Hygroscopic Earthstar

Young specimens of A. hygrometricus have roughly spherical fruit bodies that typically start their development partially embedded in the substrate. A smooth whitish mycelial layer covers the fruit body, and may be partially encrusted with debris. As the fruit body matures, the mycelial layer tears away, and the outer tissue layer, the exoperidium, breaks open in a star-shaped (stellate) pattern to form 4–20 irregular "rays". This simultaneously pushes the fruit body above ground to reveal a round spore case enclosed in a thin papery endoperidium. The rays open and close in response to levels of moisture in the environment, opening up in high humidity, and closing when the air is dry. This is possible because the exoperidium is made of several different layers of tissue; the innermost, fibrous layer is hygroscopic, and curls or uncurls the entire ray as it loses or gains moisture from its surroundings. This adaptation enables the fruit body to disperse spores at times of optimum moisture, and reduce evaporation during dry periods. Further, dry fruit bodies with the rays curled up may be readily blown about by the wind, allowing them to scatter spores from the pore as they roll.

The fruit body is 1–8 cm (0.5–3 in) in diameter from tip to tip when expanded. …

Astraeus hygrometricus is an ectomycorrhizal fungus and grows in association with a broad range of tree species. The mutualistic association between tree roots and the mycelium of the fungus helps the trees extract nutrients (particularly phosphorus) from the earth; in exchange, the fungus receives carbohydrates from photosynthesis. In North America, associations with oak and pine are usual, while in India, it has been noted to grow commonly with chir pine (Pinus roxburghii) and sal (Shorea robusta). The false earthstar is found on the ground in open fields, often scattered or in groups, especially in nutrient-poor, sandy or loamy soils. It has also been reported to grow on rocks, preferring acid substrates like slate and granite, while avoiding substrates rich in lime. In Nepal, fruit bodies have been collected at elevations of 3,000 m (9,800 ft). Fruit bodies typically appear in autumn, although the dry fruit bodies are persistent and may last up to several years. Gelatinipulvinella astraeicola is a leotiaceous fungus with minute, gelatinous, pulvinate (cushion-shaped) apothecia, known to grow only on the inner surface of the rays of dead Astraeus species, including A. hygrometricus. The genus has a cosmopolitan distribution except for arctic, alpine and cold temperate regions; it is common in temperate and tropical regions of the world. …

This earthstar has been used in traditional Chinese medicine as a hemostatic agent; the spore dust is applied externally to stop wound bleeding and reduce chilblains. Two Indian forest tribes, the Baiga and the Bharia of Madhya Pradesh, have been reported to use the fruit bodies medicinally. The spore mass is blended with mustard seed oil and used as a salve against burns. The Blackfoot of North America called the fungus "fallen stars", considering them to be stars fallen to the earth during supernatural events.

Astraeus hygrometricus has been given a number of colloquial names that allude to its hygroscopic behavior, including the "hygrometer earthstar", the "hygroscopic earthstar", the "barometer earthstar", and the "water-measure earthstar". The resemblance to Geastrum species (also known as true earthstars) accounts for the common name "false earthstar". The specific name is derived from the Greek words ὑγρός (hygros) 'wet' and μέτρον (metron) 'measure'. The German Mycological Society selected the species as their "Mushroom of the Year" in 2005.