24 Plant Genetics 



resisting plants, whose seeds, however, the next year 

 developed a race with enormous resistance. He gives 

 the following theoretical explanation of his results : 



Either (i) the so-called unit character of resistance was present 

 in undeveloped form and becomes stronger from year to year 

 under conditions of disease; or (2) there never was any character 

 present which is entitled to be called a unit character, but it began 

 to develop the first year the parent plant came in contact with the 

 disease, and the protoplasmic nature of the ancestors of the plants 

 which we now have has been such that they accumulated more 

 and more the resisting power from year to year, just as they had 

 opportunity to develop resistance against a constantly acting 

 factor of disease, which, when too powerful, acts as an eliminating 

 factor. 



BoLLEY inclines to the second alternative. This 

 general conception seems to explain why homegrown 

 seed is regularly more resistant than seed from the 

 same variety which has had a vacation away from 

 home for several years. It has kept in training like a 

 football player. Bolley says that if these conclusions 

 are correct there are probably no unit characters which 

 are not fluctuating and there are no fluctuating characters 

 which may not readily be fixed. 



These results are striking enough, but their signifi- 

 cance depends entirely upon the purity of the strains 

 which were used originally and also upon the preserva- 

 tion of purity during the experiment. Bolley's phrase 

 "ehmination factor," which he uses repeatedly, suggests 

 selection from an impure strain. If his conception is 

 true it could be demonstrated by developing a large 

 majority of resistant individuals among the non-resistant 

 plants which were first subjected to disease attack, 

 rather than merely "a few scrubs." In other words, the 



