S. F. Armstrong 81 



Many plants were purposely taken from cultures 15/5, 24/3, 29/15, 

 57/8, etc. (Table III), in which rust-resistance had apparently been dis- 

 turbed or "broken down" under the adverse conditions prevailing in 

 1919. A few of these extracted jPg's had, indeed, been badly rusted, 

 e.g. 15/5/15, 24/3/18, etc. Nevertheless in 1920 all the F^ plants proved 

 to be either completely rust-free or highly resistant. 



Table XI. Progress uf infedion bi/ Yellow Rust on the various F-^ cidlmes 

 in 1919. (Cross No. 120.) 169 cultures*. 



Percentage of cultures on wliich infection had occurred 

 for the weekly periods ending 



t\ Number May June July Final examination 



cultures of ^''3 , ^' , '• , , * , July 28th to 



1919 cultures 17th 24th 31st 7th Uth 21st 2Sth 5th 12th August 1 1th 



DD\ 5.3* 13-2 340 773 94-3 100 — — — — — 



DK'a 'Mi 3-1 15-6 22-4 49-0 61-4 IJ4-5 80-2 81-2 93-7 100 



lilCs 20 Of 90 



* One culture omitted because no record of date of first attack was made. 



t Only one small pustule seen on one plant by this date out of a total of 817 [ilants. 



Section- 111. .SOME OF THK FACTORS WHICH JIAV IXCREASIC Oil 

 DIMINISH SUSCEPTIBILITY TO RUST ATTACK. 



Introduction. 



In the previous section of this paper it was seen that a sharply defined 

 1:2:1 ratio was not directly found in the F.^ generation in 1918, 

 although the Fg results showed that it undoubtedly existed. The varia- 

 tions observed in the F^, and from season to season, appear to indicate 

 that, in addition to the inherited factors which primarily determine 

 resistance, etc., there exist other factors which may tend either to 

 increase or reduce a plant's predispo.sition to attack. 



The now generally accepted view is, that immunity is due to the 

 production of specific toxins or anti-toxins which have the power to 

 neutraUze the action of the attacking fungus. If this is correct, an im- 

 mune plant is one whose normal metabolism provides such a substance, 

 whilst a susceptible plant is one which is either unable to form such a 

 substance at all, or can only do so to an ineffective extent. But even 

 in the case of a plant which is able to produce such protective substances, 

 it is obvious that such production may be subject to modification as 

 regards quantity, rate of formation, etc. There is, indeed, evidence to 

 show that a state of complete immunity depends not only upon the 

 inherited factor for resistance, but also upon a properly balanced con- 



Joura. of Agric, Soi. xu 6 



