426 Illinois State Laboratory of Natural History. 



appendages, that it is impossible to maintain specific distinc- 

 tions. Other short-appendaged forms have been called M. ex- 

 tensa var. brevis by Peck; and Berkeley (Notices of N. A. Fungi) 

 has referred specimens on oak (probably of this character) to 

 M. jyenicillata, Lev. (M. alni, (DC.) Winter). In fact, except 

 that the spores are usually a little larger, it is almost impossi- 

 ble to distinguish some of our specimens on oak, from forms of 

 this latter species; and some specimens of M. alni on Corylus 

 show appendages so long as to resemble the form of M. extensa, 

 Peck. This all goes to show that the two species are very 

 nearly related, and that some forms of each approach the other 

 so closely as to make it difficult to draw the line between them. 

 In the aggregate, however, the forms on oak differ so widely 

 from those of M. ahti, that they must be considered distinct. 

 Where the fungus occupies both sides of leaves that are woolly 

 or hairy beneath, as in Q. imbricaria, etc., there is often consid- 

 erable difference between perithecia from the upper and lower 

 sides; but where both sides of the leaf are smooth, as in Q. alba, 

 very little difference is noticeable. 



M. densissima, (Schw.) Peck, cannot be distinguished 

 by its perithecia from the ordinary form on Q. rubra; but it 

 presents some peculiarities of the mycelium, which, if constant, 

 would entitle it to specific distinctness. It has not been found 

 in Illinois. 



All the other observed forms on Querciis must be con- 

 sidered as forming one widely variable species, and it becomes 

 a question of some difficulty to decide under what name it 

 should be known. If it were merely a matter of preference, 

 the choice would easily be in favor of M. extensa, C. & P., both 

 from its appropriateness, and because it is so well known. 

 Schweinitz, however, (N. A. Fungi, p. 270) has described a 

 species on oak as follows: — 



Erysiphe quercinum L. v. S., sero autumno non rara in foliis querci- 

 nis praesertim Bannisteris, Pennsylvania. Hyphasma occupans fere 

 totum folium — expansum candicans, tenuissimum, floccis vix distinctis. 

 Sporangiolis raris, minutissimis, sparsis nigris, Praesertim loco dis- 

 tinguenda species. 



This description is not, perhaps, sufficient in itself to 

 enable us to determine positively what specimens he had in 



