ORTON: north AMERICAN SPECIES OF ALLODUS 175 



The genus Uromycopsis is exactly parallel with Allodus. They 

 differ in no way except in the number of cells in the teliospore. 



Exceedingly complex questions arise as to the relation and inter- 

 relation of the eight genera in the tribe Dicaeomae. If the classi- 

 fication of the rusts proposed by Arthur^ has accomplished nothing 

 more, it has certainly opened up the whole subject from a new 

 standpoint and presented the rusts in such a manner that many 

 of these more difficult problems can be investigated more clearly 



than ever before. 



Genetic relationship 



Kern- has pointed out that it appears more probable to sup- 

 pose that the short cycle {Dasyspora) forms of rusts have been 

 evolved at one step from the long cycle {Dicaeoma) forms by 

 dropping out the intermediate spore forms. Olive^ on the other 

 hand thinks that the long cycle forms have been evolved from the 

 short cycle forms by a process of amplification. 



The writer's work on Allodus and his study of this genus and 

 the other genera of the tribe Dicaeomae indicates that some 

 species of Allodus hold an intermediate position between Dicaeoma 

 and Dasyspora and that in some cases the transition between the 

 two extremes has been gradual rather than abrupt. It seems most 

 logical to accept the view that Allodus has been evolved from 

 autoecious species of Dicaeoma and that some species of Dasyspora 

 have in turn originated from species of Allodus. The remark- 

 able similarity of certain species of rusts in these three genera upon 

 the same or closely related hosts, indicate such a condition as 

 outlined above. Further prima facie evidence of such evolutionary 

 tendencies is found in certain species of Allodus where occasional 

 urediniospores occur in the telia. These urediniospores are in 

 every detail like the urediniospores of its correlated species of 

 Dicaeoma. 



Bullaria appears to have developed from Dicaeoma in a manner 

 parallel with Allodus. It seems possible that certain meteoro- 



1 Arthur, J. C. Eine auf die Struktur und Entwicklungsgeschichte begriindete 

 Klassifikati'on der Uredineen. Result. Sci. Congr. Bot. Vienna 331-348. 1906. 



2 Kern, F. D. The genetic relationship of parasites. Am. Jour. Bot. 2: 116 131. 



3 Olive, E. W. Origin of heteroecism in the rusts. Phytopathology i: 139-148. 

 1911. 



