P.aquaticus (or amphibium?) (Polygonom) -- I found this plant on a Labor day weekend trip to Michigan's upper Penninsula two years ago. Unfortunately, as a consequence I can't go back and check for particulars to further identify it. Fortunately there are several people who wrote to me to help identify it as "smartweed", probably "water smartweed". It is a member of the buckwheat family. (Photographed in MI)
Thanks to Susan J. Meades, Jean Krieg, Angella Moorehouse, and Cathie Mann for identifying it for me!