58 Immunity in Cereals 



last 26 races belong to vaiieties with black and grey grains (Howering 

 glumes), and in general they are morphologically different from sus- 

 ceptible races (IG, pp. 15—18, 94—95). 



The various examined races of wild oats, belonging to the species 

 A. fdtuu Ij., A. Ludoviciana Dur., and ^1. sterilis L., all pioved to be 

 very susceptible to ciown rust. 



Aveiui brevis Roth., A. striyosa Schreb., and A. inula L. var. biu- 

 ristata A.seiir. and Or. are relatively immune to this rust. 



A different result was obtained with black rust. Of S50 examined 

 sorts of cultivated and wild oats, belonging to nearly all known botanical 

 varieties, only two races of the species A. sativa L. proved to be 

 relatively immune, (^ne of them (more resistant) belongs to the var. 

 bruiinea Kcke., the other (less resistant) to the var. montana AL, two 

 varieties with dark flowering glumes, and both these races are morpho- 

 logically very different from the other susceptible races of the same 

 varieties ; for instance, they are very low plants and are characterized 

 by very thin straw; practically both are of small value. All other 

 cultivated and iuitd varieties belonging to six species are badly attacked 

 by P. graiiiinis. 



In other words, as a result of these observations, we come to a 

 simple statistical conclusion as to the very slight probability of plant- 

 breeders' finding oats resistant to black rust. 



This conclusion will be rpiite logical if we remember that black 

 rust of oats is a very weakly specialized fungus, whieii lives freely not 

 only on th(^ genus Aveiai, but also on Alopecurus, Milliuni, Bromus, 

 Lamarckiana, I'htdaris. Koeleria, Festiica and other genera of ^t/'«- 

 mirieae. For genetists it is (juite natural to conclude that if the 

 fungus does not distinguish generic differences, there is ver}- little 

 probability that it will sharply distinguish racial differences in the 

 species A. sativa L.' 



' A similar argument may be applied to the ergot of cereals — Chiviceps purpurea Tul. 

 The same biologic race of this fungus, according to Stiiger's experiments, lives on rye, 

 barley, wheat, Aniliojtinllium, HierochUm, Arrlii'inaherum, Daclylin, Foa, Ilri:a anil other 

 genera of Griimineae. Theoretically, therefore, there is very slight probability for plant- 

 breeders to find a great difference among races of rye, barley and wheat in their 

 susceptibility to this fungus. The great difference between rye, barley and wheat in 

 the degree of infection by ergot (the two latter are very rarely attacked by ergot), is 

 evidently connected with tlie different modes of Howering of these cereals. Rye usually 

 flowers with opened glumes, wheat and barley with closed glumes ; and the closed mode 

 of flowering prevents the two latter from being infected by ergot. Eventually all the 



