Myriostoma is a fungal genus in the family Geastraceae. Basidiocarps resemble earthstars, but the spore sac is supported by multiple columns (instead of a single column) and has multiple ostioles instead of a single, apical ostiole. Until 2017, the genus was thought to be monotypic with a single, widespread species, Myriostoma coliforme. Recent research has, however, shown that at least six species occur worldwide.
Nicaise Auguste Desvaux first defined and published the genus Myriostoma in 1809, with the single species Myriostoma anglicum (an illegitimate renaming of James Dickson's original Lycoperdon coliforme).[1] In 1821 Samuel Frederick Gray described the superfluous genus Polystoma for it.[2]
Myriostoma was classified in the family Geastraceae until 1973, when British mycologist Donald Dring placed it in the Astraeaceae[nb 1] based on the presence of trabeculae (stout columns that extend from the peridium to the central core of the fruit body) in the gleba, and the absence of a true hymenium.[4] In his 1989 monograph, Stellan Sunhede returned it to the Geastraceae.[5] Molecular analysis of DNA sequences has confirmed the traditional belief that Myriostoma and Geastrum are closely related.[6][7]
Recent molecular research, based on cladistic analysis of DNA sequences, has shown that the genus, previously thought to be monotypic, comprises at least six species worldwide.[8][9][10]
The generic name is from the Greek words μυρίος, meaning "countless" and στόμα, meaning "mouth" (the source of the technical term stoma).[11][12]
The fruit bodies start their development underground or buried in leaf debris, linked to a strand of mycelium at the base. As they mature, the exoperidium (the outer tissue layer of the peridium) splits open into 7 to 14 rays which curve backward; this pushes the fruit body above the substrate. Fully opened specimens can reach dimensions of 2–12 cm (0.8–4.7 in) from ray tip to tip. The rays are of unequal size, with tips that often roll back inward. They comprise three distinct layers of tissue. The inner pseudoparenchymatous layer (so named for the resemblance to the tightly packed cells of plant parenchyma) is fleshy and thick when fresh, and initially pale beige but darkening to yellow or brown as it matures, often cracking and peeling off in the process. The exterior mycelial layer, often matted with fine leaf debris or dirt, usually cracks to reveal a middle fibrous layer, which is made of densely packed hyphae. The base of the fruit body is concave to vaulted in shape, and often covered with adhering dirt. The roughly spherical spore sac (endoperidium) is supported by a cluster of short columns shaped like flattened spheres. It is grey-brown and often minutely roughened with small warts. There are several to many evenly dispersed mouths, the ostioles, mainly on the upper half of the endoperidium. They are roughly circular with fimbriate edges.[13][14]