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- © Britton, N.L., and A. Brown. 1913. An illustrated flora of the northern United States, Canada and the British Possessions. Vol. 3: 533. 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/2012/10/30/06/59899_orig.jpg