May 12, 1923 
Resistance to Stem Rust in a Cross of Common Wheat 461 
of the resistant hybrids were immune. There were, therefore, only two 
classes, i. e., immune and susceptible. 
An intensive study was made by testing a large number of F3 plants 
with a single known biologic form to which the Marquis parent is sus¬ 
ceptible and the Kanred parent is resistant. Two questions then natur¬ 
ally arise: How is the reaction tendency of the host to all of these biologic 
forms inherited ? Is the reaction due to the presence of a single genetic 
factor, or, if several factors are concerned, are they linked in the process 
of segregation ? An attempt was made to solve these questions by inoc¬ 
ulating various F3 selections, which were homozygous in their reaction 
to the first form studied, with 12 other biologic forms of stem rust. 
pass I u III IV y VI vii Vj ii ix 
Sfir/ng Intcrmecflates Winter 
Parent Parent 
Fig. I. —Diagram showing the segregation for time of heading in the spring-sown F2 of a Marquis 
X Kanred hybrid. 
EXPERIMENTAL RESULTS 
THE MODE OF inheritance OF GROWTH HABIT 
The first generation material was sown in the fall. Because of the 
mildness of the season, only 2 out of 75 plants were killed during the win¬ 
ter. Had the winter iDeen severe, it is possible that a higher percentage 
of the first generation plants would have been killed. No F^ plants were 
grown from spring-sown seed. 
All of the seed from the F^ was sown in the spring of the following 
year, producing an Fg population of 5,250 plants. Of this number 4,808 
plants headed during the summer and 442 plants did not. Of the former, 
980 plants headed at the same time as did the' spring parent. In the 
second weekly period 1,502 plants headed, which is the greatest number 
to fall into any one heading period. From the second period on to the 
eighth, there was a gradual decrease in the number of plants heading. 
In the eighth period there were but 19 plants. The total number for 
each class is shown in figure i. 
It is very evident from the chart that the segregation of the plants for 
growth habit characters in the Fg is of a complex nature. All of the plants 
in the first 5 classes matured like any ordinary spring wheat variety. 
It would be quite fair, then, to assume that in general, the plants in these 
