130 C. A. KNIGHT 



although it is true that this relationship may be reversed by changing hosts. 

 It may be germane to mention here that the presence of about 0.5 % 

 mutants in a plant virus preparation, while constituting a factor to be 

 seriously considered in some types of investigation, can be neglected for 

 most of the chemical and physical analyses. 



D. Significance of Mutation 



When a virus mutates, the resulting strain introduces new potentialities 

 of disease. Virus diseases cause enormous damage to crop and ornamental 

 plants, and whatever disease situation is prevalent, it cannot be considered 

 stable. Mutation can occur to provide strains which are more or less invasive, 

 more or less destructive, more or less readily transmitted by vectors, and 

 more or less confined to a particular host plant. Furthermore, mutation may 

 nullify the benefits of breeding "virus-resistant" varieties of plants by pro- 

 ducing strains of virus to which the new hosts are not really resistant. As 

 mentioned previously, it is true that a particular host provides a selective 

 environment so that it is not dominated by one virus strain after another. 

 Nevertheless, chance events do occur in nature which can favor the ascen- 

 dancy of a mutant with all its potentiaHties. 



Despite the considerable importance of the mutation phenomenon in the 

 disease problems associated with plant viruses, mutation may be of even 

 greater significance for its relevance to certain fundamental problems. One 

 of these is the mechanism of virus multipHcation. Since mutation is a dis- 

 continuous alteration of the normal process of virus reproduction, the study 

 of strains may yield vital clues to the basic nature of virus reproduction 

 itself. At least such studies should provide restrictions to which any compre- 

 hensive theory of virus duplication must conform. 



Another area in which the study of plant virus mutation holds forth great 

 promise is in the field of genetics. The plant viruses seem to be chemically 

 and morphologically the simplest entities with genetic function which can be 

 obtained in the quantities and in a state of purity suitable for extensive 

 chemical and physical investigation. Viral strains are ideal here, for they 

 possess different biological properties which it should be possible to relate 

 to specific chemical structural differences. Thus, the \Trus mutants provide 

 systems for a direct investigation of the chemical basis of heredity. 



II. Plant Virus Strains 



A. Characteristics of Strains 



Variants of plant viruses have been shown to differ in a variety of ways 

 from the parent strains (Kunkel, 1947). With respect to disease, a strain may 



