PREVENTION OF BUSTS. 71 



and healthy for a long time, giving abundant nourishment to the 



parasite. Marryat (70, pp. 129-137) had similar results in working 



with two wheats, American Club, a resistant wheat, and Michigan 



Bronze, a highly susceptible variety. She concluded: 



We are forced to fall back upon the theory that immunity to disease is due in these 

 cases to the production of certain toxins and antitoxins by host or parasite, or both, 

 which are mutually destructive. 



Salmon (91, p. SS), working on the barley mildew, was similarly 

 led to believe that disease resistance is due to physiological and not 

 structural peculiarities. Bolley (2G, pp. 180-182) is not certain 

 whether disease resistance is due to structural or physiological char- 

 acters, but believes it to be due to the latter, from having been able 

 to develop resistance in every strain of potatoes, flax, or wheat with 

 which he has worked. He further maintains: 



Under uniform conditions of rust infection, all wheats rise rapidly to a stage of 

 marked resistance to general uredospore infection, whether caused by Puccinia grami- 

 nis or P. rubigo-vera, which resistance seems to be characteristic for each variety 

 concerned * * * . The facts point quite clearly to the probable influence of 

 chemical agencies, perhaps toxins, arising from the direct existence of fungous attacks 

 upon the hosts. In my mind there is not the slightest doubt but such attacks origi- 

 nate heritable resistance. 



Biff en (16, p. 128), after making numerous hybrids between varie- 

 ties resistant and susceptible with respect to rusts and studying the 

 first and second generations, concluded that "immunity is independ- 

 ent of any morphological character." Orton (81, p. 457), in analyz- 

 ing the nature of resistance of varieties, similarly concluded that 

 "resistance is due to a specific protective reaction of the host cell 

 against the parasite." To whatever the resistance may be due in 

 the last analysis, it seems to be a peculiar, delicately balanced con- 

 dition of the host against specific parasites, a balance wliicli is not 

 maintained in the same way toward any two species or varieties and 

 wliich may be easily upset b}" change in environment of the host. 



SELECTION AND BREEDING OF RESISTANT VARIETIES. 



It has long been known that disease resistance is inheritable to a 

 greater or less degree, and on tliis basis selection of resistant varieties 

 and strains has been going on for some time. Biff en (15, p. 40; 16, 

 pp. 109-128) has recentl}^ brought forth experimental results to prove 

 that resistance and susceptibility of cereals to rust are Mendelian 

 characters^ and are inherited in Mendelian proportions. He collected 

 a large number of wheat and barley varieties of various degrees of 

 resistance to the three rusts, Puccinia glumarum, P. graminis, and 

 P. triticina, common in Englantl, and then made crosses between 

 resistant and susceptible varieties. The hybridizing was done in 1904, 

 and results of growing these in 1905 and 1906 were reported. With 



216 



