Reference

Hindrik J. Huizing, Theo M. Malingré, Th. W. J. Gadella, E. Kliphuis (1983): Chemotaxonomical Investigations of the Symphytum officinale Polyploid Complex and S. asperum (Boraginaceae): Phytosterols and Triterpenoids – Österreichische Botanische Zeitschrift = Plant Systematics and Evolution – 143: 285 - 292.