In 1886 Adametz,[1] investigating the biochemical changes occurring in soils, isolated several species of fungi. It was, however, only with the work of Oudemans and Koning,[17] in 1902 when forty-five species were isolated and described, the majority as new to science, that the real study of the fungus flora of the soil commenced. There is now no doubt that fungi form a large and very important section of the permanent soil population, and certain forms are found only in the soil. Indeed, Takahashi[22] has reversed the earlier ideas by suggesting that fungus spores in the air are derived from soil forms. The majority of investigations on this subject fall, perhaps, into one or more of three classes: (_a_) purely systematic studies such as those of Oudemans and Koning,[17] Dale,[5] Jensen,[9] Waksman,[25a] Hagem,[8c] Lendner,[12] and others, which consist in the isolation and identification of species from various soils: (_b_) physiological researches, such as those of Hagem[8c] on the Mucorineæ of Norway, or the many investigations on the biochemical changes in soils produced by fungi, such as those of Muntz and Coudon,[15] McLean and Wilson,[15] Kopeloff,[11] Goddard,[7] McBeth and Scales,[14] and others: (_c_) quantitative studies, such as those of Remy,[20] Fischer,[6] Ramann,[18] Waksman,[25c] and Takahashi,[22] which involve numerical estimates of the fungus flora in soils. QUALITATIVE STUDY. With very rare exceptions soil fungi cannot be examined in situ, and the necessary basis of any qualitative research is the isolation of the organisms in pure culture. Most soil forms belong to the _Fungi imperfecti_, and often show considerable plasticity on artificial media. This makes it very difficult to determine them by comparison with type herbarium specimens or published morphological diagnoses. In consequence many soil fungi have not infrequently been given new specific names, as _humicola_, _terricola_, and so forth, which is very unsatisfactory, and means that the determinations have little significance.