in the B tomes and their Brown Rust. 279 
posed in a garden or even in a greenhouse or laboratory 
already contaminated with such spores, the same kinds of 
difficulties and dangers may arise as in any other work where 
highly contaminating organisms have access to the experi- 
ments. 
On putting together the total results of these preliminary 
infections of garden seedlings, removed from the beds to pots 
and then infected, we get the following : — 
In no case apparently did the spores taken from the leaves 
of B. mollis infect B. inermis , and in no case did they fail 
to infect B. secalinus , and with a few doubtful exceptions 
B. mollis. 
As regards B. aspev , B. arvensis , and B. sterilis the results 
were less consistent. 
Similarly with the spores from B. sterilis . They failed to 
infect B. mollis , B. arvensis , and B. inermis in all cases : they 
infected B. asper and B .secalinus, however, in the only case tried. 
On looking more carefully into these preliminary trials, 
I found that several sources of error needed careful examina- 
tion. In the first place, the seedlings themselves had been 
growing for some time in the open, and there was no 
guarantee that their leaves were free from wind-borne spores 
at the time of transplanting. As matter of fact, on searching 
the tufts of seedlings in the garden during the next two or 
three weeks I found the Uredo on B. asper , B. secalinus , 
B. mollis , and B. sterilis , and subsequently on others. 
Secondly, I had collected both B. mollis and B. sterilis , with 
spores on them, from the same fields, and, as already stated, 
was not at the time fully alive to the existence of any 
physiological difference between the two forms : morpho- 
logically the two forms are identical. It is true I kept the 
two kinds separate in my collecting tins, but considering the 
quantity of spores already forming on B. mollis in the fields, 
it would not be surprising if the leaves of B. sterilis had 
already been dusted with them. 
Thirdly, it soon appeared certain that an experimental error 
had crept in in the case of at least one of the experiments. 
