The Basidiomycetes are divided into 2 parallel series, the Heterobasidiomycetes and the Homobasidiomycetes. The orders Tremellales, Uredinales, Ustilaginales and possibly Septobasidiales are recognized within the Heterobasidiomycetes. The order Tremellales retains the largest number of primitive characters and through forms within it satisfactory transitions may be made to the more specialized Homobasidiomycetes on the one hand, and the rusts on the other. The phylogenic relationships of these groups are based on the assumption that the nature of the basidium is the fundamental character in the Basidiomycetes, to which all other characters are subordinate, that basic groups in general will exhibit more simplicity and flexibility than derivative groups, that the basidiocarp becomes increasingly significant in derivative saprobic groups in which the basidium tends to become stabilized, and that parasitic nutrition indicates relative specialization as compared to saprobism.