Older classifications of fungi frequently divided these organisms into four categories--parasites, saprophytes, facultative parasites, and facultative saprophytes, but the further mycological studies are carried the more clearly it is seen that these groups are entirely artificial. There are probably few fungi that cannot, under particular conditions, invade living tissues, and it only seems a question of time before at all events the vast majority of fungi will be grown on synthetic media in the laboratory. From our present point of view the importance of this lies in the fact that fungi living saprophytically in the soil may, given the right conditions or the presence of some particular host plant, become parasites or symbionts, and conversely well-known pathogens may live a saprophytic existence. Thus Cucumber Leaf Spot is caused by _Colletotrichum oligochætum_, and Bewley[3] has repeatedly isolated this fungus from glasshouse manure and refuse of various kinds. In his early studies, Butler[13] isolated many parasitic species of _Pythium_ from Indian soils, and the presence of _P. de Baryanum_ as a soil saprophyte has been confirmed by Bussey, Peters, and Ulrich.[11] De Bruyn[17] has recently found that most species of _Phytophthora_, including _P. erythroseptica_ and _P. infestans_ may live as saprophytes in the soil, whilst Pratt[53] has isolated from virgin lands and desert soils various fungi, which cause disease in potatoes. In 1912 Jensen[29] gave a list of twenty-three “facultative parasites” isolated from soil, and these are but a moiety of those which could be listed to-day.