Folk drama of English farm life; Fen. Play by Caryl Churchill. Directed by Les Waters.

As the audience assembles in Public/LuEsther Hall, the misty stage is occupied by a raggedly clad, barefoot boy who periodically shakes a clapper. The clapper is to scare crows away from the surrounding potato field. When the lights go up, the field is briefly occupied by a gentleman from Japan who extols the investment possibilities and beauties of ''the Engrish countryside.'' Thus does Caryl Churchill span a long-term evolution in the development of British agriculture. And thus does her folk play, ''Fen,'' begin.

The Churchill overview encompasses several generations in the history of the Fens (a rural area less than 100 miles north of London). The author interweaves accounts of the often drab and sometimes tragic experiences of typical country people with larger socioeconomic unfoldments as seen from a particular viewpoint. She alludes to such matters as the coming of unions, the mechanization of agriculture, and the transfer of farmland from individual ownership to corporate control. The result is a fascinating mosaic in theatrical terms.

The play's tragic personal theme concerns a landworker named Val (Jennie Stoller) who leaves her family for an illicit affair with Frank (Bernard Strother), a local farm laborer. Miss Churchill relates the effects of the unhappy woman's decision on her two children (Amelda Brown and Linda Bassett), on Frank himself, and on the immediate community. Since members of London's Joint Stock Theatre Group, which produced ''Fen,'' spent time living in a fen village and talking with its people, their play is also a kind of anthropological study on a person-to-person level.

With the potato field and its soft earth as a visual constant, the play moves forward in a succession of vignettes: domestic scenes, a prayer meeting, a gleaner's wretched plight, a great-grandmother's tales, a pub encounter, a spectral aftermath. At one point all gossip and chitchat is drowned out by the momentary screech of overhead supersonic military jets.

The compelling power of ''Fen'' is due equally to the authenticity of its material, the skill with which the elements have been drawn together by Miss Churchill, and the performance of a remarkable troupe. Each member of the versatile company plays at least two parts, and, in most cases, as many as four.

Designer Annie Smart's many costumes are a great help pictorially. But the vitality and validity of the portrayals spring from a common denominator of shared understanding, caring observation, and perceptive insights.

In addition to those already mentioned, the fine Joint Stock Theatre Group cast includes Cecily Hobbs and Tricia Kelly. The performance directed by Les Waters (which ends Saturday) springs naturally from the topsoil with which Miss Smart has covered the stage of LuEsther Hall and which, when the mist rises, comes into clear perspective under Tom Donnellan's lighting.