The acorn was chosen as a symbol for the 14th Corps, Thomas's. The story goes that after the arrival of Hooker's troops to the Army of the Cumberland at Chattanooga that it was decided that the AoC would adopt the symbols as well, so when Thomas was asked what the symbol for the 14th Corps would be, he said he wanted an Acorn, since the 14th Corps had stood like an oak at Chickamauga. The Acorns are on most of the monuments along battleline road, not all are Ohio, you have Wisconsin and Indiana, and Illinois monuments with them. Hope this helps.

Lee