The collection consists primarily of the manuscript volumes Observationes Botanicae de Plantis Americae Septentrionalis and Fungi Lancastriensis. Observationes Botanicae de Plantis Americae Septentrionalis previously cataloged in the library, QK110 M95, describes North American plants including documentation for herbarium specimens, Muhlenberg's notes about specimens collected at Philadelphia gardens and on plants cultivated from Meriwether Lewis’s seed collections. The collection also includes typescript copies of correspondence to Stephen Elliott.