This Callixylon newberryi plant fossil is of the late Devonian period. Fossils like this are considered some of the oldest petrified woods in the world. This piece was found in Bullitt County, Kentucky in the New Albany Shale.
See this post for pictures of larger section of fossil wood on display at the Indiana State Museum. Also this post about a really large fossil trunk on display at the Smithsonian National Museum of Natural History. The Kentucky Paleontological Society has a very nicely prepared and photographed magnified sample seen here.
Below are pictures of the fossil shown at the beginning of this post. It is a magnified view of a cut and polished section of one end.
This last slide shows a 100x magnification of the long grain view of the wood.