394 Illinois State Laboratory of Natural History. 



limitations prevailed, the older names were apt to be disregarded 

 By further study, and the comparison of numerous specimens 

 of each of the so-called species, it becomes apparent that in 

 very many cases the differences between them in the number 

 and length of the appendages, the number and size of the asci 

 and spores, and the appearance of the mycelium, — all so easily 

 recognized in single instances, — are not constant; that these 

 parts are exceedingly variable, and that it is frequently 

 impossible to maintain distinctions based on them. This 

 necessitates the putting together of forms often considered 

 specifically distinct, and a corresponding change in specific 

 descriptions. 



This wider view of the limits of species sometimes throws 

 new light on the work of the earlier mycologists, enabling 

 us to decide with reasonable certainty whether or not their 

 names should be adopted for the species as now understood. 

 The process leads to the abandonment of some familiar names, 

 which is always to be regretted; but the true interests of a 

 stable nomenclature demand the adoption of the earliest specific 

 name given to any form of the species. (See Bull. 111. State 

 Lab. Nat. Hist., Vol. II., p. 149.) 



In his admirable revision of the fungi for " Rabenhorst's 

 Kryptogamic Flora," Winter has very carefully and thoroughly 

 performed this labor for the European species of Erijsiphece, 

 and his nomenclature is adopted, for the most part, in this 

 paper, for those species common to both continents, such 

 changes only being made as are suggested by the study of 

 numerous American specimens. The distinctively American 

 species are usually much less encumbered with synonyms than 

 those that also occur in Europe. In a number of cases, how- 

 ever, names have been given to forms that cannot now be 

 considered distinct, and, in some cases, owing to the difficulty 

 of interpreting his descriptions, the names given by Schweinitz 

 have been disregarded. The attempt is here made to clear up 

 these difficulties, so far at least as our Illinois species are 

 concerned. 



Some of the species of Erijsiphecp are of practical interest 

 from the injuries they do to cultivated plants. The mildew of 



