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- © Britton, N.L., and A. Brown. 1913. An illustrated flora of the northern United States, Canada and the British Possessions. 3 vols. Charles Scribner's Sons, New York. Vol. 2: 265. Courtesy of Kentucky Native Plant Society. Scanned by Omnitek Inc.
- Copyright licence: Marked as being in the public domain
- Provenance: http://media.eol.org/content/2015/03/20/13/50051_orig.jpg