The art of dance, far from being immune to the "roots" syndrome, is deep in its throes. On every side, lately we have been seeing reconstructions and reinterpretation of a hitherto "lost" heritage.

Martha Graham, after years of steadfast refusal to look back, is suddenly hastening to retrieve the masterworks of her pioneers days, and will be appearing at Wolf Trap this week with some of the latest results of her quest.

Annabelle Gamson was in town recently displaying her striking versions of the solo dances of Isadora Duncan. Within the past five years, no less than three celebrated choreographers - the late Jose Limon, Sir Frederick Ashton and Maurice Bejart - have paid tribute to Isadora in works of their own.

In the '60s, dancers, like other artists, shut their eye to the past In the '70s, they are embracing it with a vengeance.

In this light, nothing could be more timely than the hour-long telecast tonight (at 9 on Channels 26 and 22) of "Trailblazers of Modern Dance." The last segment in the second season of public television's "Dance in America" series, the program surveys the evolution of modern dance in the United States from the turn of the century through the early 1930s, using still photos, graphics, old film clips and videotaped contemporary performances as illustrative material.

Among this material is an extraordinary new discovery - brief footage of a woman dancing at a garden party, believed to be the only extant film of Isadora Duncan.

The progress may well be the most impressive and valuable contribution the "Dance in America" series has yet turner out. It would be treasurable alone for the illuminating glimpses it provides into the sources and motivations of American dance. But it is also an esthetic event in its own right - not just instructive, but beautiful.

In seeking its ancestry, dance is hampered to a degree unknown in the other arts by its own transitory nature. Painting, sculpture and literature, including dramatic literature, persist in relatively durable media; motion pictures, though perishable, can be preserved and many have been; music has phonograph recordings over a 100 years' aspan, and at least written scores before that. Dance alone has evaded most past attempts to save it for posterity, and the largest part of its legacy has evaporated.

All the more amazing, then, that "Trailblazers" has succeeded in presenting a vivid, multifaceted portrait of modern dance origins. There are some disturbing ommissions in the narrative - Mary Wigman and Hanya Holm, for instance, who had a vast influence on the American scene both as models and teachers; Lester Horton, the West Coast pioneer who initiated he idea of a racially integrated dance troupe, among other things; and Louis Horst, resident musician for Denishawn and Graham and godfather to a generation of choreographers.

On the whole, however, given the length of the program and the paucity artifacts, the production seems a triumph of scholarship and imagination. It begins at the logical beginning, with Isadora - Rosemary Harris intones Duncan's visionary words as we see a haunting still of the dancer's face - "I have discovered the art which has been lost for 2,000 years." Film clips show us the dance milieu Isadora rebelled against - a vaudeville "artiste" balancing a chair in her mouth, a "ballerina" in a strip-tease routine.

Other films, stills, posters and drawings trace her revolutionary career; Gamson, in her own solos, and Lynn Seymour, in Ashton's enrich our image of what Isadora's presence must have meant.

The historical path leads onward to Ruth St. Denis and Ted Shawn and the dance progeny they spawned - Graham, Doris Hurmphrey and Charles Weidman, with a sidelong glance at Helen Tamiris. Through it all, the passion and prophetic spirit of these "trailblazers" beat like a quickened pulse.