and Asia.
A more or less spherical spore case sitting atop pointed rays that fold over the spore case in dry conditions.
1–1.5 cm across; more or less spherical; dry; matted-fibrillose; papery; rupturing at the top with maturity; whitish becoming brownish.
White and fleshy when young; becoming chocolate brown and powdery.
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.
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.
Astraeus hygrometricus Taxonomy & Etymology
The Barometer Earthstar was described scientifically in 1801 by Christiaan Hendrik Persoon, who placed it with the other 'earthstars' in the genus Geastrum as Geastrum hygrometricum. In 1889 the American mycologist Andrew Price Morgan (1836 - 1907) transferred this species to the genus Astraeus, thereby establishing its currently-accepted scientific name as Astraeus hygrometricus.
Synonyms of Astraeus hygrometricus include Lycoperdon stellatus Scop., Geastrum hygrometricum Pers., Geastrum vulgaris Corda, and Astraeus stellatus (Scop.) E. Fisch.
When in 1885 American mycologist Andrew Price Morgan first described this genus, it was surely the star-like rays of these fungi that prompted him to make Astraeus the generic name. (At the time only one or possibly two species were thought to exist, but molecular studies have now separated at least six Astraeus species.)
In Greek mythology, Astraeus, one of the Titans, was the god of dusk (when the stars come out). Astraeus married Eos, goddess of the dawn, and among their children were the four winds and the five planets of our sun known to exist at that time but considered to be 'wandering stars'.
The specific epithet hygrometricus means 'water measurer', a reference to the way the rays of this earthstar respond to changes in atmospheric humidity in much the same way that a barometer responds to fluctuations in atmospheric pressure.
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