Dead Man's Fingers
Xylaria polymorpha
Swollen black clubs standing upright from decaying hardwood stumps like blackened fingers, tough and carbon-crusted outside with startling white flesh inside. It is a wood-rotting flask fungus present year-round.
Overview
Dead Man's Fingers is one of the most memorably named fungi in temperate woodland, and the name is unusually literal: a cluster of thick, blunt, blackened clubs pushing up from a rotting stump does look uncomfortably like a hand emerging from the ground.
It is a pyrenomycete, one of the flask fungi, in the family Xylariaceae. Its fruiting body is a stroma - a hard compound structure of fungal tissue - within whose blackened outer rind hundreds of tiny flask-shaped chambers called perithecia are embedded. Each perithecium opens by a minute pore at the surface and slowly releases dark spores, giving the surface a finely pimpled or sandpapered texture visible with a hand lens.
The epithet polymorpha means many-formed, and it is well chosen: clubs may be slender or grossly swollen, simple or lobed, flattened or clubbed, and the species as traditionally circumscribed almost certainly covers several closely related taxa. Its toughness means fruiting bodies persist through the whole year, blackening and hardening rather than rotting away.
How to identify it
Fruiting body
Upright clubs 3-10 cm tall and 1-3 cm thick, simple or occasionally lobed and forked, usually swollen and blunt-tipped though sometimes tapering, and often flattened or irregular where crowded. They arise singly or, more typically, in tight clusters of three to ten from the same point on dead wood, jostling each other into finger-like arrangements.
Surface and colour
Young stromata are pale - whitish, greyish or bluish-white at the tips - and often dusted with a powdery layer of asexual conidia. They darken with age through brown to matt black or charcoal grey, developing a hard, roughened, carbon-like crust. Under a hand lens the mature surface is finely warted or pimpled by the openings of the embedded perithecia.
Interior
The key contrast: cut a club across and the interior flesh is startlingly white, firm, tough and fibrous, sharply bounded by the thin black rind. Just inside the rind lies a ring of tiny dark cavities - the perithecia - visible as a dotted line in cross-section.
Base
The club narrows below into a short, sometimes rooting sterile stalk which attaches into the rotting wood or runs down into buried root material.
Spore print
Black. Spores are dark brown to black, spindle-shaped, smooth and asymmetric, and are released gradually rather than in a single flush.
Where and when it grows
Xylaria polymorpha is a fungus of dead broadleaf wood. It fruits on rotting stumps, at the bases of dead and dying trees, on fallen logs and on large buried roots, and it can appear to come straight out of the soil when the wood it is colonising is underground. Beech is a classic host, along with maple, apple, elm, oak, ash, poplar and other hardwoods; it is uncommon on conifers.
Fruiting bodies can be found in every month of the year because they are tough and persistent. New stromata typically appear in spring as pale clubs, mature and blacken through summer, and continue releasing spores into autumn and winter, with old blackened remains lasting a year or more. The species is common and widespread across temperate Europe, North America and Asia.
Ecological role
This is a wood-decay fungus, mainly saprotrophic on dead hardwood but capable of behaving as a weak parasite in the roots and butts of stressed or wounded living trees. It causes a soft rot of the wood, degrading cellulose and hemicellulose and leaving a soft, pale, crumbly residue, and it is a significant agent in the breakdown of stumps and root systems after a tree has died or been felled.
In wood already colonised, Xylaria mycelium often produces conspicuous black demarcation lines - the zone lines seen in spalted timber, where competing fungal colonies wall themselves off from one another with pigmented barriers. The spore-releasing strategy is unusually slow and steady. Rather than a single flush, perithecia discharge dark spores gradually over months from the hardened stroma, spreading the risk across a long period and a wide range of weather. The species also produces powdery asexual conidia on young pale stromata, a second dispersal route earlier in the cycle.
Similar species
Candlesnuff Fungus (Xylaria hypoxylon) shares the black base and white interior but is far slenderer, flattened and antler-branched, with white powdery tips like a snuffed wick, and it grows on small sticks and stumps. Dead Moll's Fingers (Xylaria longipes) is more slender and cylindrical with a distinctly long, well-defined stalk, and grows characteristically on fallen sycamore and maple branches.
Cramp Balls or King Alfred's Cakes (Daldinia concentrica) belong to the same family but form hard black hemispherical balls on ash and beech, and cutting one reveals concentric silvery-black growth rings rather than white flesh. Earth tongues (Geoglossum species) are superficially similar dark clubs but are much smaller, softer, flattened and rubbery, growing from soil and moss in grassland rather than from wood, and they lack the hard carbon crust and the white interior.