PLANT VIRUS ASSAY 2 1 



where pv is the mean number of aggregates consisting of one or more 

 virus particles that enters to cause infection at a single one of the A^ 

 points. The relation between the number of aggregates causing in- 

 fection and the number of component infectious virus particles is given 

 by the equation 



K(pvx)2 + pvx — pnx = (3) 



V 1 + 4K.pnx — 1 

 I.e. pvx= - 



2K 



The value K gives an estimate of aggregation. Methods of evaluat- 

 ing the maximum number of lesions, A^, the aggregation factor, K, and 

 the concentration factor pn are best outlined by Lauffer and Price. 



The postulates on which this formulation is based are the same as 

 for equation i, except that aggregates of one or more than one virus 

 particle are assumed to have the same chance of entering the plant cells 

 to cause infection. As in the simpler case, however, it is assumed that a 

 single virus particle is capable of causing infection. 



These equations are applicable to the end-to-end aggregation of 

 virus particles, or to any other form of linkage involving only two faces 

 on a virus particle. It has not been proved that an equation of this form 

 may be applied, for example, to aggregation of virus particles in a three 

 dimensional lattice. The equation may not be applicable to such viruses 

 as bushy stunt which crystallise in this way. If equation 2 is applied to 

 a virus which aggregates in an unknown or in a different manner, K 

 should be considered rather as a measure of the distortion of dilution 

 series than as a measure of aggregation. 



This was the point of view adopted even for tobacco mosaic virus 

 before there was good evidence of end-to-end aggregation. However, 

 recent work on the viscosity and sedimentation rates of virus suspen- 

 sions (e.g. by Schachman) provides independent testimony in favor of 

 end-to-end association of tobacco mosaic particles in moderately dilute 

 suspensions and their dissociation on further dilution. Such physical 

 studies on tobacco mosaic virus have provided evidence in support of 

 the original postulate: reversible aggregation of the virus particles does 

 contribute to distortions of the lesion-concentration relationship. 



An alternative explanation for such distortion has recently been 

 suggested by Kleczkowski, who has attempted to fit the results of lesion- 

 virus concentration trials by a single hypothesis. Kleczkowski suggested 

 that the infectable areas on leaves exposed by inoculation with a virus 

 suspension differ in infectability. The chance of infection is considered 

 in terms of the numbers of virus particles that must make contact with 



