VARIATION AND ITS CHEMICAL CORRELATES 137 



This test was used nicely by Holmes (1941) to help demonstrate that a 

 Plantago virus was actually a strain of tobacco mosaic virus. 



6. Similar Resistance to DestrvMion of Infectivity 



The earhest studies on the properties of plant viruses (Stanley, 1938; 

 Smith, 1939, 1957; Bawden, 1950) showed considerable disparity in the 

 resistance of different viruses to chemical and physical agents. However, 

 viruses of the same group show remarkably similar, though often not 

 identical, ability to survive treatment with various chemical and physical 

 agents. This, then, becomes another standard by which strain relationship or 

 lack thereof can be judged. This criterion, like those of similarity of method 

 of transmission and similarity of host range, is less specific than the immuno- 

 logical standards, but when pooled with other evidence can be of consider- 

 able weight. 



Some of the treatments to which viruses might respond in characteristic 

 manner include subjection to heat, pressure, irradiation, desiccation, and 

 chemicals (especially salts of heavy metals, oxidizing agents, formaldehyde, 

 enzjnnes, acids, and bases). 



7. Coincidence of Specific Chemical and Physical Properties 



Plant viruses possess a variety of sizes and shapes, including forms which 

 appear in electron micrographs as stiff or sinuous rods, spheroids, and par- 

 ticles with hexagonal contour (Williams, 1954). However, individual plant 

 viruses or strains of a virus are characterized by particles which are so 

 similar in size and shape that they are not distinguished at present by 

 behavior in the analytical ultracentrifuge. X-ray scattering, or electron micro- 

 scopy (Stanley, 1943; Bernal and Fankuchen, 1937, 1941; Knight and Oster, 

 1947; Steere, 1953). It is possible, of course, for two unrelated viruses to have 

 particles of the same size and shape, although very few instances of this kind 

 are known at present; therefore, with this reservation in mind, a coincidence 

 of particle size and shape for two viruses is a good piece of evidence for strain 

 relationship. 



No highly purified plant virus appears at present to contain constituents 

 other than nucleic acid and protein. Just as different viruses vary in size and 

 shape, so also do they differ in their proportions of nucleic acid and protein. 

 The viruses of the tobacco mosaic group contain only about 5 % nucleic acid, 

 whereas turnip yeUow mosaic and tobacco ringspot viruses contain about 

 35 %; other viruses contain intermediate amounts. The significant point is 

 that virus strains appear to contain the same proportions of nucleic acid and 

 protein, and hence analyses for these constituents can help to characterize a 

 virus. 



