179 
The following communication is taken from the Annals of 
Botany, Vol. xv., No. lx., Sept., 1901. 
THE BROMES AND THEIR RUST-FUNGUS (Puccinza 
dispersa).—I have for some time been occupied with an investi- 
gation which has involved a comparative examination of the 
“seeds” and seedlings of all the species and varieties of the 
genus Bromus that I could obtain, and a study of the conditions 
of infection of these grasses by the Uredo-form of Puccznza 
dis persa, the Rust-fungus so common on certain species of them. 
The details of the results will, I hope, appear in due course, 
but some points of interest may be summarized now. 
The uredo-spores germinate at all temperatures between about 
10-12° C. and 25-27.5° C., which may be considered the minimum 
and maximum cardinal points; the optimum is about 18-20° C., 
and many failures in infection were found to be due to the fact 
that the leaves of the grass may be at temperatures above the 
maximum. Uredo-spores in water frozen for ten minutes into 
solid ice germinated on thawing, but freezing for two hours seems 
to kill them. 
Infection experiments show that spores originating on a given 
species—e.g., B. mollis—easily infect that species, but not 
necessarily another species, even if closely allied, and indeed may 
not infect all varieties of that species. Thus spores from B. 
stertlis easily infect B. steri/is and the closely allied B. 
madritensis, but not B. mollis or its allies B. secalinus, B. 
arvensis, &c,, and not even the more closely allied B. maximus. 
Broadly speaking, the uredo-spores, growing on a species belong- 
ing to the Serrafalcus group, infect other members of that group 
—in varying degree, however, and not necessarily all—but fail 
to induce the development of pustules on species belonging to 
another group—e.g., Stenobromus or Festucoides. 
There is evidence, however, going to show that an occasional 
adaptation to another species may occur in cases where infection 
does not usually succeed. In such cases, the Uredo having once 
established itself on another species, its uredo-spore progeny 
will thenceforth readily infect that species. 
The rule seems to be that the uredo-spores infect most easily 
the species and variety on which they have been developed, less 
easily varieties or species more remote, and fail altogether to 
gain a hold on more distant ones. 
This does not appear to be a matter depending merely on the 
