May 12,1923 Resistance to Stem Rust in a Cross of Common Wheat 465 
There were 1,020 individuals in the 32 families which were heterozy- 
pus in their reaction to Biologic Form I. Of this number, 798 were 
immune, while 222 plants were clearly susceptible, an approximate ratio 
of 3 resistant plants to i susceptible. In some cases there were a few 
plants which failed to become infected in families which, judging from 
the majority of the plants inoculated, should have reacted as homozy¬ 
gous for susceptibility. Upon reinoculation it was found that the plants 
really were susceptible and had merely escaped infection. In the case 
of the heterozygous families it is not surprising, therefore, that the 
number of resistant plants is a little larger than expected. Apparently, 
there should have been a simple ratio of 3 resistant plants to i suscep¬ 
tible. There is a deviation from the expected of 33 ± 9-33. 
From these data it is very evident that the segregation for resistance 
and susceptibility to this one biologic form of stem rust is very simple. 
Many desirable types were obtained in the Fg which are homozygous for 
spring habit or growth and are immune from Biologic Form I. 
Several Fg selections, homozygous in their reaction to Form I, were 
inoculated with 12 other biologic forms. The results obtained were very 
striking and consistent, and are presented in Table V. 
Table V .—The reaction of Marquis^ Kanredy and F^families of the cross between Kanred 
and Marquis, to ij biologic forms of stem rust 
1 S^Completely susceptible; I—immune; R=resistant, a type of infection intermediate between that of 
S and I. 
It will be noticed in Table V that as far as the reaction of the two 
parents is concerned, the 13 biologic forms of rust may be placed into 
2 groups. The first group is typified by Form I, to which Marquis is 
susceptible and from which Kanred is immune. 
The second group is represented by Form III, to which both Marquis 
and Kanred are susceptible. Here it will be seen that all of the progeny 
are as susceptible as either parent and identical in their reaction in this 
respect. 
The hybrid families may also be placed in two groups; {a) those whose 
rust reactions are similar to the Marquis parent, and (6) those that are 
similar to the Kanred parent. Family 41, for example, is identical with 
Kanred in its rust reaction to all of the forms used, and family 181 is 
identical with Marquis in its rust reaction to all forms used. 
These results very definitely demonstrate that resistance and sus¬ 
ceptibility to several biologic forms of stem rust may be carried either 
in a single genetic factor or in different factors linked in the process of 
segregation. 
