58 Mendelian Inheritance and Yelloiv Rust In Wheat 



Section I. INTRODrCTIOX. 



The present paper may, at first sifjht, a])pear to contain nuicli that is 

 irrelevant to tlie subject as indicated in the title. But, whilst the primary 

 object throughout was to trace the inheritance of susceptibility and 

 resistance to rust attack, experience shows that a study of the environ- 

 mental conditions is also essential for a correct understanding of this 

 problem. 



The unusual weather, and other circumstances in 1919, led to such 

 an abnormal growth of the wheat cultures, that it was felt necessary to 

 present all the available evidence which would throw any hght upon 

 the condition of the host plants, the spread of rust, and the greater 

 severity of its attack in that year. For these reasons, details which are 

 usually ignored in the treatment of a subject from the genetic point of 

 view, e.g. such as refer to soil, manuring, and weather, have been in- 

 cluded. 



(a) Historical. 



A large number of investigations have been carried out bearing on 

 the question of the susceptibihty and resistance of plants to rust attack, 

 and the more common cereal rusts es])ecially have received much atten- 

 tion. A great deal of this work has been concerned with the histology of 

 the rusts, in which the phenomena of inoculation and infection have 

 been studied under various conditions, and attempts made to discover 

 the primary causes of immunity and susceptibility. Our knowledge of 

 this branch of the subject is based largely upon the investigations of 

 Ward (15-20), Pole Evans(5), Miss Gibson(7), Miss Marryat(iO), and Eriks- 

 son (4). 



The suggestion that immunity depended upon certain slight ana- 

 tomical differences, e.g. thicker cell walls, fewer or smaller stomata, more 

 or longer hairs, etc., was proved to be incorrect by the researches of 

 Marshall Ward(i9) on the Brown Rust [Puccinia dispersa) in the genus 

 Bromus. Ward showed that the curves of infectibihty and those indi- 

 cating the various anatomical differences, not only failed to correspond, 

 but showed no relation whatever. This led to the conclusion tliat im- 

 munity and susceptibility are not directly determined by the anatomy 

 of the host plant, but depend u])on physiological reactions between the 

 protoplasm of the parasite and the cells of the host. 



From the experimental evidence obtained, Ward concluded that 

 "infection and resistance to infection depend on the power of the fungus 



