Concept of the day

Doctrine of signatures

The old belief that a plant’s shape or colour shows which ailment it treats.

Mandrake (Mandragora officinarum): purple flowers and leaf rosette

The concept’s plantMandrakeMandragora officinarumThe legend of uprooting it with a dog already appears in the writings of the historian Josephus in the 1st century AD.Plant page →

The doctrine of signatures is the belief that a plant’s shape, colour or appearance is a sign of which organ or illness it can help. Its roots go back to Pliny and Dioscorides; it was developed above all by the Swiss physician and alchemist Paracelsus and became widespread in the West especially in the 16th and 17th centuries ^[1]. In his “Phytognomonica” of 1588, Giambattista della Porta pictured eyebright, which resembles an eye, and maidenhair fern against hair loss ^[1].

Examples are as many as the plants: bloodroot with its scarlet root was thought to treat the circulation, yellow dandelion jaundice and the liver, and hepatica, whose lobes resemble the liver, liver ailments ^[2]. The root of Mandrake was likened to male genitalia and so held to cure infertility ^[2]. Mandrake is poisonous; it appears here only as a cultural record.

According to the biologist Bradley Bennett there is no evidence that a plant’s form ever led to the discovery of its medicinal properties; “signatures” are explanations added afterwards, not predictions made beforehand ^[3]. Bennett showed that of 80 species chosen at random from plants with heart-shaped leaves, only 3 were used for the heart ^[1]. Still, once taste and smell are included the doctrine has some truth: strongly scented or bitter plants appear often in old pharmacopoeias. In Bennett’s view, the doctrine worked best as a memory aid that made it easier to pass knowledge down the generations ^[3].

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