“Grant Wood and the American Farm,” on view through Saturday, was organized around “Spring Turning,” Wood’s only painting owned by Reynolda House — a bird’s-eye-view of a cultivated farm field in midwestern hill country, painted in 1936. Another inspiration for the show is the Reynolda estate’s original function as a state-of-the-art model farm. The overarching focus is the central place of agrarian life in this country from 1850 through the mid-20th century.