A text accompanying “Rebels Are Reasonable,” Peter Fend’s second solo at Essex Street, describes national flags as “highly abstracted landscape paintings.” On a group of aluminum panels, Fend has superimposed flags onto maps of the countries they represent. These works meld symbolic representations of a place with depictions of its geographic features. For Fend, both categories are unstable and sites of struggle. A subtly ominous video of the ocean is a reminder of how the rising sea is reshaping the land and a nearby group of aluminum panels feature texts that call for activism in a form that is part concrete poetry, part political slogan.