July 8,1918 Can Biologic Forms of Siemrust Change Rapidly ? 
113 
and susceptible parentage, and that such sharp 3 to 1 segregation could 
not occur in the F s generation if plants with the constitution RR similar 
to that of the immune parent could be infected by rust from the DR 
plants. His F 8 resistant plants were still as resistant as those of the 
F 3 generation. He directs attention to the fact that Rivet wheat, one 
of the oldest English varieties, is still resistant to P. glumarum , and 
that einkom, possibly the first wheat to be cultivated, is still resistant 
to most rusts. He concludes that if varieties lose their immunity, 
either on account of some change in their own nature or on account of 
increased virulence of the rust, the change is too slow to affect the work 
of plant breeders. 
EXPERIMENTAL METHODS 
In the experiments with Puccinia graminis tritici-compacti , unless 
otherwise specified, the grains used were Haynes Bluestem wheat, 
(Minnesota 169, Triticum vulgare) ; Manchuria barley (Minnesota 105, 
Hordeum vulgare) ; Swedish rye (Minnesota 2, Secale secale ); and Im¬ 
proved Ligowa oats (Minnesota 281, Avena sativa). The methods used 
were similar to those described by Stakman and Piemeisel (jo, p. 
431-432). 
In estimating the degree of infection, more significance was attached 
to the character than to the number of uredinia. The total number of 
uredinia produced is no adequate index of resistance unless the size be 
considered together with flecking or yellowing of the leaf. All the 
inoculated leaves of a highly resistant variety often become infected, 
but the character of infection is quite different from that in a susceptible 
variety. 
On resistant varieties the uredinia are usually smaller, less confluent, 
and are usually surrounded by a more or less definite yellowed or whitened 
area, while the uredinia on susceptible plants produce more spores, 
become larger, and tend to coalesce. This difference is shown clearly 
in Plate 14, in which A and B show a susceptible form, while C and D 
represent a partly resistant variety. The differences between the sus¬ 
ceptible Marquis shown at B, Plate 16, and the resistant Kubanka shown 
at C and D may appear indistinct at first glance; but close observation 
shows clearly the much more pronounced whitening of the tissues near 
the uredinia on the partly resistant Kubanka. Usually the uredinia on 
Marquis are larger than those shown in B, while those on the Kubanka 
at C and D are about normal. An almost immune form is shown at D 
and E, Plate 15. 
The difference between moderately susceptible and completely sus¬ 
ceptible hosts, such as that between club wheats infected with P . 
graminis tritici-compacti and barley infected with the same rust, may be 
shown in degree of infection only. Barley is infected fairly normally, 
but not so heavily as club wheats. 
