58 
E. S. Salmon. 
constitutional peculiarities shown by the two plants in the com¬ 
parative inoculation experiments with the four forms of the fungus. 
Conidia of the fungus on B. arduennensis were sown six times on 
B. mollis , but no infection resulted ; sown on B. “ hordeaeeus,” 
infection occurred eight times out of nine. Again, conidia of the 
fungus on B. commutatus sown thirteen times on B. mollis produced 
no infection ; sown on B. “hordeciceus ” infection occurred eight times 
out of nine. Similarly with the fungus on B. secalinus and on 
B. adoensis. 
These results lead us to conclude that the morphological species 
B. mollis includes two “races” or sets of individuals possessing 
distinctive physiological (or constitutional) characters, that is to say, 
with regard to the forms of the fungus in question, an immune and 
a susceptible race. These may be termed two “ biologic forms ” of 
a host-plant. 
/Ul 
/'tatcwuriud 
Fig. 64 . 
Diagram illustrating the position of B. “ hordeciceus"’ as a “bridging 
species.” B. “ hordeaeeus ” is infected by the forms of E. Graminis on 
B. racemosus, B. interruptus, and B. arduennensis, and the fungus occur¬ 
ring on B. *■ hordeaeeus" is able to infect B. commutatus. Assuming that 
the fungus produced on B. “ hordeaeeus ” by inoculation with conidia 
from B. racemosus, B. interruptus, and B. arduennensis is able to infect B. 
commutatus, B. “ hordeaeeus " will serve as a “ bridging species ” enabling 
the forms of the fungus on these three host-plants to pass on to B. 
commutatus, a species which they are unable to infect directly. 
