Nuttall's sensitive-briar (Mimosa nuttallii), Chickasaw National Recreation Area, 2014.

Before blooming, these tiny flowers look a lot like small green bramble fruits. They occur in bunches as seen in the photo. Like a few other Mimosa species, leaves of this plant fold up when touched or disturbed. The species name nuttallii is in honor of Thomas Nuttall, the famous English Botanist.