Doctrine of Signatures, the concept that the key to humanity's use of various plants was indicated by the form of the plant. The red sap of the bloodroot ( Sanguinaria canadensis ), for instance, was believed to cure diseases of the blood, while the fused leaves of boneset ( Eupatorium perfoliatum ) were used to heal broken bones. The concept was employed by the herbalists of the Renaissance, and was accepted until the latter part of the 19th cent.
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