Tom Volk's Fungus of the Month for April 2000

This month's fungus is Xylaria polymorpha, dead man's fingers.

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Xylaria polymorpha fruiting bodies This month's fungus is an inhabitant of the forest, growing from the bases of rotting stumps and poking up through the ground like a dead man's fingers. I think it's very appropriately, although morbidly, named. "Xylaria" refers to growing on wood and "polymorpha" means "many forms." This species has a very variable fruiting body, sometimes with many separate "fingers" and sometimes with the fingers fused into something more like a hand. This is actually a "species complex," which is what mycologists like to call a group of closely related species that we can't (yet) easily distinguish from one another. There may be as many as 5 to 10 species masquerading under the name Xylaria polymorpha. Incidentally I took this picture last summer in the Missouri Botanical Garden. Xylaria polymorpha fruiting bodies

The location of the fruiting bodies growing on wood leads one to believe this is a wood decay fungus, which in fact it is. (sometimes you can follow your instincts...) However, like most wood decay Ascomycota, it causes neither a brown rot nor a white rot. See this page for a discussion of white and brown rots. Briefly recounted, the main components of wood are cellulose and lignin, with some glucans and other materials acting as glue. Brown rot fungi digest the cellulose and leave the brown lignin behind; white rot fungi (in their simplest form) digest the lignin and leave the white cellulose behind. Neither touches the glucans. The wood-decay Ascomycota cause a soft-rot, digesting the glucan and other glues and leaving the cellulose and lignin behind. Without the glue the structure of the wood collapses and leaves a "soft" rot. The Ascomycota are thus less efficient at removing all the nutrients from the wood. However, they still seem to be pretty successful and are very common in the environment.

Xylaria polymorpha fruiting bodies cross section far and closeIf you start to examine the fruiting body, you might be surprised to find that it is white inside, with black "dots" all around, as seen in the cross section to the left. Looking more closely at this cross section to the right, we can see these dots are actually flask-shaped structures called perithecia. The perithecia have a layer of asci (shown below), which contain ascospores. It's a little difficult to conceive how these ascospores get out of the ascus and through the small hole (ostiole) in the top of the perithecium. It's actually a rather ingenious mechanism. The asci individually elongate (grow) into the ostiole, and individually forcibly discharge their enclosed ascospores to the outside world. It takes quite a while for this to occur with all of the individual asci in a perithecium. Thus the fruiting body disperses its spores over a long period of time, sometimes several months or even years. The fungus is "hedging its bets" by this long term dispersal-- chances are better that sometime during that dispersal period the conditions will be favorable for spore germination and mycelial growth. Most mushrooms that you are familiar with release their spores for only a few hours or a few days and they're done for the year.
Xylaria polymorpha fruiting bodies cross section microXylaria polymorpha is a member of the Ascomycota, tentatively placed in a class called the Pyrenomycetes, all of which form perithecia. This classification system is oversimplified, and not all perithecial fungi are closely related to one another, but this will do for now. In this case the perithecia are embedded in the white stroma, but there are some perithecia in other species that are borne naked on the substrate. In the spring this fungus often produces a layer of white asexual spores, called conidia, all over its surface. This is called the "candlensnuff" stage. Look for it this spring.

There are many other species of Xylaria and many other Pyrenomycetes that grow on wood. You've probably seen Daldinia concentrica (carbon balls), Hypoxylon, and many others. For more technical information on one of these groups see this link at the Field Museum of Natural History in Chicago. It's a site on the Lasiosphaeriaceae put up by Sabine Huhndorf, who was my office mate for a year when I was at the Forest Products Lab in Madison. I recommend a visit.

I hope you enjoyed this month's fungus. Be on the lookout for them next time you're in the woods. They're actually quite common, but because of their black color they are often difficult to see. Most importantly, I hope you find these fungi rather than real dead man's fingers...

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