:i75 



The Unattached Aecial Forms op Plant-rusts in North 



America. 



By a. G. Johnson. 



Ever since the definite establisliment of lieteroecism iu tlie Uredinales 

 by DeBary in 1864 and 18(35, many diCt'erent aecial forms liave been, one 

 by one, properly connected with their respective telial forms, so that now 

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

 otlier 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 Caeoma, Perklcrmium, Roestelta and Aecidium. In this paper 

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

 dium. 



The genus Aecidium. was established by Persoon iu Linne, Systema 

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

 (membranacea) utrinque glabra seminibus nudis non cohaerentlbus 

 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 boi"ne 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 

 recognized 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, DeBary 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 



