MiCKOSPHAERA ' 137 



certain that in M. alni, as in many species of the Erysipliaceae, the 

 mycelial characters are too variable to be of any systematic value. 

 In an interesting series of specimens in the Kew Herbarium, from 

 the herbarium of W. R. Gerard the mycelium on some leaves is 

 persistent in suborbicular patches, on other leaves subpersistent 

 and more or less effused, finally on others the mycelium is wholly 

 evanescent and the perithecia are seated on the discolored patches 

 of the leaf. Occasionally, too, on other hosts than Quarus^ M, 

 aim shows a persistent mycelium forming definite spots as is seen in 

 certain specimens on VibiirjiuDi, Alnus^ etc. The definite subor- 

 bicular patches of persistent mycelium could not therefore, even if 

 constant, be considered as distinctive of *' J/, dcnsissima.'' 



Howe*s plant, originally published as M. sparsa in the Journal 

 of Botany (91) and later- (168) as M. vihurni (Schwein.) agrees 

 with specimens of Erysiphe viburni Schwein. from Schweinitz's 



herbarium 



M, alni. The same 



is the case with E. ceanothi Schwein., E. syringac Schwein., AL 

 platani Howe, and M, ncmopanthis Peck. 



M, Ravcnclii Berk, (on Glcditschia triacanthos)^ since its publi- 

 cation in Grevillea in 1876, has been kept distinct as a species by 

 all authors, but neither in the type-specimen at Kew, nor in the 

 numerous specimens so-named in American collections that I have 

 examined, can I find any characters separating it from M, ahii. 



_ * 



Often the apex of the appendages is only 3 times dichotomously 

 branched (the tips of the ultimate branches are always regularly 

 recurved) and the plant is then identical with the usual European 

 forms of M. alni ; frequently, however, the apical branching is 

 more elaborate, sometimes even extremely ornate, and then re- 

 sembles that of American forms of M, alni on Qucrais, Cormts, 

 etc. In the type-specimen the appendages are colorless, or some- 

 times tinged amber-brown at the base. Burrill says "appendages 

 usually hyaline, occasionally colored for a distance, the color end- 

 ing at an abrupt line like a septum." As mentioned below 

 the occurrence of partially colored appendages is found sporadically 

 in many forms of M. a!)n^ and no systematic value can be attached 



to this character. 



J/, semitosta Berk. & Curt., on Ophalanthus^ has, since its pub- 

 lication in Grevillea in 1876, been maintained as a distinct species by 



