Julys, 1918 Can Biologic Forms of Stemrust Change Rapidly? 
US 
in the form of a fraction, the denominator indicating the total number of leaves inocu¬ 
lated and the numerator the number which became infected. The number of leaves 
which became distinctly flecked but did not develop uredinia, is given after the 
semicolon. The degree of susceptibility is indicated after each transfer as follows: 
(s), very susceptible; (s—), moderately susceptible; (r—), moderately resistant; (r), 
very resistant; (i), immune. Although the word “immune” is used, it is quite 
likely that oats may be very rarely, indeed, and very weakly infected. 
Diagram 1 .—Effect of barley on the parasitic capabilities of Puccinia graminis tritici-compacti 
Puccinia graminis tritici -, 
compacti from Elymus' 
glaucus. 
R-; 7(0 
11 7 
,54 
54 ' 3*' ' 40 ' 
• f>(r\ 
68 1 
W|; 6(r) 
CW-g(s) 
O—(i) 
2>r J 
B 2 - 4 (s-)- 
24 v ; 
b|( S -)- 
B^(s— )— 
I 4 V > 
B—(s—)— 
31 
B|(s-> 
W i6 ; 3 (r) 
°r 3 (i) 
W§(r) 
W—(r) 
14 ' 
W^; 11(f) 
W-; io(r) 
22 v 1 
R r ; 
6 (r) 
O-(i) 
It will be seen from diagram 1 that club wheat and barley are suscep¬ 
tible to the rust, which develops normally on both, although it grows 
more luxuriantly on club wheat. It is equally clear that barley did not 
enable the rust to attack Haynes Bluestem wheat any more readily than 
it could before having been transferred to barley. The rust was con¬ 
fined to barley for seven successive “urediniospore generations” cover¬ 
ing a period of four months, and at the end of that time it had not 
acquired any additional virulence whatever on wheat. The same is 
true of its action on oats and rye. Neither did it lose any virulence as 
a result of its sojourn on Marquis wheat on which it developed very 
weakly. Incidentally the results also showed that the rust did not 
adapt itself to more luxuriant development on barley, on account of 
its long association with it. As indicated, the rust does not develop as 
luxuriantly on barley as on club wheat, and it never acquired that 
ability. In this case at least barley does not act as an intermediary or 
bridging host. 
57665°—18 - i 
* Marquis wheat. 
