Black snakeroot stem

This is the characteristic purple stem of black snakeroot, Actaea racemosa, a shade-tolerant, moisture-loving woodland perennial native to the eastern and midwestern states, also called black cohosh. Branches split off from the main stem in a lily-like hooded fashion. Note that the foliage in the background is not black snakeroot, but non-native euonymous.
 
For more about black snakeroot and native plants for the Midwest, especially those that do well in shade/moist areas, see the "book" on this website, Gardening with Native Plants in Shady Evanston.