His earliest major work was on the moss flora of the
Kola Peninsula (Brotherus and T. Saelan. 1890. Musci Lapponiae Kolaensis.
Acta Societas pro Flora et Flora Fennica 6: 1-100.) His other major European work was
Die Laubmoose Fennoskandias (1923). He also studied, through collections sent to him by botanists abroad, the mosses of
Turkmenistan,
Africa,
Australia,
Brazil, and
New Guinea, among others, and was known as an authority on extra-European mosses. His work on the Musci of
Die Natürlichen Pflanzenfamilien covered
Archidiales,
Andreaeales, and
Bryales, and continued to the second edition. By invitation of
Heinrich von Handel-Mazzetti, he authored the section on Chinese mosses in the
Symbolae Sinicae. His collaborations and correspondence with other bryologists of the day were extensive. In particular, he was well acquainted with
Max Fleischer, and used Fleischer's new 'natural' system of moss classification, which was outlined in the latter's
Die Musci der Flora von Buitenzorg, in his own systematic description of the mosses in
Die Naturlichen Pflanzenfamilien. Brotherus's unique achievement was his synthesis of
moss taxonomy for the world-wide distribution, and his mastery of the identification and classification of the estimated 20 000 species of mosses then known to him. ==Miscellaneous==