134 C. A. KNIGHT 



6. Similar resistance to destruction of infectivity by chemical and physical 

 agents, such as heat, desiccation, hydrogen ion concentration, irradiations, 

 pressure, chemicals, etc. 



7. Coincidence of specific chemical and physical properties: 



a. Particles same size and shape 



b. Same proportions of protein and nucleic acid 



c. Same proportions of nucleotides in nucleic acid components 



d. Same kind of iV-terminal and C-terminal residues 



e. Same number of terminal residues and, hence, same number of sub- 

 units in protein. 



It should be noted that similar, though less extensive, standards were 

 summarized by Findlay (1939). A brief explanation will now be given of each 

 of the above criteria, together with selected references for more detailed 

 information. 



1. Serological Tests 



Precipitin reactions, complement fixation, anaphylactic response, and 

 neutrahzation of infectivity have aU been used to test for the structural 

 similarities which might denote strain relationship between plant viruses. 

 The actual methods are weU described in Bawden's book (1950), 



The precipitin test has been most widely used, probably because of its 

 relative simplicity and directness. If the viruses are closely related, this is 

 indicated by copious precipitation when one of the viruses is mixed with 

 antiserum for the other, and the reverse. If the relationship is not close, 

 lesser amounts of precipitate wiU be observed up to the pomt of no precipi- 

 tate and hence no serological relationship. One of the simplest apphcations 

 of the precipitin test is illustrated in Table I with data obtained with some 



TABLE I 



Peecipitation of Some Strains of Tobacco Mosaic Vmus and of CtxcirMBER 

 Viruses 3 and 4 with Antiserum against Tobacco Mosaic Virus** 



" Degree of precipitation is indicated by the signs. 



