PLATE I 
Classes of host reaction: Resistant (R), susceptible (S), and intermediate (X), as 
indicated by different types of infection (o, i, 2, 3, 4, and X) produced by biologic 
forms of stem rust on different varieties of oats. 
Class R (resistant), including 3 subclasses corresponding to infection types o, i, 
and 2. 
A. —Practically immune (o). No uredinia are developed, but sharply defined 
hypersensitive flecks are usually present. 
B. —Extremely resistant (i). Infection very light; uredinia minute and scattered 
and surrounded by very stiarply defined necrotic areas. 
C. —Moderately resistant (2). Infection light; uredinia usually small and scattered; 
hypersensitive areas varying from sharply defined necrosis to pronounced chlorosis. 
Class S (susceptible), including two subclasses corresponding to infection types 
3 and 4. 
D. —Comparatively susceptible (3). Infection moderate; uredinia midsized with a 
tendency to coalesce; true hypersensitiveness absent, but light chlorotic areas usually 
present. 
E. —Completely susceptible (4). Infection normal and heavy; uredinia large, 
numerous and confluent; hypersensitiveness entirely absent, but chlorosis may be 
present when cultural conditions are unfavorable. 
Class X (intermediate), representing the heterogeneous (X) type of infection. 
(No subdivisions of this class are recognized.) 
F. —Uredinia very variable, apparently including all types and quantities of in¬ 
fection, often of the same blade; no mechanical separation seems to be possible, as, 
on reinoculation, spores from small uredinia may produce large ones, and vice versa. 
In general, the infection is ill-defined. 
