H'2 Meiuh'luni I iiln rlttnicr ((ml Yellow Rf(st in Wlmd 



ditioii of the plant's normal nu'taholisni. For example, American Club, 

 thoufjh normally immuiio, has been found to develop pustules of 

 Yellow ]?ust under abnormal conditions of gro\vth(iO) or nutrition (12). 

 It is therefore probable that any factor or factors which bring about a 

 condition of growth unfavourable to the production of those substances 

 upon which immunity depends, may lead to a greater or less degree 

 of susceptibility. Ward(20) especially emphasized the fact that "the 

 physiological condition of the host is always a factor of prime import- 

 ance" in considering the beliaviour of the host towards the parasite. 



Since all physiological processes are dependent upon both internal 

 (inherited) and external factors, in matters of this kind we must be 

 careful to distinguish between the inherited and non-inherited factors 

 concerned in bringing about the net result. Consequently, in trying to 

 discover the causes of the "disturbance in rust resistance" as observed 

 for example in 1 919, we nuist recognize the possible effects of: 



(1) External environmental factors. 



(2) Inheritable factors (other than those prinuirily concerned in 

 causing resistance, etc.) which lead to fresh combinations of parental 

 features in the hybrid and its descendants. These will now be con- 

 sidered. 



(a) Effect ok climatic conditions. 



The greater severity of attack in 1919 is partly explained by the 

 different weather in the two seasons. That of 1918 favoured a regular 

 and normal development of the plants. On the other hand the cultures 

 of 1919, while still in a backward condition, were subjected first to a 

 7 weeks" drought, and afterwards to G weeks of almost continuous dull, 

 cool, wet weather. 



During both years records were taken of the extent of the Yellow Rust 

 attack on 13 distinct varieties of wheat grown in the cages. In eight 

 cases the attack was more severe in 1919 than in 1918; on four varieties 

 it was of about the same intensity, and in one case it was slightly less 

 severe. This and other evidence support the behef that the abnormal 

 weather of 1919 was partly responsible for the more pronounced attack 

 in that year. 



(h) Food supply. 



It was, however, clear that the greater intensity of attack on the 

 F^ cultures in 1919 was not due solely -or even chiefly — to tlu' different 

 weather conditions. The increased severity of attack was far moie pro- 

 nounced on these than on the standard varieties growing alongside. 



