2 literature references:

Author: Hegi, G.
Year: 1981
Title: Illustrierte Flora von Mittel-europa, Band 1, Teil 2 [third edition]
Source: Verlag Paul Parey, Berlin - Hamburg
Author: Wrobel, M. & Creber, G.
Year: 1996
Title: Elsevier's dictionary of plant names
Source: Elsevier, Amsterdam