80 Mendeliau Inheritance and Yellow Btisf in Wheal 



{b) Direct aiial/jsis of un F., Ihroiujh the F^ results, 1920. 



A small Fn crop of the same parentage (Wilhelmina x American 

 Club) consistiiij; of 86 plants was raised in 1919. No attempt was made 

 to grade these for rust attack as it was decided to grow an F.^ culture 

 from each plant and to use the F^ data for placing each F.^ plant in its 

 proper category for susceptibihty or resistance. These 86 F^>^ were 

 raised in 1920. In 22 of these cultures all the plants were readily su.s- 

 ceptible; 20 others contained only rust-free or highly resistant plants, 

 while the remaining 44 cultures consisted of readily susceptible and 

 resistant plants. The infeience is that the A', grown in 1919 consisted 

 of 20 rust- resistant plants, 44 impure susceptible and 22 pure susceptible 

 individuals. In four of the pure susceptible F-^ cultures all the plants 

 were excessively attacked. 



(c) Results from F^ cidliires. 

 (Cross No. 120. Wilhelmina x American Club.) 



A considerable number of F^ cultures were grown in 1920. Twenty- 

 seven of these were raised from plants taken at random out of the wholly 

 susceptible F^ cultures of the previous year. Without exception these 

 F^ plants (433 altogether) were badly rusted. 



Thirty-.six F^ cultures were grown from plants which had been 

 extracted from obviously segregating F^ cultures. Space will not allow 

 a full iircount of the results, but they showed definitely that the three 

 genetic types — pure resistant, pure susceptible and impure susceptible 

 —were present in the F^ cultures. At the same time they indicated 

 again that some allowance must be made for fluctuations in suscepti- 

 bihty due to season and other external conditions. The 19 F^'f. raised 

 from rust-free F^ plants consisted entirely of rust-resistant individuals. 

 Similarly all the extracted badly rusted F^ plants produced badly rusted 

 F^ cultures, while slightly or moderately rusted F.-^ j)lants gencniJlN- 

 produced cultures in which rusted and rust-free plants occurred. 



Two hundred and eighty-eight F/^ cultures were raised from F.^ 

 plants which were the offspring of rust-free F.;, individuals. 



Some of these were picked from cultures like 66/9, 11/21, 2/10 and 

 19/12 (Table III) in which all the plants were completely or almost 

 completely rust-free in 1919. 



The F^ plants nimTbered several thousands and it was impossible 

 to examine each individual very closely, but a general inspection showed 

 that all the plants were either rust-free or possessed a very high degree 

 of resistance. 



