Reference

Hindrik J. Huizing, Th. W. J. Gadella, E. Kliphuis (1982): Chemotaxonomical Investigations of the Symphytum officinale Polyploid Complex and S. asperum (Boraginaceae): The Pyrrolizidine Alkaloids – Österreichische Botanische Zeitschrift = Plant Systematics and Evolution – 140: 279 - 292.