Parasitism in the Erysiphaceae. 59 
There remains now to be considered the case of certain host- 
species which serve as “ bridging species.” In Fig. 64 we have 
represented in the first place, the relations which exist between the 
forms of the fungus on B. secalinus, B. interruptus, B. racemosus, 
B. commutntus and B. arduennensis and their common host-plant, 
B. “ hordeaceus’ ; secondly, the relations which exist between the 
fungus on B. “hordeaceus ” and B. secalinus, B. interruptus, B. 
racemosus, B. commutatus, and B. arduennensis ; and, thirdly, the 
relations (so far as they have bee'n ascertained) which exist between 
any one of the forms of the fungus infecting B. “ hordeaceus ” and 
the host-plants of the other forms of the fungus infecting B. 
“ hordeaceus .” 
We see that in every case but one the fungus occurring on 
B. “ hordeaceus ” is able to infect the host-plant of the forms of the 
fungus which infect B. “ hordeaceus .” Further we see that the 
forms of the fungus which are able to infect B. “ hordeaceus” are 
not capable of reciprocal infection among themselves. 
With the fact of the existence of such inter-relations as these 
before us, the question at once suggests itself, do not such species 
as B. “ hordeaceus ” act as bridges affording a passage for certain 
forms of the fungus to species of host-plants which they are unable 
directly to infect ? 
I have obtained proof that B. “ hordeaceus ” does in at least one 
case act in this way as a “ bridging species.” 
Experiments have proved (see Fig. 64) that the fungus on B. 
racemosus is incapable of infecting B. commutatus, while it never 
fails to cause full infection on B. “ hordeaceus and further, that 
the fungus found growing on B. “ hordeaceus ” is able to infect 
B. commutatus. 
In the experiments carried out last summer I took spores of the 
Diagram illustrating the result of the experiments mentioned over-leaf, 
B. “ hordeaceus ” is here proved to serve as a “bridging species,” 
affording the fungus on B. racemosus a passage to B. commutatus, a 
species which this fungus is unable to infect directly. 
