in the Brontes and their Brown Rust. 287 
The spores from B. mollis also infect B. arvensis readily, 
and B. secalinus , B. interruptus , B. racemosus , and B. velutinus 
less readily. 
On the other hand, while the spores of B. sterilis rapidly 
and easily infect that species (incubation period ten days), 
they are unable to successfully attack B. mollis even after 
twenty-one days. 
Now this, put in other words, seems to mean that the 
spores of the brown Uredo (P. dispersa) if grown on B. mollis 
are able to infect successfully practically any member of the 
group Serrafalcus , and but rarely or not at all any member 
of the other sections so far examined of the genus Bromus ; 
whereas the same fungus grown on B. sterilis readily infects 
members of its own group Stenobromus — e. g., B. sterilis , 
B. madritensis , and B. maximus — but is debarred from the 
Serrafalcus group. 
The matter appeared so important that I proceeded to try 
to get results even more conclusive and free from possible 
sources of error. 
I here add the results given in Table XVI, though they 
teach nothing new except to show the kind of error which 
may creep into such experiments, in the case of the one plant 
of B. erectus which appeared to have been successfully infected 
by spores from B. mollis , which it was practically certain was 
not the case. 
The procedure adopted in the next series of experiments 
was to start the germination in new pots under glass in the 
garden \ as before, but to allow only a small definite number 
of plants to each pot. When old enough to infect — i.e. 
when at least one leaf was completely formed and expanded 
— I infected a certain definite proportion of the seedlings 
at known spots on marked leaves, and recorded the results. 
The uninfected seedlings thus served as controls. In most 
cases also I carefully tested the germinating power of the 
spores used in two ways : in cells and in open watch-glasses, 
1 It has been already stated that there were no Bromes or Brome Rust in the 
garden here referred to, except such as I had under experimental control. 
