Exidia repanda

Exidia repanda is a species of fungus in the family Auriculariaceae. In the UK, it has the recommended English name of birch jelly. Basidiocarps (fruit bodies) are gelatinous, orange-brown, and button-shaped. It typically grows on dead attached twigs and branches of birch (Betula species) and has been recorded from Europe, North America, and Japan.

Taxonomy
The species was originally described from Sweden in 1822 by mycologist Elias Magnus Fries. Molecular research, based on cladistic analysis of DNA sequences, has shown that Exidia repanda is part of a complex of species including E. recisa and E. crenata.

Description
Exidia repanda forms reddish brown, gelatinous fruit bodies that are firm and button-shaped, around 3 cm across. The fruit bodies typically grow gregariously, but do not normally coalesce. The upper, spore-bearing surface is smooth and shiny, whilst the undersurface is smooth and matt. Fruit bodies are attached to the wood at a point, but do not have a stem. The spore print is white.

Microscopic characters
The microscopic characters are typical of the genus Exidia. The basidia are ellipsoid, septate, 12.5 to 15 by 7.5 to 10 μm. The spores are allantoid (sausage-shaped), 12 to 14 by 2.5 to 3.5 μm.

Similar species
In Europe, fruit bodies of E. recisa are similarly coloured and microscopically indistinguishable. The fruit bodies are conical and pendulous, however, and the species typically occurs on willow. Fruit bodies of E. umbrinella are also similar, but the species only occurs on conifers and is uncommon. Exidia brunneola is also uncommon and occurs on poplar. In North America, E. crenata is similar, but has conical to pendulous fruit bodies and typically occurs on oak. The widespread E. glandulosa has much darker, blackish brown fruit bodies with sparse warts or small, peg-like projections on their surface.

Habitat and distribution
Exidia repanda is a wood-rotting species, typically found on dead attached twigs and branches of birch. It is widely distributed in Europe and has also been reported from North America and Japan.