5o8 Journal of Agricultural Research voi. xix, no. w 



but if enough spores are germinated the promycelia will be found. 

 The spores of some collections produce them more often than those of 

 other collections. In general it may be said that promycelia are pro- 

 duced more abundantly by aeciospores collected late in the season. 

 They are not entirely absent, however, from cultures made with spores 

 collected early in the season. The possibility of these promycelia being 

 produced by spores of the short-cycled rust that have contaminated 

 the cultures has been excluded in most instances. In order to do this, 

 the aeciospores were taken from caeomas that had not yet opened. 

 Moreover, many of the spores were collected and germinated in parts 

 of the country where the short-cycled rust is not known to occur. If 

 the promycelia appeared only in cultures from aeciospores collected 

 in the South where the short-cycled rust is abundant, mixed infection 

 might offer a possible explanation. But since they also occur in cul- 

 tures of spores collected in the North where the short-cycled rust has 

 never been found, this explanation is unsatisfactory. In the vicinity 

 of Glen, N. H., orange-rust has been collected each spring since 1913. 

 Spores taken from a number of different places in this vicinity have 

 been germinated, but the short-cycled rust has not been found. In 

 both 1 91 7 and 191 8, cultures made at Glen were studied and found 

 to contain a few promycelia. Promycelia have also been observed 

 in cultures of the aeciospores of the long-cycled rust collected at Old 

 Forge, N. Y., where several seasons' search has failed to reveal the 

 presence of the short-cycled rust. They have been found in cultures 

 of aeciospores taken from the black raspberry at Rouses Point, N. Y., 

 French Creek, W. Va., and at points in the vicinity of Washington, 

 D. C. Promycelia were also found in cultures of aeciospores collected 

 at Mountain Lake, Va., on Rubus alleghaniensis. 



When spores of the short-cycled rust are incubated at room tem- 

 perature (about 25° C.) on a favorable medium, they produce promycelia- 

 bearing sporidia within 24 hours. Spores of the long-cycled rust placed 

 under similar conditions produce long germ tubes within 24 hours. 

 Promycelia have seldom been found in these cultures after so short a 

 time. They occur in cultures of Gymnoconia only after a rather long 

 period of incubation. They can usually be found after 3 or 4 days. 

 In order to study the production of promycelia by the aeciospores 

 of Gymnoconia, it is best to incubate cultures at a fairly low tem- 

 perature. Temperatures varying from 10° to 15° are favorable. 

 Promycelia are always slow to make their appearance in cultures of 

 this rust. If incubation temperatures are high, many germ tubes die 

 before they have time to develop into promycelia. Moreover, cultures 

 kept at high temperatures are usually overgrown by saprophytic mold 

 fungi and bacteria after a few days. Low temperatures check the 

 growth of these organisms. Promycelia can be found most easily in 



