During her time as an art student in New York City, Mary Keithan never imagined that one day she would drive the state of Michigan's labyrinthine back roads in search of architectural subjects for her photographs. But, in 1990, shortly after acquiring an 8" x 10" view camera, she began just such an odyssey. In the process she captured on film and preserved images of the rural landscape's most endangered visual treasures, its aging, historic barns.       Many of her "subjects" are in poor condition; many no longer stand. And while we might expect a photographic series on aging barns to be a sad chronicle of America's rural decline, instead Keithan gives us a visual story of endurance and perseverance, of a way of life that in our modern times continues to thrive.       What Keithan has captured with her camera and presented in Michigan's Heritage Barns is enriched with her own narrative, often including interesting histories from the barn owners themselves. Photographs from most of Michigan's eighty counties are included to create a collection that celebrates Michigan's rural heritage as no other does. To use Theodore Roethke's words, it is a visual hymn to "nobility of soul at odds with circumstance."