An Eco-sustainable World
Species Fungi

Astraeus hygrometricus

Astraeus hygrometricus

The hygroscopic earthstar or barometer earthstar, false earthstar (Astraeus hygrometricus (Pers.) Morgan, 1889) is a mushroom belonging to the Diplocystidiaceae family.

Systematic –
From a systematic point of view it belongs to:
Eukaryota domain,
Fungi Kingdom,
Agaricomycotina Division,
Class Agaricomycetes,
Subclass Agaricomycetidae,
Order Boletales,
Family Diplocystidiaceae,
Genus Astraeus,
Species A. hygrometricus.
The terms are synonymous:
– Astraeus hygrometricus f. decaryi (Pat.) Pat.;
– Astraeus hygrometricus f. ferrugineus V.J.Stanĕk;
– Astraeus hygrometricus var. anglicum Pers.;
– Astraeus stellatus (Scop.) E.Fisch.;
– Astraeus stellatus var. duplicatus (Chevall.) Hollós;
– Elaphomyces sulphureopallidus Vaek, 1949;
– Geastrum argentum Desvaux;
– Geastrum castaneum Desvaux;
– Geastrum commune Desvaux;
– Geastrum decaryi Pat.;
– Geastrum diderma Desv., 1809;
– Geastrum fibrillosum Schwein.;
– Geastrum hygrometricum Pers.;
– Geastrum hygrometricum subsp. anglicum Pers.;
– Geastrum hygrometricum var. paucilobatum Wettst.;
– Geastrum stellatum (Scop.) Wettst.;
– Geastrum vulgare Corda;
– Geastrum vulgaris Corda;
– Lycoperdon stellatum L., 1753;
– Lycoperdon stellatus Scop..

Etymology –
The term Astraeus comes from the Greek ἄστριος ástrios stellato (= ἀστέριος astérios): for the star-like shape with the carpophore.
The specific epithet hygrometricus comes from the Greek ὐγρόϛ hygrόs humid and from μέτρον métron measurement: which absorbs water and gives its measure.

Geographic Distribution and Habitat –
Astraeus hygrometricus is a mushroom with a cosmopolitan distribution as it is common in tropical and temperate regions of the globe, but it is not found in arctic areas, and in colder areas or with an alpine climate. It can be found in Africa, Asia, Australia, Europe, North America and South America.
This species grows in association with a wide variety of trees. The mutualistic association between the roots of the tree and the mycelium of the mushroom helps the former to extract nutrients from the soil (in particular phosphorus), and the latter receive carbohydrates from photosynthesis. In North America, associations with oaks and pines are widespread, while in India they commonly grow in symbiosis with Pinus roxburghii and Shorea robusta. In Nepal some of these mushrooms have been found at an altitude of 3,000 meters. Fresh specimens can often be found next to specimens that are even a few years old.
Its habitat is that of various types of forest, not linked to particular environments, in clearings, on bare, poor, dry ground, normally in small groups, but also single, initially semi-underground then emerging, from spring to autumn, very common. It mostly grows in the sandy areas of the woods, mostly on bare ground, between September and December. It has also been observed growing on rocks, preferring an acidic substrate such as slate or granite, while avoiding soils rich in lime.

Recognition –
Astraeus hygrometricus is a mushroom whose young specimens have a spherical carpophore and begin development partially buried in the substrate. A thin whitish mycelial layer covers the carpophore, which may be partially encrusted by debris.
When ripe, the superficial layer disappears, and the outermost layer, the exoperidium, opens like a star, with 4-20 ocher petals on the outside and light on the inside. This causes the carpophore to emerge from the ground, revealing the spherical endoperidium in the center. The “petals” open and close depending on the level of humidity in the environment: they open in the presence of a lot of humidity, and close on themselves if the air is dry. This is possible because the exoperidium is made up of many different layers of tissue; the innermost, fibrous layer is hygroscopic, causing the entire layer to curl if it loses moisture from the surrounding environment.
It is an adaptation that allows the fungus to disperse its spores at the best time and to preserve internal liquids in the driest periods. Furthermore, carpophores with closed petals can be easily transported by the wind, thus allowing the diffusion of spores from the central hole.
This mushroom, with open petals, has a diameter of 1–8 cm. The exoperidium is thick and the rays are typically areolate (divided into small areas distinguished by cracks and fissures) on the upper surface, and vary from gray to black. The central spherical body is sessile (as it lacks the stem), 3–5 cm in diameter and dark brown in colour. The apex of the sphere is pierced by a crack or pore. Small hair-like filaments called rhizomes extend from the base into the substrate.
The rhizomes are fragile, and often break once they reach maturity.
The exoperidium is made up of four different layers of tissue: the mycelial layer contains branched hyphae of 4 – 6 µm in diameter; the hyphae of the fibrous layer are branched and 6 – 8 µm thick; the collenchymal layer has branched hyphae of 3 – 4 µm; the soft layer, however, contains hyphae 3 to 6 µm thick.
The meat is thin; it has a greyish or brownish color and is divided into niches (a characteristic that allows these mushrooms to be distinguished from those of the Geastrum genus). When ripe it becomes brown and dusty; It also has no odor or taste.
Under the microscope, round spores are observed, reddish brown in color, covered with warts and thorns. The dimensions vary from 7 to 11 μm; warts are about 1 µm long. The spores are non-amyloid and do not stain with iodine with Melzer’s reagents. Scanning electron microscope analyzes showed that the spines are 0.90 – 1.45 µm long, narrow, tapered, rounded at the tip and sometimes joined together. On the surface of the spore there are hyaline and branched capillary filaments of 3.5 – 6.5 µm in diameter.
The basidia have a variable number of spores from 4 to 8, with very short sterigmata. The basidia are arranged in groups on long chains; each basidium measures 3 – 5.5 µm.

Cultivation –
Astraeus hygrometricus is not a cultivated mushroom.

Customs and Traditions –
Astraeus hygrometricus is a mycorrhizal species, which grows in association with numerous plants, especially on sandy soils. It has a wide distribution, and is common in temperate and tropical regions.
Despite its general appearance, the A. hygrometricus is not related to the mushrooms of the genus Geastrum, although they have been confused in the past. The species was first described by Christian Hendrik Persoon in 1801 as Geastrum hygrometricus. In 1885 Andrew Price Morgan proposed, given the differences in microscopic characteristics, to create the new genus Astraeus distinct from the genus Geastrum; this proposal was later not universally accepted. Some Asian populations, thought to be A. hygrometricus, were renamed in the 2000s after phylogenetic analyzes revealed that they belonged to different species, including Astraeus asiaticus and Astraeus odoratus. Research has identified the presence of numerous bioactive chemical compounds in the carpophore. Guides usually list A. hygrometricus as inedible, although they were once used by Indian populations and the Blackfeet tribe in North America.
North American sources in fact list the A. hygrometricus as inedible, in some cases due to its toughness. However, they are regularly consumed in Nepal and southern Bengal, where “local people consume them as delicious food”. They are collected from the wild and sold in markets in India.
This mushroom is not toxic, but has no food value (it is woody and tasteless), although other North American guides classify it as edible. In India they are collected in the forests and then sold at the market.
However, it must be taken into consideration that the differences of opinion regarding the edibility of this mushroom date back to sources published before it was discovered that the specimens from North America and those from Asia were not always of the same species; in some cases specimens collected in Asia have been identified as new species, such as A. asiaticus and A. odoratus.
Among the similar species we remember the Astraeus pteridis which is similar in appearance, but larger; it is found in North America and the Canary Islands.
Although A. hygrometricus bears a superficial resemblance to members of the “true earth stars” Geastrum, it can be easily differentiated from most by the hygroscopic nature of its rays. Hygroscopic terrestrial stars include G. arenarium, G. corollinum, G. floriforme, G. ricolligens, and G. kotlabae. Unlike Geastrum, the young fruiting bodies of A. hygrometricus do not have a columella (sterile tissue in the gleba, at the base of the spore sac). Geastrum tends to have the spore sac opening surrounded by a peristome or disc, in contrast to the single lacerated slit of A. hygrometricus. There are also numerous microscopic differences: in A. hygrometricus the basidia are not arranged in parallel columns, the spores are larger and the hair threads are branched and continuous with the hyphae of the peridium. Despite these differences, older specimens can be difficult to distinguish from Geastrum in the field. One species of Geastrum, G. mammosum, has thick, fragile rays that are moderately hygroscopic and could be confused with A. hygrometricus; however, its spores are smaller than A. hygrometricus, typically around 4 µm in diameter.

Preparation Method –
The edibility of Astraeus hygrometricus, although it is not a toxic mushroom, is also controversial due to some reports prior to the reclassification of species that were believed to be A. hygrometricus. However, it is a mushroom of no particular value also due to its woody consistency and being tasteless.

Guido Bissanti

– Wikipedia, the free encyclopedia.
– GBIF, the Global Biodiversity Information Facility.
– Cetto B., 2008. Mushrooms from life, Saturnia, Trento.
– Pignatti S., 1982. Flora d’Italia, Edagricole, Bologna.
– Conti F., Abbate G., Alessandrini A., Blasi C. (ed.), 2005. An annotated checklist of the Italian vascular flora, Palombi Editore.

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Attention: Pharmaceutical applications and food uses are indicated for informational purposes only, they do not represent in any way a medical prescription; we therefore decline any responsibility for their use for healing, aesthetic or food purposes.

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