PLANT VIRUS ASSAY IQ 



equally effective in penetrating host tissues. Aggregation was assumed 

 to be reversible; as a concentrated virus suspension is diluted, the ag- 

 gregates break apart. Within the limits of the experimental error the 

 modified equation fitted many dilution series not fitted by the simple 

 equation. An independent summary and discussion of these formula- 

 tions and the ideas on which they were based is given by Lauffer and 

 Price. 



The simple equation for the relations between relative virus concen- 

 tration and numbers of lesions is: 



y = N(i — e-P'^) (i) 



where y is the number of lesions, and A^ the maximum number of 

 lesions that can be produced by increasing the concentration of virus 

 particles in the inoculum. This maximum is equal to the number of 

 entry points, or infectable areas, created by inoculation on the inocu- 

 lated leaves, with which the inoculum effectively makes contact. Its 

 value is supposed to be the same whether or not virus gains access and 

 causes infection through all A^ entry points. The value, e, is the base 

 of natural logarithms, and the composite value pn represents the mean 

 number of virus particles from a sample of undiluted inoculum enter- 

 ing and causing infection at a single one of the TV entry points. The 

 value X is the relative concentration, the inverse of the dilution. 



The postulates on which this formulation rests are ( i ) a single virus 

 particle can cause infection, and (2) the sites where the final acts of 

 infection occur are all equally susceptible. For agreement with the 

 Poisson distribution within the usual limits of experimental error the 

 second postulate need not be strictly true. The following hypothetical 

 series are obtained on the assumption that three types of discrete infec- 

 table areas exist in equal mmibers on a series of leaves (see Table I). 



The numbers of virus particles needed to cause single infections 

 within these three types of susceptible areas are in the ratio i: 2: 4. 

 Series 1 is derived from numbers of lesions produced by inoculation 

 with a series of dilutions of one virus sample on the leaves bearing 

 these three types of infectable areas. They are represented as per- 

 centages of the maximum possible number of lesions (A^). A perfect 

 fit with the Poisson distribution for each type of infectable site is 

 assumed. Series 1 is the mean of three such series with values of 

 pn, 10.24, 20.48, and 40.96. It departs slightly but definitely from 

 series 2, having the same value of A^ and a mean (pn) equal to the 

 average of the mean for the three series (23.89). Suppose, however, 

 that the numbers of lesions from which the values in series 1 were de- 

 rived had been subject to random variation as in series 3. Series 3 was 



