444 SCIENCE PROGRESS 



which, since both characters it bears are recessive, may be counted 

 on to breed true. It does. 



Another illustration of a character other than colour is 

 that which has been studied by Mr. R. H. Biffen. One of 

 the most disastrous pestilences which affect the wheat is the 

 fungoid pest known as Yellow Rust (Puccinia ghimarum). 

 Some varieties of wheat are more susceptible to it than others ; 

 for example, the ; variety Michigan Bronze is hardly ever free 

 from it. Another variety grown by Mr. Biffen may be regarded 

 as practically immune, for, although it was grown for four 

 years in close proximity to the most rust-stricken varieties, 

 it was not affected by the disease. This immune variety was 

 crossed with Michigan Bronze. The hybrids were all so 

 badly affected by the disease that one could not find an area 

 an eighth of an inch square unattacked by the rust. No 

 difference in respect of their susceptibility to the disease 

 could be discerned between the hybrid and Michigan Bronze. 

 But the hybrid did differ from it by the fact that whereas 

 the susceptible parent hardly set a grain, the hybrid gave a 

 moderate crop. 



"Every available grain was sown in plots alongside the 

 parent varieties, part on land which had been exhausted on 

 a previous crop of wheat, and part on land which had carried 

 a crop of clover the previous season and consequently was in 

 high condition. The difference in cultivation, however, made 

 no difference in the results of the experiment. The rust was 

 late in appearing that season, but again every plant of the 

 susceptible parent was stricken, and all of the immune type 

 escaped entirely. The hybrid plots were badly attacked, and 

 when the epidemic seemed to be well advanced, the plants were 

 sorted into two groups — namely, those which were attacked, 

 and those showing no signs of the disease even on the withering 

 basal leaves. On some plots the diseased plants were cut 

 out, leaving those free from infection for a subsequent 

 examination. A few individuals, which had escaped the attack 

 at the early stage, became infected later, and these were then 

 added to the total of susceptible plants. The statistics showed 

 that 1,609 diseased plants were present and 523 immune, or a 

 ratio of 3*07 : 1. There cannot, I believe, be any question 

 that these latter were really immune, for they were surrounded 

 by plants covered in rust, whose leaves were continually 



