375 



The Unattached Aecial Forms of Plant-rusts in North 



America. 



By A. G. Johnson. 



Ever since the detinite establisliiueut of lieteroecism in the Urediiiales 

 by DeBary in 1804 and ISGO, many different aecial forms have been, one 

 by one, pro])erly connected with their respective telial forms, so that now 

 the proper relationships are definitely Icnown for a large number. On the 

 other hand, there still remain a considerable number of aecial forms whose 

 telial connections are still unknown. 



The aecial forms of Uredineae are included mainly under the form- 

 genera of Cacoma, Pcridcrmiton, Roestclia and Aecidium. In this paper 

 the treatment will be limited to the last named form-genus, viz : Aeci- 

 dium. 



The genus Aecidiuin was established by I'ersoon in Linne, Systema 

 Naturae 2 :1472. 1791, by the following brief generic description : "Theca 

 (membranacea) utrinque glabra semiuibus nudis non cohaereutibus 

 plena." As now most generally accepted the diagnostic characters of the 

 genus are: a more or less cupulate peridium, rupturing at apex, within 

 which spores are borne in chains. 



The genus was at first considered distinct and independent by the 

 early botanists, yet practical farmers had for a long time observed and 

 lecognized the connection between rusted barberry bushes and rust on 

 wheat in the fields, and were very certain that the former was a direct 

 cause of the latter. To the end of protecting wheat from the disease due 

 to this origin, a strict law providing for the destruction of all barberry 

 bushes in Massachusetts was enacted as early as 1755, the same to take 

 effect in 17G0 and be in force for practically four years. Following this, 

 various observations were made and experiments performed by different 

 men, with varying degrees of conclusiveness. While observations and 

 experiments had been previously made by Schroeter in 1816, DeBai'y was 

 the first to show conclusively the exact succession of spore-forms in the 

 life history of a heteroecious rust. He showed definitely by experiments 



