S. F. Armstrong 8:1 



while the only known difference in environmental conditions was that 

 of food supply. The rust-cultures had all received a heavy dressing of 

 nitrate of soda, whereas the other plots were unmanured. The nitrate 

 was carried to the roots of the plants when their Very existence was in 

 the balance at the end of the drought (June "20th). Thus the nece.s.sary 

 supply of moisture, and also a large quantity of available nitrogen, were 

 simultaneously presented to the plants. It is very important to note, 

 therefore, that the change from a critical condition to a state of active 

 growth was as sudden as it was pos.sible to be. The "second growth" — 

 which also occurred on the unmanured wheats — was stimulated to an 

 enormous extent on the F^'s, and the cultures assumed the very dark 

 green colour characteristic of plants receiving an excess of nitrogen. 

 The maturation of the plants was also considerably delayed. These 

 conditions evidently in some way afforded a greater opportunity for 

 rust attack, for it was precisely during this period of delayed maturation 

 that the great epidemic of the season developed. 



Nitrogenous manures, especially when greatly in excess of other 

 fertihzers, are well known to be very effective in increasing the severity 

 of rust attack. Biff'en(3) has pointed out that, on the Rothamsted wheat 

 plots, rust attack is invariably encouraged where ammonium salts or 

 nitrates are continuously applied in heavy doses. Spinks(i2) also showed 

 that susceptibility to Yellow Rust is increased by the use of large quan- 

 tities of available nitrogen, while plants which are semi-starved as regards 

 nitrogen may exhibit a considerable degree of resistance. Further, as 

 Spinks and others have shown, some salts, e.g. salts of potassium and 

 especially lithium salts, may markedly reduce susceptibihty. The 

 question of food supply is therefore certainly of great importance in 

 connection with the observed fluctuations in susceptibility. 



In connection with this question of food supply, it may be noted 

 that during the present experiments there appeared to be a difference 

 in susceptibility between "interior" and "exterior" plants in the same 

 pure susceptible cultures. Records of the first plants to be attacked were 

 made on 23 of the homozygous susceptible cultures given in Table VII. 

 At the time of "first infection" these cultures contained 642 "interior," 

 and 124 "exterior" plants. By June 7th the number of infections noted 

 on "interior" plants was 35, i.e. 5-4 per cent., while at the same date the 

 infected "exterior" plants numbered 20, i.e. 16-1 per cent. Similarly, 

 in 12 other pure susceptible cultures, by the end of May it was found that 

 7-3 per cent, of the "exterior" plants were attacked, but only 2-3 per 

 cent, of the "interior" plants. The number of early infections were 



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