Methodus qua omnes detectos lichenes

Methodus qua omnes detectos lichenes is an 1803 book on lichen classification by the Swedish naturalist Erik Acharius, with a supplementary volume issued shortly thereafter. The book revolutionized lichen classification by abandoning Carl Linnaeus's earlier approach, which grouped all lichens into a single genus called Lichen, and instead established 23 separate genera organized into orders. Acharius based these groupings primarily on the visible shape and position of apothecia (the spore-producing fruiting structures) rather than on features requiring a microscope.

Working in relative isolation in the Swedish town of Vadstena, Acharius built an extensive network of correspondents who sent him specimens from across the world, including material gathered during Archibald Menzies's voyages around the globe. The Methodus ultimately described 536 species. Political tensions between Britain and Sweden during the Napoleonic Wars delayed the book's distribution, but once it reached England, the botanists Dawson Turner and James Edward Smith promoted it enthusiastically, recognizing it as essential for organizing the rapidly growing knowledge of lichen diversity.

Although later scientists criticized Acharius's reliance on external physical features rather than internal or microscopic anatomy, the work established principles that remain fundamental to modern lichen classification. His annotated specimens continue to serve as important reference material for researchers working to understand lichen relationships.