Inheritance of Disease-Resistance in Plants. F. T. Brooks. 75 
Of 19 cultures grown from Fy, plants which had been badly 
rusted in 1918, 15 proved to be homozygous susceptibles, and 
the others gave evidence of segregation. 
The 24 cultures raised from very badly rusted F, plants all 
proved to be homozygous susceptibles. 
In this generation, not only did the intensity of attack vary 
in the different cultures, but also the time of attack. Thus, in 
general, homozygous susceptibles were attacked earlier than 
heterozygous susceptibles and the latter were attacked earlier 
than the “genetically immune” where it happened that the 
last-named were attacked at all. 
Taking the 56 cultures in which segregation in F was clear, 
it was found that out of a total of 3045 plants, 2385 were either 
moderately or badly rusted, while 660 were only slightly rusted 
or not attacked at all. The 3:1 Mendelian ratio would be 
represented by 2284 : 761, and if it exists here, it follows that 
ror plants out of a possible 761 recessives were rusted beyond 
the slight extent indicated by grade 1, i.e. 13:2 %. This degree 
of “disturbance in rust resistance,’’ as Armstrong calls it, is 
comparable with that occurring in certain homozygous “‘im- 
mune’”’ cultures of the same series in which 11 % of the plants 
were rusted beyond grade 1. This disturbance is attributed 
to the operation of environmental factors previously mentioned. 
Seeing that a plant which shows traces of rust attack in one 
season may be shown by its offspring to have been “ genetically 
immune”’ and that a rust-free plant in the F, generation may 
be actually an impure susceptible that has escaped infection, 
Armstrong realised the importance of applying the F, results 
to the statistics of the F#, generation. In considering the Fy, 
results in these researches, it was seen that the I : 2: 1 ratio 
was not closely reached, although it probably existed, as is 
shown by applying the results of the F, cultures to the Fy, 
statistics. In this way, the probable composition of the autumn- 
sown F’, crop was: 
202 (homozygous susceptibles) : 419 (heterozygous 
susceptibles) : 208 (homozygous immunes), 
the expected numbers being 207 : 414 : 207. 
Similarly, the estimated composition of the spring-sown part 
of the F, generation was 190 : 378 : 163, the expected numbers 
being 183 : 365 : 183. Taking the whole of the F, crop together 
after adjustment with the F, results, the following composition 
is indicated: 
392 homozygous susceptible individuals. 
797 heterozygous Pe a 
371 homozygous immune i 
