8. F. Armstrono 93 



depend primarily upon definite factors which are inherited according 

 to the simple Mendehan law. 



3. That segregation leads to the occurrence in the F^, generation of 

 susceptible and immune individuals in tlie proportion of 3 : 1. 



4. That the immune F2 plants breed true to that character. 



5. That one-third of the susceptible F.^ plants are homozygous for 

 susceptibihty and breed true, and that the remaining two-thirds are 

 heterozygous for susceptibility and give rise to offspring in which rusted 

 and rust- resistant plants occur as in the ^''2 • 



6. That homozygous susceptible plants are distinguishable from 

 heterozygotes quite as much by an earlier and more rapid spread of 

 infection as by the final extent of attack. 



7. That a plant's predisposition or resistance to attack is, neverthe- 

 less, subject to greater or less modification from the interaction of various 

 other causes. There is evidence for this in the fact that a badly rusted 

 plant may prove to be heterozygous for susceptibility, whilst a moderately 

 rusted individual may be shown to be homozygous for the same feature. 



8. That the relative difference in resistance between homozygous 

 susceptible and "immune" plants remains approximately the same even 

 when the external conditions are favourable to an extremely severe 

 attack. 



9. That while new combinations of characters may lead indirectly to 

 increased susceptibihty, there is evidence to show that the reverse effect 

 may be produced. In some instances the inherited immunity was in 

 some way so stabihsed that the most favourable conditions for rust 

 attack failed to bring about the shghtest degree of infection. 



10. That breeding for rust resistance may proceed with every as- 

 surance of success, and that even the production of new races as stable 

 for immunity as American Club itself is by no means an impossible task. 



The evidence further indicates: 



11. That a cross between a normally immune and a susceptible 

 wheat produces a hybrid in which the inherited susceptibihty is probably 

 of an intermediate nature. 



12. That the causes referred to above under clause 7 probably include 

 any factor which is capable of modifying the plant's metabohsm to an 

 appreciable extent, and embrace 



(a) Inherited factors leading to fresh combinations of morpho 

 logical or physiological features, and 



(6) Non-inherited environmental factors. 



