In 1843 Sullivant traveled with Asa Gray through the Allegheny Mountains collecting mosses. He presented his findings in a bound two-volume folio, Musci Alleghaniensis (1845,1846), containing dried specimens of the mosses he had collected along with accompanying text in Latin for each species. He also authored the sections on mosses and liverworts in Gray's Manual of the Botany of the Northern United States (1848). Sullivant's contribution was later published separately under the title, The Musci and Hepaticae of the United States, east of the Mississippi River (1856).