398 BROOKLYN BOTANIC GARDEN MEMOIRS 



would expect. Agrostis alba, however, seems to harbor a distinct race 

 of rust and so the result recorded is not in line. 



Taking the results of aecidiospore inoculations as a whole, there 

 seems to be no good reason for assuming that aecidiospores from the 

 barberry, produced by teleutospores from a known grass, have any 

 greater range of hosts than uredospores from the same grass. It 

 appears that the racial strains of the black stem rust are not so sharply 

 fixed in their host restrictions in either the uredo or aecidial stage. 

 Further, the nature of the specialization is different in Europe from 

 what it is in America. It is not surprising, then, that these races are 

 able to grow on other hosts. There is, however, no clear indication 

 that the barberry acts in any way as a bridging host and that it 

 enables the races on different grasses to increase their range. 



The possibility of the aecidial host serving as a means for extending 

 the host range of specialized races is quite apparent in Piucinia 

 coronata Corda. Miihlethaler (102) records Rhamnus Imeretina as an 

 aecidial host for specialized races of three of the main subgroups of 

 the crown rust: Puccinia coronata (Corda )Kleb., P. coronifera Kleb. 

 and P. alpinae coronata Miihlethaler. Rhamnus Purshiana is likewise 

 an aecidial host for the races on two subgroups — P. coronata (Corda) 

 Kleb. and P. alpinae coronata Miihlethaler. There is, however, no 

 evidence at hand to indicate that, as a matter of fact, these species do, 

 in any way, act as bridging hosts. 



There are many other cases where a particular species of plant is 

 a host for two or more specialized races of a parasite and it might be 

 possible for these to enable the different races to extend their host 

 range. A few cases of this sort may be mentioned. According to 

 Jaczewski (68) Agropyron re pens and A. caninum are hosts for the 

 races Tritici and Secalis of Puccinia graminis and this might serve to 

 enable one race to pass over on to the hosts of the other. There is, 

 however, no experimental proof in support of the suggestion. Stak- 

 man and Piemeisel (149) record a number of hosts as common to several 

 or all of the six races they studied. However, no bridging occurs, 

 each race being distinct. According to Miihlethaler (102), Festuca 

 elatior is a host for races Festucae and Lolii of Puccinia coronata. 

 Phalaris arundinacea is the only uredo and teleuto host for the special- 

 ized races of Puccinia sessilis with their aecidial stages on Liliaceae, 

 Orchidaceae, Amaryllidaceae and Araceae. Several species of Ribes 

 are common aecidial hosts for the specialized races of Puccinia Ribesii- 

 Caricis. Similar conditions are found among a large number of other 

 forms — Uromyces Dactylidis, U. Poae, U. Fabae, U. Scirpi, Coleo- 

 sporium Campaniilae, Melampsora Larici-epitea, M. populina, M. 

 Tremulae, etc. 



