34 A Monograph of the Erysipiiaceae 



variation in the 



crease in both the number and length of the appendages, until they 

 cannot be separated from the more common long-appendaged 

 American form. P. vmtor Howe {Microsphacra fulvofitlcra Cooke) 

 on Spiraea tonicntosa also shows much the same 



length of the appendages. 



These short-appendaged forms show^ the range of variation of P. 

 oxyacanthae in one direction, in the other we meet with a ver>^ long 

 '* appendaged form on Vacciniuni tiliginositm, which has been de- 

 scribed as ' P. myrtilliuavdiW major jxxtV '' Professor J uel has kindly 

 sent me a specimen (now in the Kew Herbarium) of this form. In 

 this specimen I find that contiguous perithecia have appendages vary- 

 ing from 2^2 to lo times the diameter of the perithecium. I have 

 seen exactly the same form — with the appendages varying in 

 len<^th from 6—10 times the diameter of the perithecium — in Rus- 

 sian specimens in Professor Tranzschel's herbarium. As this 

 great variation occurs in perithecia on the same leaf, it is obviously 

 impossible to treat the length of the appendages as a character of 

 diagnostic value, and in other respects the form shows no differ- 

 ences. 



In conclusion, it appears then that we must allow to the present 



species — P. oxyacanthae — a range of variation in the length of its 

 appendages of from less than the diameter of the perithecium to 

 ten times exceeding it The existence of a perfect series of con- 

 necting links justifies this treatrpent, as does also the fact that simi- 

 lar variation in the same characters is found in other species. In 

 the var. tridactyla of the present species the appendages vary from 

 1—8 times the diameter of the perithecium. In the series of forms 

 of Micros phacr a alni, the one long known as " M, Hedwigii'' cor- 

 responds exactly in the short, few appendages with the form of 

 P. oxyacanthae on Crataegus OxyacantJia and Mcspihis, and my- 

 cologists now admit that " M, Hedzvigii'' must be united with the 

 longer appendaged forms of M. aha. 



In some examples of American P, oxyacanthae on Crataegus 

 the perithecia form densely matted patches about the midrib of the 

 leaf Perhaps the plant described by Thiimen (350) as Podosphaera 

 clandestina var. ramiilicola is a similar form. The following de- 

 scription is given : '* Periitheciis dense aggregatis, numerosissimis, 

 pulviniformibus ; mycelio candido, non evanido ; ascis sporisque 



