Seedbox, a native species, has been added to the USWildflowers database (08/11/2011.) Scientific name is Ludwigia alternifolia. Photo below was taken in Walker County, GA on July 22, 2011. Go to the Seedbox detail page for more photos and information.
Seedbox, Smooth Seedbox, Bushy Seedbox, Rattle-box, Square-pod Water-primrose – Ludwigia alternifolia
I appreciate your excellent photos and information. I also have numerous photos, particularly on Flickr.