136 A Monograph of the Erysiphaceae 



F 



M. qiiercina (Schwein.) Burr, is maintained as a species by 

 Burrill in Ellis and Everhart's N. Amer. Pyrenomycetes. This 

 species on American oaks has been built up out o{ Erysiphe qjicr- 

 cvia Schwein., Microsphacra extensa Cooke and Peck, and NL ab- 

 breviata Peck. 



M, abbrcviata Peck, occurring on certain oaks, was described 

 (279) as '* allied to M. Hedivigii [= M. alnf^ , from which it is sep- 

 arated because of the short scabrous appendages." These char- 

 acters prove wholly insufficient to distinguish this form, and *' M. 

 abbrcviata'' may safely be considered a synonym of M, alni. 



M. extensa Cooke and Peck, however, is a marked plant in its 

 typical form with the long flaccid appendages, and although con- 

 nected by intermediates with M, alni^ I consider it is w^orthy of 

 being separated as a variety. 



M, qucrcina (Schwein.) Burr, has always been recognized as a 

 somewhat unsatisfactory species. Burrill, e, ^., says (60, p. 29), 

 ** It must be acknowledged that it is well nigh impossible to dis- 

 tinguish some forms referred to M. alni from certain specimens 



M. 



M 



By separating M. extensa as a variety, and uniting the other forms 

 on oaks with M. alni, from which they do not differ except in 

 occasionally showing a slightly more elaborately branched apex, 

 we get a more natural arrangement of the American forms of 

 Microsphaera on Qnercns. 



M. densissuna (Schwein.) Cooke and Peck (91) must certainly 

 be referred to M. alni. The plant is thus described : " Hyphasma 

 very dense, between filamentose and himantioid, indefinite suborbi- 

 cular patches ^-2 in. broad, somewhat radiating at the margin, 



persistent ; concepticles few, scattered ; appendages 6-10, sporangia 

 4-S ; sporidia 8. Remarkable for the definite orbicular patches of 

 mycelium. On leaves of Qiierais:' In Cooke and Peck's type 

 the persistent mycelium forms definite suborbicular spots on which 

 the perithecia are seated; in all other respects the characters 

 shown are those of ordinary M. alni. Curiously in the specimen 

 of " Bysiphc dcnsissima " from Schweinitz's herbarium, in Berke- 

 ley's herbarium at Kew (which is identical in other respects with 

 Cooke and Peck's specimen, and is accepted as the same species 

 by Cooke), the mycelium is completely evanescent. It is quite 



