32 A Monograph of the Erysiphaceae 



missa, P. domcstica, P. Pennsylvanica (382), P. Per ska (382), P. 

 pumila, P. serotma, P. Virgmiana, Pynis Auaiparia, P. coronaria 

 (97) (137)' P- Cydonia, P. Gcrmanica, P. Mains, Spiraea befidifolia, 

 S. discolor (159), S. salicifolia, S. tomcntosa, Vaccinium inter- 

 in ediiun (319), V. Myrtillus, V. uliginosiun. 



Distribution. — Europe : Britain, France, Belgium, Nether- 

 lands (253), Germany, Switzerland, Italy, Austria-Hungary, 

 Denmark, Norway, Sweden, Finland, Russia. Africa : Algeria. 



r 



Asia: Siberia (Minussinsk) (350), Japan. North America : United 

 States — Maine (163), Vermont, Massachusetts, Connecticut, New 

 York, New Jersey (53), Virginia, North CaroHna, Ohio (71) (324), 

 Michigan, Indiana, Alabama, lUinois, Wisconsin, Missouri, Iowa, 

 South Dakota, Nebraska, Kansas, Montana, Idaho, Utah (363), 

 Wyoming, Colorado, CaHfornia. Canada — Ontario, New Bruns- 

 wick, Greenland (300). 



Examination of a very large series of specimens has convinced 

 me that the plants hitherto known as Podosphaera oxyacanthae 

 and P, myrtillina are not specifically distinct. The distinguishing 

 characters relied upon have been these : P, oxyacanthae^ ap- 

 pendages 8 or more, shorter than or about equaling the diameter 

 of the perlthecium ; P. myrtillina, appendages 4-10, spreading on 

 all sides, or even deflexed, 3 or more times exceeding the diameter 

 of the perithecium. In Europe these two forms, P. oxyacanthae 

 on Crataegus and Mespilns, and- P. myrtillina on Vaccinium Myriil- 



Ins and V, uliginosuvi^ could perhaps, taken by themselves, be 

 kept distinct as varieties, for the distinctive characters are scarcely 

 of specific value, besides occasionally showing a tendency to fail. 



F 



On dealing with other material, however, it Is seen very clearly that 



there exists every intermediate link between these two forms. 



Let us take first the European form on Pyriis Aucuparia^ de- 

 scribed by Eriksson as a new species. P. aucitpariae^ with the fol- 

 lowing diagnosis: **HypophyIIa. Mycelium evanidum. Perithecia 

 sparsa, sphaeroidea, minuta. Appendices paucae (4-6), diametrum 

 perithccii ter superantes, e parte superlore perithecil radiatim diver- 

 gentes * * * Mea species P, aucupariae differt a P. oxyacantJiac 

 appendicibus longioribus, a P. myrtillina Kze. appendicibus 

 paucis, a P. tridactyla (Wallr.) de Bary appendicibus radiatis, ab 

 his utrisque peritheciis hypophyllis.'' 



