26 INVESTIGATIONS OF RUSTS. 



approach other groups, and, indeed, after further experiment, may 

 have to be placed in some other group. The following species have 

 given interesting results in culture experiments. 



Rust of Cocklebur {Pacdnia wanthii Schw.). 



Observations and culture experiments of the writer show that the 

 rust of cocklebur is probably limited to one host and is distinct from 

 the species on Ambrosia, and also justify the suspicion that it lies very 

 near the border of the Lepto-uredinea?, and may belong to another 

 group. 



On March 1, 1897, the first water-drop culture made gave an excel- 

 lent germination in forty-eight hours. Long promycelia were pro- 

 duced, but no sporidiola. On February 13, 1897, at Washington, 

 D. C.j inoculations were made on seedlings of cocklebur and Ambrosia 

 trifida, resulting in an infection of the former in eighteen days, but 

 not of the latter. On March 12 a second experiment resulted in a 

 large number of infections of cocklebur seedlings in fourteen days. 

 In all these cases spermogouia preceded the teleutospores in the 

 infected spots. On October 8 of the same year an secidium was found 

 on cocklebur in considerable abundance, associated with the teleuto- 

 sori, at Perkins, Okla. An inoculation on cocklebur seedlings, made 

 at Lincoln, Nebr., on February 16, 1898, resulted in the production 

 of spermogonia in ten days. The inoculating material had been col- 

 lected in October, 1897. An SBcidium on cocklebur was again found 

 in abundance at Las Cruces, N. Mex., on July 11, 1899. Attempts 

 should be made to infect the cocklebur with this secidium. Doctor 

 Farlow says an secidium on Xanthium in Massachusetts is frequently 

 followed by Puccinia xanthii. a 



The ease with which artificial infections are made with this rust is 

 at first surprising. So long as there is warmth and moisture, germina- 

 tion occurs under almost any condition and at any time. 



Rust of Velvet Leaf (Puccinia heterospora B. and C). 



In the last-mentioned experiments the inoculating material was 

 taken each time. from dead leaves. The same was true in one experi- 

 ment with the rust of velvet leaf, November 11, 1896, at Manhattan, 

 Kans., in which seedlings of the host were infected in twelve days. 

 For all these experiments the average time of incubation was about 

 twelve days. In December, 1896, the infected plants of the last 

 experiment were transferred from Manhattan to Washington, D. C, 

 and material from these was used to inoculate new seedlings, which 

 resulted in an infection in nine days. It appears, therefore, that the 



«Proc. Amer. Acad. Arts and Sci., Vol. XVIII, p. 75. 



