26 A STUDY IN CEREAL RUSTS 



for the former to develop into the latter. Eriksson ( 1902, p. 657) also 

 expresses this view in a somewhat modified form. Ward (1902-1) 

 shows that adaptation of Puccinia dispersa takes place. Klebahn ( 1904, 

 pp. 152-167) cites numerous experiments to show that this may be 

 the case. Miss Gibson (1904, pp. 184-191) grew a number of suc- 

 cessive generations of rust on resistant host varieties, but, in that time, 

 found little adaptational tendency. Massee (1904, p. 17) explains the 

 resistance and susceptibility of various plants to parasitic fungi on the 

 ground of the presence or absence of chemotactic substances in the 

 host. He contends that saprophytes can be educated to become para- 

 sites. This would also apply in large measure to biologic forms. Sal- 

 mon (1905, p. 183) grew Erysiphe graminis from wheat on Hordeum 

 sylvaticum for five generations and found no diminution in the power 

 to infect the original host. Freeman and Johnson (1911, p. 28) con- 

 clude that "the host plants exercise a strong influence, not only on 

 the physiological and biological relationships, but in some cases even 

 on the morphology of the host." 



An attempt was made to test this matter. The object was to de- 

 termine the change, if any, in the physiology of the rust as evidenced 

 by its power of infection and in its morphology as evidenced "by changes 

 in spore dimensions. For this purpose Puccinia graminis tritici was 

 grown on Minnesota No. 163, a susceptible wheat, and on einkorn 2433. 

 The einkorn, in the first few trials, was apparently one of the most 

 resistant of the Triticums to the wheat stem rust. The rust was trans- 

 ferred to einkorn and grown on this host through successive genera- 

 tions for 19 months, transfers being made, on an average, once every 3 

 weeks. After one year on einkorn the rust seemed to be much more 

 virulent than the original wheat rust had been. Einkorn plants were 

 inoculated with this einkorn rust and with uredospores from wheat. 

 In both cases there was 100 per cent of infection, but the virulence of 

 the infection was quite different. On the leaves inoculated with rust 

 from einkorn, leaf areas one to two centimeters long were affected, 

 being well covered with pustules shedding spores in great abundance. 

 Individual pustules were from one to three millimeters long and nearly 

 as broad, giving every indication of a severe rust attack. On the plants 

 inoculated with wheat rust, on the other hand, the pustules were al- 

 ways smaller, although fairly numerous. Few of them were as much 

 as one millimeter long and many of them did not rupture at all. 



Some wheat plants were inoculated with rust from einkorn and 

 others with the rust taken directly from wheat. Here again there was a 

 considerable difference in virulence of infection. The pustules developed 

 from einkorn-rust inoculations were fairly large and numerous. Some 

 of them were two millimeters long, although the average length was 

 less than this. The infection on wheat inoculated with wheat rust was 



