Heartfelt essay on a disappearing rural America; Foxfire. Starring Jessica Tandy, Hume Cronyn, Keith Carradine. Play by Susan Cooper and Hume Cronyn, with songs by Jonathan Holtzman. Directed by David Trainer.

Susan Cooper and Hume Cronyn have gone to the heart of southern Appalachia for a heartfelt dramatic essay on a disappearing rural America. ''Foxfire,'' at the Ethel Barrymore Theater, is a gentle and loving as well as staunchly affirmative tribute to the country folk whose sturdiness and character it celebrates. A Playbill preface calls them ''the last of the indomitable pioneers who carved out of unwelcoming mountain soil not only a living but a tough, joyous way of life which has almost vanished now.''

The play's inspiration was the vast folkloric treasury collected by Eliot Wigginton and his high-school students in Rabun County, Ga., for their Foxfire magazine (published in book form by Doubleday). For theatrical purposes, the adaptors have concentrated on the members of one mountain family.

Annie Nations (Jessica Tandy) is a somewhat frail but sturdily self-reliant widow whose late husband, Hector (Mr. Cronyn), is more than a ghostly presence. Hector comes doubly to life, so to speak, in flashback scenes as the action alternates between present and past. In either state of existence, Hector is a mountain man to be reckoned with.

Annie has lived alone for five years since Hector's passing. But even before that, the Nations' children had left Rabun County for the beckoning opportunities of the outside world. Young Dillard Nations (Keith Carradine) has made it as a country singer. Returning to the area for a concert gig, he takes the opportunity to visit Annie, urge her to sell the family farmstead to eager developers, and make her home with him in Florida.

The premise opens the way for Miss Cooper and Mr. Cronyn to create the slight dramatic situation into which they introduce some of the rich Foxfire material. The flashbacks recall Dillard's conflicts with his stern father over everything from the son's frowned-upon musical activities to his challenging with modern agricultural theories the older man's unshakable faith in almanac ''signs.''

But the core of '''Foxfire'' is not so much in its narrative as in the characters and their way of life. Near the beginning of the play, an insistent real estate agent (Trey Wilson) calls just as Annie is preparing a particularly repellent hogshead for eating. In a scene reflecting the way the Foxfire material was collected, a young teacher-to-be (Katherine Cortez) tape-records the Nations' reminiscences. In one of the play's loveliest moments, Miss Tandy recaptures in a few graceful steps the mood of the night of the dance at which Hector proposed.

It is in these moments - tender, touching, richly comic - that ''Foxfire'' makes its strongest bid as a theater work to be treasured. The whole may not, in every sense, equal the sum of its parts. But more than a few of its parts are so lovely and genuinely affecting that their poignancy touches the heart.

Miss Tandy's Annie is a moving figure of serene dignity and maternal compassion - a supple contrast to the rugged, hard-rock strength of Mr. Cronyn's Hector. Mr. Carradine, as the son whose musical success has been accompanied by marital failure, makes an appealing impression as both actor and singer. (The attractive incidental songs and musical direction are by Jonathan Holtzman.) A cast that includes James Greene as a philosophical doctor has been directed with sensitive insights by David Trainer.

David Mitchell's setting, with its verdant photo-mural projection, frames the Nations' small log house against a misty backdrop of Appalachia. Linda Fisher designed the costumes, and the subtle lighting is by Ken Billington.

According to Playbill, foxfire is a lichen that lives on dead fallen trees and ''glows in the dark in the forests of Appalachia.'' At the Barrymore Theater , ''Foxfire'' glows on Broadway.