Rounded Earthstar

Earthstars begin life as small onion-shaped balls buried in litter. In Geastrum saccatum the outer wall then splits into four to eight pointed rays that fold back and downward, lifting the fungus slightly and exposing a pale, thin-walled inner spore sac in the middle. The result is the classic star on a saucer that gives the group its name.

What sets the Rounded Earthstar apart from several near-identical relatives is the detail around the mouth. The apical peristome is fibrillose — made of converging fibres rather than neat grooves — and, crucially, it is set within a clearly delimited circular area, a slightly sunken pale disc or court, sharply outlined against the rest of the sac.

The sac itself is sessile, sitting directly on the star with no stalk, and the rays often curl until the tips touch the ground, cradling the sac in a shallow bowl. This is one of the most frequently recorded earthstars in the world, common in the tropics and warm temperate zones as well as in northern woodlands.

Star and rays

Expanded specimens are 2–5 cm across. The outer peridium splits into 4–8 (usually 5–7) triangular rays which recurve strongly, often to the point where the fungus rests on their tips. The inner face of the rays is pale buff to pinkish-tan when fresh, drying darker brown and sometimes cracking. The outer face is usually encrusted with soil, litter and mycelial felt.

Spore sac

The spore sac is 1–2 cm across, globose to slightly flattened, pale grey-brown to buff, smooth or very finely mealy, and sits directly on the star with no stalk, collar or apophysis. There is no pinched neck beneath it.

Peristome

The mouth at the top of the sac is conical and fibrillose, formed of fine converging fibres, and is surrounded by a distinctly bounded circular area — a pale, slightly sunken court — whose edge is clearly visible against the surrounding sac surface. This delimited court is the single most useful field character.

Interior and spores

Inside, the young sac is white and firm before ripening into a mass of dry brown spore powder around a slender central columella. Spores puff out through the mouth when the sac is struck by raindrops or brushed. The whole fruit body is tough and can persist as a weathered star for months.

The Rounded Earthstar grows on the woodland floor among leaf litter, needle duff, mossy banks and rotting woody debris, and also appears in gardens, shelterbelts, plantations and shrubbery mulch. It favours humus-rich, well-drained ground and often fruits in small scattered groups.

Fruiting is mainly from summer through autumn, with fresh material appearing after rain, and the dried stars persisting long afterwards. The species is close to cosmopolitan, recorded from North and South America, Europe, Africa, Asia and Australasia, and is particularly abundant in warm-temperate and tropical woodland.

Geastrum saccatum is a saprotroph, decomposing leaf litter, fine roots and buried woody debris in the upper soil layers. Its mycelium threads through the humus horizon, and the fruit bodies develop underground before the outer wall splits and forces them into view.

Spore release is by bellows action: each impact of a raindrop on the thin sac compresses it and drives a small puff of brown spores out through the fibrous mouth. The recurved rays help by holding the sac clear of the litter surface so that puffed spores are caught by air movement.

The Fringed Earthstar (Geastrum fimbriatum) is the closest match, also sessile with a fibrillose mouth, but its peristome is not enclosed in a sharply delimited circular court — the fibres simply merge into the sac surface. The Collared Earthstar (Geastrum triplex) is larger and its thick fleshy inner layer cracks to leave a raised collar or bowl encircling the base of the sac.

The Rosy Earthstar (Geastrum rufescens) is bigger, with rays that flush pinkish-red on the inner surface when fresh. Stalked species such as the Crowned Earthstar (Geastrum coronatum) and Beaked Earthstar (Geastrum pectinatum) are separated at once by the distinct stalk raising the sac above the star, which G. saccatum never has.