September 26, 2007

In Praise of the Stinky Green Phallus

What's five inches long, shaped like a penis, half-covered with greenish slime, and smells like roadkill soaked in chlorine?

No, it's not Bill O'Reilly!

Give up?

It's Phallus ravenelii!

If you live anywhere east of the Mississippi and have a garden wherein you use woodchips as mulch, you're likely to see one or more of these amazing organisms at some point. I use the word "amazing" here without the slightest hint of sarcasm, because despite the degree to which many people find this fungus offensive to nose and eyes, it is truly a wonder of the biological world.

One of the questions I get most frequently from someone who has found P. ravenelii or other members of its clade is, "Is this stinky, penis-shaped thing hurting my plants?" The answer is an unequivocal no. In fact, if you look carefully at the third photo, you'll see a couple of root-like structures at the base of the fungus that, oddly enough, we mycologists call "rhizomorphs." This word comes from the Greek and means "a thing shaped like a root." Very technical, no? In any case, those rhizomorphs branch into finer and finer filaments in the soil and network with the roots of your plants, which trade off some of the sugar they produce via photosynthesis to gain the advantage of an extended root system. The fungus, you see, is helping the plant to collect phosphorus and nitrogen compounds. So despite the offensive odor and perhaps worrying about your children seeing a big olive-and-white Phallus sticking out of the dirt, stinkhorns like this one are a very good thing to have around, indeed.

Now, why the terrible smell? Thank mutation and natural selection for that one. Yep, good old evolution has been working overtime on this marvelous, chitinous critter. It makes the bad smell to attract flies. The flies land on the slimy spore mass and the spores stick to its feet and abdomen. Now, the fly can't actually eat the fungus, so it flies off still hungry carrying the spores along with it. You can probably guess what happens next; when that fly does finally come upon something to eat, it lands... and the spores fall off. Flies are attracted to materials containing large proportions of soluble nitrogen compounds which, oddly enough, the fungus also likes. The fly has become the vector for the stinkhorn's spores — the only one, in fact. You're looking at a clever specialist, indeed, when you see that P. ravenelii sticking out of your wood chips. It has evolved to take advantage of relationships with two entirely different groups of organisms. Moreover, each stinkhorn species smells a bit different, thus attracting a different selection of flies. Some smell like fecal material and some like decomposing meat and all of their scents are tuned to take advantage of the preferences of a few fly species, and so the stinkhorns spread.

In the first photo, you can see another interesting feature of this organism. It doesn't simply pop out of the ground all at once like most mushrooms do. No, it's far too clever for that. Instead, what it does is form an "egg" that stays underground, waiting for conditions to be just right to maximize the reproductive effort. The egg has a leathery outer membrane that helps protect the fungus from drying out. Inside that membrane is a highly viscous slime that also keeps it well-hydrated. When soil temperature and moisture are in the right range, the egg ruptures and the fruiting body protrudes upward. You can see the remnants of the egg in the third photo, which leave a structure reminiscent of the Amanita's volva around the base of the stinkhorn.

Whenever someone asks what they should do about the stinkhorns in their garden, I always tell them to just let it sit there and do its thing. Yeah, the smell isn't wonderful, but it will only last for a few days. In the meantime, it's mycelium is helping your plants maximize nutrition and, in any case, it really is an amazing sort of living thing. It just so happens not to care one bit about human aesthetics, like most other organisms, for that matter. That smell is what evolutionary success smells like.

I love the smell of Phallus ravenelii in the morning. It smells like victory.

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