112 
Journal of Agricultural Research 
Vol. XIV, No. a 
HISTORICAL REVIEW 
Ward (jj), 1 as a result of his extensive work with biologic forms of 
the rust of bromes (Puccinia dispersa Erikss.), first suggested the pos¬ 
sible occurrence of bridging hosts. He suggested that bridging hosts 
might be hybrids or varieties taxonomically linking one predisposed 
species with another. Freeman (4) obtained results similar to Ward’s. 
Salmon (7) concluded that biologic forms of Erysiphe graminis DC. on 
the genus Bromus might be broken down by the use of bridging hosts. 
Freeman and Johnson (5) and Johnson (6) obtained evidence that 
bridging hosts might also, in some cases, enable a biologic form of P. 
graminis Pers. to infect a host plant which was normally immune. 
Stakman (8) and Stakman and Piemeisel (9), on the other hand, could 
not get any evidence of bridging, nor of rapid changes in the parasitic 
capabilities of biologic forms of P. graminis Pers. 
Although Ward first suggested the possible significance of hybrids in 
connection with bridging, Pole Evans (5) first called attention to the 
actual effect of such hybrids. He found that when immune and sus¬ 
ceptible wheats were crossed, the resulting hybrid, whether infected 
naturally or artificially, was more susceptible to Puccinia graminis than 
the susceptible parent. In addition, he found that the rust from the 
hybrid was more virulent on the susceptible parent than the rust from 
that parent itself, and had even acquired the power to infect the im¬ 
mune parent. It therefore acted as a bridge between the susceptible 
parent and the immune parent, and intensified the action of the rust on 
the susceptible parent. The significance of these results is obvious, and 
if they were of universal application the outlook for breeding and suc¬ 
cessfully growing rust-resistant wheats would be discouraging. 
Biffen (j) had previously shown resistance in wheats to the yellow- 
rust Puccinia glumarum Erikss. and Henn. to be a recessive Mendelian 
character and found that the relatively immune forms bred true. After 
the publication of Pole Evans’s results, Biffen (2) published the results 
of extensive experiments, showing that F 3 individuals resulting from 
crossing susceptible and immune wheats show definite segregation. An 
analysis of the F 3 plants proved that 25 per cent of the individuals were 
relatively immune. These bred true, at least from 1907 to 1911. Some 
of the susceptible forms bred true, while others produced offspring 
which segregated into immune and susceptible forms in the ratio 1 to 3. 
Biffen also states that some of the susceptible F 3 plants are more sus¬ 
ceptible than the susceptible parent, but that even if plants of the F x 
generation can act as a bridge between the susceptible and immune 
forms, the effects in the field are negligible. In support of his statement 
he points to the facts that the immune parents remained practically 
rust-free for eight seasons, even when growing near 1P 1 plants of resistant 
1 Reference is made by number (italic) to "Literature cited,” p. 122-123. 
