In his three volumes of "Systema Mycologicum" published between 1821 and 1832, Elias Fries put almost all of the fleshy, gill-forming mushrooms in the genus "Agaricus". He organized the large genus into "tribes", the names of many of which still exist as common genera of today. Fries later elevated several of these tribes to generic level, but later authors—including Gillet, Karsten, Kummer, Quélet, and Staude—made most of the changes. Fries based his classification on macroscopic characters of the fruit bodies and color of the spore print. His system had been widely used as it had the advantage that many genera could be readily identified based on characters observable in the field. Fries's classification was later challenged when microscopic studies of basidiocarp structure, initiated by Fayod and Patouillard, demonstrated several of Fries's groupings were unnatural. In more recent history, Rolf Singer's influential work "The Agaricales in Modern Taxonomy", published in four editions spanning from 1951 to 1986, used both Fries's macroscopic characters and Fayod's microscopic characters to reorganize families and genera; his most recent classification included 230 genera within 18 families. Singer treated three major groups within the Agaricales "sensu lato": the Agaricales "sensu stricto", Boletineae, and Russulales. These groups are still accepted by modern treatments based on DNA analysis, as the euagarics clade, bolete clade, and russuloid clade.