394 BROOKLYN BOTANIC GARDEN MEMOIRS 
From this data it is evident that Bromus Krausei and B. pendulinus 
are susceptible to the rust on both B. mollis and B. sterilis. From the 
standpoint of bridging hosts, however, what we need to know is the 
infecting capacity of uredospores from B. Krausei and B. pendulinus, 
produced by inoculation with uredospores from B. mollis and B. sterilis. 
It may well be that B. Krauset and B. pendulinus are merely hosts for 
the rust on both B. mollis and B. sterilis. At least the evidence is 
not complete for proving that they are bridging hosts. 
Freeman and Johnson (57) conclude that barley is a bridging host 
enabling the specialized races of Puccinia graminis on wheat and rye 
to infect oats. They find that the rust on wheat will not infect oats, 
but will infect barley; the same is true of the rust on rye. When, 
however, the rust on barley, produced by inoculation from either 
wheat or rye is sown on oats, infection occurs to a very slight extent. 
The data upon which this conclusion is based are as follows: (1) uredo- 
spores from wheat to barley (26/31),4 to barley (28/42), to barley 
(16/16), to oats (2/54); (2) uredospores from rye to barley (23/31), 
to oats (1/22). As noted before, the barely rust is able to infect all 
four cereals, but rye and oats less completely than wheat and barley. 
The indications are that wheat rust and rye rust, as a consequence of 
growing on barley for one or more generations, are able to infect oats. 
It is evident, however, that the data obtained are rather meager and 
very much more extensive series of inoculations should be carried out. 
Evans (44) has crossed a wheat (Bob’s Rust Proof) resistant to 
rust (Puccinia graminis) with another wheat (Wol Koren) which is 
highly susceptible. Evans found that the hybrid, although of more 
vigorous growth than either parent, was much more severely attacked 
by rust than the susceptible parent. It was also noted that in pot 
cultures in the greenhouse the hybrid produced an abundance of 
teleutospores, which rarely occurred on either parent under the same 
conditions. 
Evans next tried to determine the infecting capacities of the rust 
after it had developed on the hybrid. He found that the rust from 
the hybrid infected the susceptible parent much more severely than 
the rust originally found on it. Not only that but the rust on the 
hybrid readily attacked the resistant parent. Accordingly it is 
suggested that hybrid plants may play an important part in the trans- 
mission of parasites from susceptible to resistant varieties by increasing 
the virulence of the parasite. 
Stakman and Piemeisel (149) record many grasses as hosts for 
more than one race of Puccinia graminis. In fact Bromus tectorum, 
4 The denominator of the fraction indicates the number of leaves inoculated 
and the numerator the number infected. 
