7 i8 SCIENCE PROGRESS 



conditions of cultivation was secured and crossed with slightly, 

 moderately, and extremely susceptible varieties. Further 

 crosses were made between slightly and extremely susceptible 

 varieties, and between extremely susceptible varieties. It is 

 difficult to give any accurate description of the differences in 

 the degree of susceptibility. In practice they have been dis- 

 tinguished with sufficient accuracy by grouping the varieties 

 into five classes, ranging from no disease, through varying 

 degrees of susceptibility, up to excessive liability. 



The degree of liability to the attacks of the rust appears to be 

 a definite and constant character. Some varieties, for instance, 

 never show more than a few scattered pustules of uredospores, 

 whilst others are so suitable a pasturage for the parasite that 

 the whole plant becomes orange-yellow with the spores, and the 

 ground below it is even coloured in the same way. During 

 the course of the experiments, the intensity of the epidemic has 

 been somewhat variable. For three seasons it has proved over- 

 average, one average, and one under-average, but the compli- 

 cation produced by external conditions of which, at present, 

 we know little, has not interfered to any extent with the results. 

 One case only need be described in any detail. A wheat 

 belonging to the subspecies Triticum compactum was found to 

 be entirely free from yellow rust, even in rust years when it 

 was grown in the midst of other varieties which were so 

 badly attacked that the whole of the foliage, and even the 

 chaff, was coloured yellow. This was crossed with a variety 

 known as Michigan Bronze, which is so excessively susceptible 

 to the attacks of this rust that it rarely produces any grain 

 under our conditions, though in some parts of the continent 

 it can be cultivated successfully. The hybrid between these 

 varieties was particularly vigorous. It was grown alongside 

 small plots of the parents, partly to ensure a plentiful supply 

 of spores reaching it from the Michigan Bronze, and partly to 

 be certain that the conditions of cultivation for parents and 

 hybrids were the same. Late in the spring the hybrids began 

 to show the first yellow flecks on the foliage which herald 

 the formation of rust pustules, and in the course of a few 

 weeks there was no difference between Michigan Bronze and 

 the hybrid as far as the rustiness of the plants was concerned. 

 All parts of the plant — the foliage, the stems, and even the 

 awns on the chaff — were smothered with the yellow pustules. 



