572 
Journal of Agricultural Research 
Vol. XXVI, No. is 
In the more recent work on breeding for resistance to stem rust in 
wheat, account has been taken of the physiologically distinct forms of 
the rust, and specific rust strains have been used. The relations between 
the various rusts and hosts are not uniform. 
Hayes, Parker, and Kurtzweil ( 18 ), using a known strain of stem rust, 
find that “in the cross where Emmer is one parent, resistance is partially 
dominant, while in the Durum-common cross, susceptibility is completely 
dominant over resistance.” 
Puttick (jo) studied the infection results of forms I and XIX of 
Puccinia graminis tritici on a cross between Mindum and Marquis. 
Mindum is immune from I and susceptible to XIX, while Marquis shows 
the reciprocal relation. In the F 2 inoculated with form XIX there was 
evidence of one main factor and other modifying factors concerned in 
rust resistance. The same plants exposed to form I show that several 
factors are involved. 
Aamodt (i) and Melchers and Parker (26) studied the reaction of the 
hybrid Kanred X Marquis to known strains of stem rust. They agree 
that in this cross immunity is dominant and that there is a clean-cut 
3 to 1 ratio in the F 2 . Aamodt states further that in this particular 
cross a single factor determines the reaction to several specialized forms 
of the rust. 
Little is known concerning the relationship of rust and host in these 
different reactions. Resistance to the rust evidently depends on different 
factors in different cases. The differences between hosts and also between 
rusts, in some cases, may be differences in degree rather than in kind. 
The work of Hursh (19) demonstrates distinct differences between two 
biologic forms of the rust as to tolerance of extremes of hvdrogen-ion 
concentration and temperature. 
A detailed cytological study of the behavior of rust and host in these 
various cases may give some insight into the nature of rust resistance. 
This paper is the second of a series undertaken in the hope of learning 
more concerning the nature of immunity. The earlier cytological studies 
of cereal rusts, by Eriksson (jj), Ward (44), Pole Evans ( 12 ), Gibson (r5), 
Marry at (, 24 ), and Stakman (33, 36), were reviewed in the first paper of 
this series (2). 
INVESTIGATIONS 
MATERIAL AND METHODS 
The specialized forms of rust were III and XIX. Viable spores of 
form III were supplied by Doctor Stakman. The other rust was found at 
Berkeley on wheat growing in the botanical garden of the university. 
It was used in former studies (2) of infections of Baart and Kanred under 
the name of Berkeley rust, and has since been identified by Mr. M. N. 
Levine as form XIX. As before, the varieties of wheat used were Baart, 
seed for which was grown in the cereal plots at Davis, Calif., in 1919; 
Kanred (C. I. No. 5146), received from Hays, Kans., in 1917; and Mindum 
(C. I. No. 5296, Minnesota No. 470). 
The following are the rust relationships, using a scale of o to 4, in 
which o represents complete immunity, and 4, great susceptibility : 
TYPE OP INFECTION 
Form III Form XIX 
Baart. 4. 4 
Kanred.... 3 o 
Mindum.... o . 3 — 
