BY LLOYD GROVE

Washington Post Staff Writer

David Farr, a mycologist by trade, which means he studies fungi, cracked a small joke:

"I always like to come here for mushrooms," he said as he crouched over a rotting log in Glover-Archibald Park near Georgetown University Hospital. "That way, if you make a mistake, you can always get quick treatment."

Farr, who works in the Department of Agriculture's research laboratory in Beltsville, ventured to Washington the other day for a wild-mushroom hunt. Along for the expedition was Securities and Exchange Commissioner Bevis Longstreth, an amateur who collects for both culinary and philosophical reasons.

"There's a nice intellectual exercise involved in identifying the different classifications," said Longstreth, a lawyer who started keeping his nose to the ground a few years ago at the behest of friends. "What's also nice about this activity is that you don't feel like you're in a field of millions, shouldering your way toward the last remaining mushroom on earth."

He's fond of quoting the late Charles McIlvane, whose almanac, "One Thousand American Fungi," is still the bible of mushroom-hunters after nearly 80 years in print. Longstreth opens to McIlvane's introduction and reads aloud, "The question often asked is: By what rule do you distinguish between edible and poisonous mushrooms? The answer usually surprises the questioner -- there is no general rule. All such rules which have been given are false and unreliable." Until the first frost, about 20 species of edible mushrooms grow wild throughout this area, theoretically there for the picking. The trick -- and it's quite a trick -- is to know just which species will please the palate and which will poison the body. No wonder wild mushrooms -- only 5 percent of which are said to be poisonous -- have long been associated with witches and magic, and still conjure strange visions in a few Indian religions. "There's a lot we still don't know about mushrooms," Farr said. "For a long time no progress was made in studying them, just because they seemed so mysterious and unnatural that people shied away." Consider the following narrative from McIlvane, a giant of mushroom classification who often sampled the forbidden fruits himself in the name of science: "The symptons of poisoning (after eating Amanita muscaria) . . .begin with cramp- like pains in the extremities, colicky pains in the abdomen, burning thirst, vomiting and purging. The pulse may be very slow and strong at first, but later becomes rapid, small and feeble. . . Extreme pallor is often noticed. . . The respirations are slow and become shallow. . . The mental state may be clear at first, but becomes dull, deepening into unconsciousness and deep coma if a large quantity has been taken. Convulsions are reported to have occurred in some cases. . ." "There's a very small margin for error," Farr said. Still, Longstreth and Farr spend much of their time outdoors braving poison ivy in search of such delectable varieties as Armillaria mellea, commonly known as the "honey mushroom" despite a soapy aftertaste when eaten raw; Polyporus frondosus, the "chicken of the woods" ("I don't know if that's because it tastes like chicken or looks like a chicken," said Farr); and strobilaceus, a black-looking fungus known as the "old man of the woods." Longstreth likes to invoke the words of an imperious Boston Brahmin he once knew who'd take his daughter into the wild, spy "the old man," and exult, "Goodness Gracious! Strobilaceus!" before plucking it out of the earth. "What often happens," Longstreth said, "is that a love for mushroom- hunting is passed down from parents to their children." In the park the other day, the hunters kept mostly to the footpaths and came upon a motherlode of honey mushrooms as well as a variety called "puff balls," distinguished by roundness, simple structure and a penchant for growing to extraordinary size -- sometimes measured in feet, Farr said. While they missed out on Polyporus sulphureus, one of Longstreth's favorites (all the available samples had gone leathery with age), they did find -- and stowed along with the others in a wicker basket -- a fungal parasite known as Entoloma abortivus, a haphazard clump in which one mushroom devours another. Farr guessed that it, along with the other fungi, had sprouted in the previous few days, owing to recent rains. Farr, who teaches mushroom identification at the Department of Agriculture Graduate School, approached the collecting clinically, wrapping samples he thought interesting in wax paper for later study. But Longstreth, wielding a Swiss army knife to free his quarry from stumps and soil, took to shouting and gasping in awe at every new discovery. "That's magnificent," he said when Farr found the abortivus -- the meat firm, white and tender. "I've eaten some like this, and they have been just spectacular." Indeed, sauteed in butter and garlic in Longstreth's kitchen after a couple of hours' foraging, the afternoon's haul seemed just the thing for a hungry hunter. Mushrooms make for a meal high in protein and low in carbohydrates, though not terribly nutritious by themselves over the long run, Farr said. He added yet another mycologist's jest: "You probably feel perfectly fine now," he said, munching, "but sometimes the symptoms don't crop up for eight to twelve hours after you eat."

STALKING THE WILD MUSHROOM

Folks with a mushrooming interest should send a stamped, self-addressed envelope to Louise Priest, Mycological Association of Washington, 9765 Bragg Lane, Manassas, Virginia 22110, for information on the doings of fungi enthusiasts. The group holds monthly meetings, the next one at 7:30 the night of December 1, at the Chevy Chase Public Library at 8005 Connecticut Avenue NW.