88 A Monograph of the Erysiphaceae 



by placing it as a variety under this species, although it is, perhaps, 

 not improbable that it is, biologically, distinct from it. The most 

 obvious presumption would be that the perithecia of the Uncinula 

 in question had been transferred by the wind to the Epilobinm from, 

 e. g., some Salix bushes growing in the neighborhood. Apart 

 from the fact that there were none of the usual host -plants of U. 

 salicis In the neighborhood one must quite reject this presump- 

 tion when one observes (under a slight microscopic magnification) 

 how firmly the perithecia of the var. epilobii are fixed to the sub- 

 stratum by means of the appendages radiating on all sides. More- 

 over, the infected shoots were densely covered along their entire 

 length with perithecia, whereas no signs of the fungus could be 

 found on herbaceous plants growing in the neighborhood." 



I have not seen a specimen of this Uncinula on Epilobium, but 

 from the diagnosis and the author's remarks there seems no reason 

 whatever for separating it from U. salicis. Certainly the charac- 

 ters chiefly relied upon, viz, the length of the appendages and 

 the evanescence of the mycelium, cannot be considered distinctive 

 of the "var. epilobii'' (cf the diagnosis of U. salicis given at p. 

 82). The question whether any form on a certain host-plant is 

 biologically distinct can only be answered by experiment, for its 

 mere assumption would lead to the establishment of a new " bio- 

 logical " variety or species at each occurrence of a species on a 

 new host-plant. Also, remembering the case of the accidental 

 occurrence of U. salicis on Artemisia (and perhaps also on Scntel- 

 laria) mentioned above, it will be best to wait until perithecia have 

 been seen growing on a mycelium on the Epilobiuui before record- 

 ing the latter as a host-plant for U. salicis, although in the present 

 case the crowded habit of the perithecia makes it seem probable 

 that the Epilobium was serving as a host-plant. 



The fungus described as parasitic on perithecia of Uncinula 

 salicis by Cocconi (75*), under the name of P/ioma laicinnlac, is 



'P 



Miyabei van nov. [Figs. 73-78] 



Amphigenous, or epiphyllous ; mycelium usually evanescent, 

 or (on the upper surface of the leaf) subpersistent, thin and effused', 

 rarely forming definite spots ; perithecia gregarious or more or 



