THE ^CIDIAL STAGE OF RUSTS. 33 



"disproportion which exists in En^hmd between the amount of 

 of mildow (rust) and the number of barberries." He further states 

 that there is a wonderful difference between the extent of injury 

 caused by "mildew" when derived directly from the barberry and 

 when derived from a urodo that has reproduced itself throu(];h several 

 generations, the former being much greater than the latter. He 

 adds: 



This is only what one would expect when the fact is taken into consideration that 

 the ascidium spore is a sexual product, whereas the uredospore is not. 



Bolley (21, p. 12) holds a similar view and says: 



The services rendered by it [the barberry] should probably be considered as that of 

 rein\agoration, much the same as that wlrich is rendered by reproduction in ordinary 

 plants. 



Arthur (3, pp. 67-69) similarly believes the aecidium is a device 

 to restore vigor to the rust fungus, the secidiospore giving rise "to a 

 much more vigorous state of the fungus than the uredospores do," 

 and, as a consequence, the prevention of the production of the 

 secidiospore by the removal of the aecidial host would reduce very 

 largely the injury which the rust is capable of producing. 



Tliis view has been greatly strengthened since Blackman's (18) dis- 

 coveries of cell fusions and the origin of the binucleated condition in 

 the aecidium of Pliragmidium violaceum on Eubus fruticosus and Gym- 

 nosporangium clavariaeforme on Crataegus and in the further studies 

 of Christman (33 and 34), Blackman and Fraser (19), and Olive (79 

 and 80), all of whom have shown that in various rust species a cell 

 fusion takes place and the conse(]uent binucleated condition arises at 

 the base of the aecidium. The authors differ in certain instances as 

 to the details of tliis fusion, and in the species studied, but generally 

 agree that this fusion is sexual. If it is functionally a sexual union 

 the fiiml step of which is the nuclear fusion in the teleutospore,^ the 

 reinvigoration of the rust as claimed by Plowright, Bolley, and Arthur 

 is to be expected as a natural consequence. 



Experiments to Determine the Vitality op Successive Uredo Generations of 



Various Grain Rusts. 



material used and methods employed. 



To test this invigoration thcor}' in part and to determine, if pos- 

 sible, whether or not the wcidial stage is necessary in the life history 

 of rusts, continuous cultural experiments from the uredospore of the 

 various cereal rusts were undertaken by the authors in 1907 and 



' Dangeard and Sapin-Troufly demonstrated the nuclear fusion in the teleutospore of rusts as early as 

 1S93 (Comptes Rendus 116, pp. 207-269 and 1304-1300) and regarded this a "pseudofecundation." These 

 studies led to further investigations on the sexuality of the Uredinese and consequent discovery of cell fusion 

 in the aecidium. 



88550°— Bull. 216—11 3 



