Of primary importance in the carbon relationships of soil fungi is the part played in the decomposition of the celluloses, which compose almost all the structural remains of plant tissues. Our first real knowledge of this subject was given by Van Iterson[28] in 1904 when he showed the wide extent of cellulose destruction by fungi, and devised methods whereby fifteen cellulose-decomposing forms, many of which have since proved to be common soil fungi, were isolated. Three years later Appel[1] published his account of the genus _Fusarium_, and showed that many of the species could destroy filter paper. A difficulty was introduced in 1908 by Schellenberg,[60] who, working with common soil forms, found that only hemicelluloses and not pure cellulose were destroyed. This has recently been supported by Otto,[48] but from the practical point of view the discussion is academic for the amount of pure cellulose in plants is insignificant. In 1913 McBeth and Scales[43] showed that a considerable number of common soil fungi were most active cellulose destroyers, pure precipitated cellulose and cotton being readily attacked. This was supported by McBeth in 1916,[42] whilst Scales[59] has found that most species of _Penicillium_ and _Aspergillus_ decompose cellulose, especially where ammonium sulphate is the source of nitrogen. Waksman[65] tested twenty-two soil fungi and found that eleven decomposed cellulose rapidly and four slowly, whilst Dascewska,[16] Waksman,[66],[67] and others have concluded that soil fungi play a more important part in the decomposition of cellulose and in “humification” than soil bacteria. Schmitz[61] has recently shown that cellulose-destroying bacteria play no important part in the decay of wood under natural conditions. In addition to the celluloses, practically all simple and complex organic carbon compounds are attacked by soil fungi, and in many cases the decomposition is very rapid.[26] Many _Actinomycetes_, _Aspergilli_ and _Penicillia_ are active starch splitters, and it is of interest to note that some of the strongest cellulose decomposers (_Melanconium sp._, _Trichoderma sp._, and _Fusaria_) secrete little diastase.[66] The _Mucorales_ apparently do not attack cellulose, but can only utilise pectin bodies, monosaccharides, and partly disaccharides.[26] Dox and Neidig[19] have shown that various species of _Aspergillus_ and _Penicillium_ are able to attack the soil pentosans. Roussy,[58] Kohshi,[24] Verkade and Söhngen,[64] and many other workers have found that fats and fatty acids are readily used as food by soil fungi, and Koch and Oelsner[33] have recently shown that tannins are readily assimilated. Klöcker,[32] Ritter,[56] and others have shown that the utilisation of many carbon compounds is to a large extent determined by the source of nitrogen and its concentration in the pabulum.