THE ORIGIN OF VIRUSES 



toiiis of Yellows in 1925. Other varieties have become affected as 

 soon as three years, or as late as fifty years, after introduction. In 

 symptoms of variegation it resembles those supposed virus diseases 

 which lead to chlorophyll breakdown in other forms ot Fragaria 

 and Riibus. But it is not transmissible by grafts; and it is transmissible 

 by seeds. 



In inlieritance Yellows resembles the rogues in peas. Thus 82 

 selfed seedlings from 5 healthy plants were aU healthy at 27 months. 



TABLE 19 



THE THREE ORDERS OF TRANSMISSION OF PROTEINS 

 AND THE TWO MODES OF ORIGIN OF VIRUSES 



But of 412 seedlings of affected plants, 21 per cent showed symptoms 

 at 4 months and 82 per cent at 27 months (Morris and Afanasiev). 

 It seems that yellowed plants undergo an irreversible cytoplasmic 

 change. Their chance of doing so must depend on the nuclear 

 genotype, for some seedlings are more quickly aiiected than others. 

 The character of their progeny also depends on whether the cyto- 

 plasm of their germ cells is healthy or diseased. Finally, external 

 conditions, although none have been proved to influence the result, 

 must account for the fact that some members of a clone succumb 

 earlier than others. At a first glance we should say that the Yellows 



219 



