#005: Xylaria polymorpha, Dead Man’s Fingers

In late October, it is not unusual to see Halloween decorations featuring bony fingers reaching up out of a grave. The natural world has its own version of this: Xylaria polymorpha, commonly known as Dead Man’s Fingers. This macabre name is an apt description of the mushrooms, which resemble burnt and dried out fingers reaching out of the ground to grab unwitting passers-by and drag them down into the depths of the earth. You can find these morbid ascomycetes on rotting logs at pretty much any time of the year.^1–4

Description

X. polymorpha is unusual among mushrooms because it develops slowly over the course of a few months. When the mushrooms first appear, they are light bluish to purplish to grayish with white tips and are short and round and usually grow in clusters.^1–3 During this stage, X. polymorpha produces asexual spores, which accumulate as a whitish film over the mushroom.^2,4 Because of this white film, this phase is called the “candlesnuff” phase.⁴

The mushrooms soon darken, becoming brownish to blackish with a white tip and finally entirely black. When fully mature, Dead Man’s Fingers appear more or less like a rounded cylinder or sausage and grow 3.5-10cm tall and 1-2.5cm or even 4cm thick. They generally stand straight up, though often become bent or irregularly warped. Their exterior is black wrinkled and bumpy and becomes cracked with age. The individual mushrooms often grow close together and may become fused at the bottom, forming a “hand” with several “fingers.”^1–3 The cluster usually has a substantial rooting base that makes it difficult to remove the mushroom from its substrate.

If you’re not convinced that these are mushrooms and not discarded human fingers, cut one open (this may be a little difficult, since the flesh is tough to hard).^1–3 You will probably be surprised to find that they are white on the inside and have small, black dots around the edges. These dots are flask-shaped structures called “perithecia” and are each lined with spore-producing cells called asci. These asci slowly elongate, growing toward a small hole (called an ostiole) in the perithecium which connects to the air outside (each bump on the mushroom’s surface is centered around an ostiole). Once the asci reach the ostiole, they shoot their spores out into the air. X. polymorpha releases its spores periodically over the course of a few months. This is very slow compared to most mushrooms, which discharge all of their spores within a few hours or days.^2–4 If you are lucky enough to collect X. polymorpha while it is actively firing off spores, the bag you put it in will end up covered in black polka-dots. Each polka-dot is a spore print deposited by a single ostiole.

Ecology

Dead Man’s Fingers begin growing in the spring, switch to producing sexual spores in the summer, and begin to decompose in the fall or winter, sometimes lasting well into the next year.^1,2,4 X. polymorpha fruits directly from or just next to decomposing hardwood in North America and Europe^1–3 (and perhaps elsewhere). The fungus causes a “soft rot” in wood, meaning it leaves the lignin and cellulose and instead decays the glucans and other molecules that hold the lignin and cellulose together.^2,4

Similar Species

There are actually three other species of Xylaria that can be mistaken for X. polymorpha. In theory, the easiest one of these to identify is X. hypoxylon, which is much thinner than X. polymorpha.⁵ The fungus X. longiana is identical to X. hypoxylon except that X. longiana has smaller spores.⁶ X. longipes is also generally thinner than X. polymorpha and usually features a distinct stipe.² However, all of these mushrooms are highly variable and cannot be reliably distinguished from one another without microscopic analysis. If you don’t have access to a microscope, you’ll usually have to settle for calling all the thick Xylaria species X. polymorpha and all the thin ones X. hypoxylon.⁵

To add to the confusion, all four of those Xylaria species are morphologically identical during their asexual stage. Until they grow larger and begin producing sexual spores, it is impossible to tell the species apart – even with a microscope.⁵

Some mushrooms in the family Cordycipitaceae are nearly indistinguishable from Dead Man’s Fingers. These mushrooms also produce fruiting bodies with pimply heads and perithecia. Many of them are black and can grow from wood (see FFF#197 for an example of a morphologically similar species that grows from the ground). However, all Cordycipitaceae are parasites, so careful excavation will reveal that these mushrooms are actually attached to the mummified remains of their host (usually an insect). If you fail to retrieve the host – a likely scenario if, for example, the mushroom is attached to a beetle deep within a fallen log – then microscopic analysis should help you distinguish between the two groups.

Earth tongues are another group of mushrooms that appear similar to Dead Man’s Fingers. Fortunately, earth tongues do not form perithecia and are therefore smooth on the outside. This makes them very easy to separate from X. polymorpha.

Edibility

Dead Man’s Fingers are usually considered inedible, which is not surprising given their macabre appearance.³ However, the mushrooms may be edible when very young and still tender (see this website). At that stage, they apparently taste mushroomy and do not cause poisoning symptoms when eaten raw in small amounts. Very few people have tried this mushroom, so it is not known whether it causes ill effects when eaten over a long period of time or how often someone has a bad reaction to it (even good edible mushrooms are not edible for everyone).⁷ Given how little is known about the mushroom’s edibility, I don’t recommend eating it – even though it would be fun to tell people, “I ate some Dead Man’s Fingers today!” If you decide to throw caution to the wind, remember to report any adverse reactions to the North American Mycological Association or another relevant organization to help others learn about this mushroom’s edibility.

Taxonomy

As I mentioned above, there are a few other species of Xylaria that can pass for X. polymorpha at some point during their life cycle. Since these fungi are all so similar, mycologists are not exactly sure how many species are in this group; some estimate that it includes as many as ten distinct species.⁴ At present, amateurs equipped with a microscope can identify only four species.⁵ Using DNA and microscopic analysis, mycologists may add to that number in the future. However, that still won’t help the casual observer because without a microscope you will still be forced to lump all the skinny species into X. hypoxylon and all the thick species into X. polymorpha.

Kingdom	Fungi
Division (Phylum)	Ascomycota
Subdivision (Subphylum)	Pezizomycotina
Class	Sordariomycetes
Subclass	Xylariomycetidae
Order	Xylariales
Family	Xylariaceae
Genus	Xylaria
Species	Xylaria polymorpha (Pers.) Grev.8