Arched Earthstar

The Arched Earthstar performs the most striking piece of architecture in the genus. Its outer wall separates into two layers: an outer mycelial layer that stays behind in the litter as a shallow cup, and an inner fibrous layer that splits into rays. Those rays remain attached to the cup at their tips, so as they flex downward they lever the whole fungus upward, leaving the spore sac vaulted several centimetres above the soil on what look like four bowed legs.

This fornicate habit — the technical term for rays that arch from a basal cup — is shared with only a handful of Geastrum species, and G. fornicatum is the largest of them. Whole specimens can stand 5–8 cm tall, dark brown and gaunt, and being tough and weather-resistant they often survive well into winter, sometimes still standing when the following season's fruit bodies appear.

It is a fungus of old, undisturbed, humus-rich ground and is classically associated with yew, though it turns up under many broadleaf trees and in shady hedge banks.

Star and legs

Expanded specimens are 5–9 cm tall and 3–7 cm across the arch. The inner fibrous layer splits into typically four (sometimes three to five) stout rays which curve downward and outward, their pointed tips fused to the persistent outer mycelial layer, which remains in the litter as a ragged, soil-encrusted cup. The rays are dark brown to blackish-brown, matt, and stiff.

Spore sac

The sac is 1–2.5 cm across, globose to slightly egg-shaped, dark grey-brown to dull brown, minutely roughened or granular, and is carried on a short but distinct stalk arising from the crotch where the rays meet. A small collar-like apophysis is often visible at the top of the stalk.

Peristome

The apical mouth is conical and fibrillose, composed of fine converging silky fibres, and is not sharply delimited by a groove or ring, nor is it furrowed. This distinguishes it from the grooved-mouthed earthstars.

Interior

Within the sac the mature spore mass is dark brown and powdery, surrounding a slender columella. Fresh, unopened fruit bodies are onion-shaped, buried, and covered in a felt of mycelium bound with soil and litter fragments.

The Arched Earthstar grows in deep, dry, humus-rich litter in sheltered places: under old yew, in shaded hedge banks, in mature broadleaf woodland, churchyards, parkland and shrubberies. It seems to favour ground that has been undisturbed for a long time and is often found under dense evergreen cover where the litter layer is thick and rain-shadowed.

Fruiting is chiefly in autumn, but the woody remains persist through winter and are frequently recorded then. The species is widespread but distinctly local in Europe, and also occurs in North America, North Africa and parts of Asia, generally in warm-temperate lowland situations.

Geastrum fornicatum is saprotrophic, breaking down accumulated leaf litter, humus and fine woody debris. Its preference for deep undisturbed litter under evergreens reflects the slow, stable decay conditions it needs to build up a substantial mycelium before fruiting.

The fornicate structure has an obvious dispersal function: by lifting the sac several centimetres above the litter, the fungus places its mouth in moving air, and each raindrop strike puffs a cloud of dark spores into the breeze rather than into the surrounding litter. The tough, lignified rays let a single fruit body keep releasing spores across many months.

The Rayed Earthstar (Geastrum quadrifidum) is the closest structural match, also arching on four rays from a basal cup, but it is markedly smaller — usually under 4 cm tall — with a paler, whitish-encrusted cup and a peristome that is clearly delimited by a pale ring or court around the mouth.

All the non-fornicate earthstars are separated by having rays that curl back but stay in one piece with the base, so there is no basal cup and no arch. Among these, the Crowned Earthstar (Geastrum coronatum) has a similar dark stalked sac but sits flat on recurved rays, while the Beaked Earthstar (Geastrum pectinatum) also has a long stalk but shows a conspicuously grooved, beaked mouth instead of a silky fibrillose one.