F. J. F. SHAW 



123 



The facts established by these pot cultures therefore appear to be as 

 follows : — 



(1) Seed 0, cernua tobacco is strongly parasitic upon tobacco, does not 



infect mustard, cabbage, and turnip, and its incidence upon 

 tobacco is not influenced by applications of sodium nitrate. 



(2) Seed 0. cernua brinjal and 0. cernua tomato is strongly parasitic upon 



tobacco and is slightly influenced by nitrate. 



(3) Seed 0. indica mustard, 0. indica cabbage, and 0. indica turnip do not 



infect tobacco but come up equally on the other three hosts ; 

 applications of nitrate delay the appearance of the " tokra." 



(4) Seed 0. indica tobacco comes up strongly upon tobacco but very 



slightly upon mustard ; nitrate has the same effect as upon the 

 other varieties of seed of 0. indica. 



The most interesting point brought out by these results is that the 

 morphological species 0. indica appears to consist of two races. One race is 

 strongly parasitic upon Cruciferce and does not attack tobacco {N. tabacum) 

 and the other race attacks tobacco and does not attack Cruciferce. In the field 

 experiments in 1916-17 it has already been mentioned that a crop of tobacco, 

 grown in a field which in the previous season had been under tomato and which 

 then contained plenty of 0. cernua and 0. indica, did not contain a single plant 

 of 0. indica, although 0. cernua was abundant. This suggests the possibility of 

 further specialisation in the parasitism of 0. indica. The details of the pot cul- 

 ture experiment are shown in the following list and in the plan (Text-figure 7). 



