Considering that it wants to celebrate the craggy virtues of a strong-willed mountain couple that has lived out the years on the "tip top of Georgia," the new Broadway-bound drama titled "Foxfire" is a fairly amorphous affair. Maybe they should call it "Will o' the Wisp," instead.

At the Morris A. Mechanic Theatre through Oct. 9, it stars Jessica Tandy and Hume Cronyn, the American theater's foremost husband and wife team, as a pair of icons to the Indomitable Pioneer Spirit. Working alongside novelist Susan Cooper, Cronyn also put his hand to the script, and while they all have managed some lovely little moments, there are simply no big ones. "Foxfire" keeps hinting at the drama of rural life on the wane, but in the end it settles for elegy.

Like the phosphorescent moss from which it borrows its title, "Foxfire" has its picturesque qualities, and perhaps never so much as when the mists rise up out of the valley, transforming David Mitchell's set -- ramshackle farm buildings profiled against an amphitheater of blue-ridged mountains -- into a backwoods Brigadoon. But if you strip away the layers of moss, you don't come upon a rock -- only a pebble.

Tandy is cast as spunky Annie Nations, a paragon of self-sufficiency, who at 79 has grown frail in the eyes of her son, but certainly not her own. To the extent that the evening has an ongoing concern, it is hers: Whether or not to leave the rocky acreage that has defined her life. Her obdurate husband, Hector, has been dead for five years, but she can't get him out of her mind -- doesn't want to, really. And so there he is, in the crusty, mud-caked, fly-haired person of Hume Cronyn, tossing his two bits into the conversation, quoting scripture, and generally refusing to stay put in his burial plot in the apple orchard.

Those of the Nations' five children who survived infancy have since gone down off the mountain to make their way in the 20th century. One of them (Keith Carradine) has become a successful country singer, fostered a family of his own and lost his wife to another man. At the start of the play, he's hiked back up the mountain to ask Annie -- "Aunt Annie" to anyone within a 10-mile radius -- to come live with him in Florida. Can she pull up her roots? Lay to rest old memories? Trade a proud tradition for the equivocations of modern-day society? As she ponders her decision, the script harks back in time to show us some of the archetypal events -- her courtship, the birth of her son, the death of her husband -- that have "glued" her to the land.

The authors have drawn much of their backwoods lore, if not their story, from the celebrated Foxfire books, those popular compendiums of the vanishing Appalachian culture. The play is studded with courtin' customs, plantin' superstitions, and the humble rituals marking a body's entrance into the world and its departure. There's even a culinary exhibit of sorts, as Tandy carves what appears to be a real boar's head into souse.

The details are all very authentic, perhaps, but "Foxfire" rarely amounts to more than a series of Samplers, passing exhibits in one of the Smithsonian's Folk Life Festivals. Old habits are giving way to new, granted. But the real drama in "Foxfire," I suspect, lies with the persistence of memories and the painful efforts we have to make sometimes to wrench ourselves from their grasp.

Resuscitating Annie's husband, so to speak, is a nice start, even if Annie alone can see and hear him. But he hovers mostly on the fringes. (At one point, when the talk about him is less than flattering, he even sputters, "I don't have to listen to this," and bolts from the stage.) The authors keep backing away from the very confrontation the play promises -- Annie, facing down her lifelong mate once and for all and moving on, as we all must move on; Hector, fighting to keep himself alive and vivid in her mind.

As it stands, Hector provides an accent of charming cantankerousness -- and Cronyn neatly balances the charm and the bile -- but not a great deal more. The evening is notable primarily for Tandy's performance, extraordinary in its range and nuance. She is dressed in a discreetly polka-dotted black dress and a few stray wisps of her blindlingly white hair dance delicately about her face, lending an air of poignant vulnerability to her stern features. The character refuses to be patronized, however. Life has taught her, in fact, that it's best to keep one's emotions to oneself. Tandy's firm, dignified presence will surely win your sympathy, although never once does she stoop to ask for it.

One flashback calls for the actress to depict Annie as a shy young maid. Hector tells her she's "awful pretty," she lets out a whoop of delight, and then, embarrassed, tries to muffle it with her hands. The tones of girlish laughter and mortification, of joy and sheer nervousness that make up Tandy's reaction intertwine like fine threads in a tapestry.

Carradine, who sings a half dozen or so songs in the course of the play, is more appealing right now as singer than actor. And the supporting players -- Katherine Cortez, James Greene and Trey Wilson -- are largely limited by the marginal functions they play in the saga, as teacher, doctor and a crass real estate developer with bright yellow socks.

Director David Trainer, who imparted such a sense of unity to "The Dining Room," another recent play about a culture on the wane, has as yet been unable to give "Foxfire" the backbone it needs. The fault is not entirely his. Some structural rework is definitely called for, if this is ever to be a full-blooded play. For the time being, it is a merely a loose-leaf scrapbook of recollections, anecdotes, and vanishing customs. The drama in those pages remains to be distilled.

FOXFIRE, by Hume Cronyn and Susan Cooper. Directed by David Trainer; music by Jonathan Holtzman; sets, David Mitchell; lighting, Ken Billington; costumes Linda Fisher. With Hume Cronyn, Jessica Tandy, Keith Carradine, Katherine Cortez, James Greene, Trey Wilson. At the Morris Mechanic in Baltimore through Oct. 9.