PLANT VIRUSES 53 



available evidence favors the association of infectious quality with the 

 approximately 15 x 300 mfx rod. The longer rods appear to arise by 

 aggregation and the shorter ones by various forms of disruption of the 

 300 m|j, rods. 



Infectious Unit of TMV. — Several million particles of TMV, or of 

 other plant viruses, are usually required to cause an infection, but this 

 is largely attributed to the inefficiency of existing techniques in intro- 

 ducing the viral particles into the susceptible cells. A mathematical 

 analysis by Lauffer and Price indicates that one viral particle is 

 probably sufficient to initiate infection (Arch. Biochem., 1945, 8, 449). 



Relationship of TMV to Host. — Apparently identical TMV is ob- 

 tained from serologically totally unrelated plants such as Turkish 

 tobacco and phlox. (Gaw and Stanley, /. Biol. Chem., 1947, 157, 765.) 

 The virus in each instance has, until the recent work of Wildman, 

 appeared to be a foreign entity quite distinct serologically and other- 

 wise from the normal constituents of the host plants. Wildman's in- 

 vestigations, if substantiated, would indicate some relationship between 

 macromolecular cytoplasmic constituents and TMV. 



Relationship of Plant Viruses to Animal Viruses. — The viruses of 

 all types investigated thus far possess the chemical common denomi- 

 nator of nucleic acid and protein. The most consistent difference be- 

 tween animal and plant viruses seems to be the invariable association of 

 lipid with animal viruses. Of the animal viruses investigated, the Shope 

 papilloma virus most closely resembles the plant viruses in composition 

 since it is almost solely nucleoprotein. No obviously successful cultiva- 

 tion of a plant virus in higher animals or the reverse has been reported. 

 However, the results of Black (Phytopath, 1949, 39, 2,) strongly sup- 

 port the multiplication of plant viruses in insects, and suggest the 

 possibility that some plant virus may yet be demonstrated to multiply 

 in some vertebrate host. 



Mutation of Plant Viruses. — Mutation is common among plant 

 viruses although the frequency of mutation seems to vary considerably 

 with different viruses. No exhaustive studies have been made on the 

 frequency of mutation of TMV, but Kunkel found that under some 

 circumstances the figure may be of the order of 0.5 per cent. (Publica- 

 tion No. 12, A.A.A.S., 22-27, 1940). The mutation of plant viruses 

 results in new strains. Mutation is sometimes followed directly. In 

 other instances, strain relationship between 2 viruses is established 

 logically although somewhat arbitrarily by demonstration of (see, for 

 instance, Holmes, Phytopathology, 1941, 3/, 1089) : 



