8. F. Ali.MSTKONG ' 75 



{h) Order of "JirsL appearance" of rust on the different cultures. It is 

 interesting to notice the order in which the i^g cultures became infected. 

 This is shown in Table XI. As early as Ma.}' 17th, 13-2 per cent, of the 

 pure susceptible cultures were infected. At that date, none of the homo- 

 zygous "immune" cultures, and only 3-1 j)er cent, of those in which 

 segregation occurred, contained any infected plants. By June 1 1th, all 

 the pure susceptible cultures were badly attacked, but as )'et no sign of 

 infection was observed on the homozygous ■'immune" cultures, while 

 an intermediate percentage of the segregating cultures were infected. 

 By July 12th, 93-7 per cent, of the last-mentioned cultures were attacketl, 

 but only a trace of rust was then present on one plant in all the homo- 

 zygous "immune "' cultures. These results are in the order of expectation 

 if susceptibihty and immunity are inherited in simple Mendehan fashion. 



The fact that the F^ cultures were of three kinds was emphasized, 

 not only by the ""dates of first infection," but also by the rate at which 

 rust spread throughout the cultures. There was not sufficient time to go 

 fully into this matter, but the figures given in Table VII will serve to 

 indicate how rapidly the infection spread among the plants in the pure 

 susceptible cultures. In these, the great majority of the plants were 

 very badly rusted by July 1th to 10th, whereas no rust was to be found 

 on the pure "immune"' cultures at that time. Although full statistics 

 were not obtained from the segregating cultures, it was clear that the 

 rate of rust spread was of an intermediate character in these. 



4. Summary of the F-^ re.sult.s. 



The general results obtained from the F.^ cultures may be briefly 

 summarized as follows : 



(A) Up to as late as July 10th the cultures were sharply divided into 

 three groups: 



1. Those in which every plant was severely attacked. 



2. Those in which no trace of attack was to be found. 



3. Those in which the extent of attack varied very markedly from 

 plant to plant, some of the plants being rust-free. 



(B) The homozygous susceptible (f .) cultures were all characterized 

 by: 



((/) A comparatively early infection. 



(b) A very rapid spread of the disease. 



(c) An exceptionally severe attack on every plant. 



