47 



VIRUSES AS ORGANISMS 



FIGURE 36 - LOCAL LESIONS SHOVm BY N. GLUTINOSA LEAVES RUBBED WITH 

 TOBACCO MOSAIC VIRUS SOLUTIONS 07 DIPFERBirT CONCENTRATIONS. (¥ . . 

 Holmes, Bot. Gaz. 87, 39 (1929) }. 



The leaf on the left was rubbed with a tobacco mosaic virus solution containing 

 about a milligram of virus per cubic centimeter. The leaf next to it was rub- 

 bed with a solution l/lO as concentrated, and so on. You can see that the num- 

 ber of local lesions produced is dependent upon the virus concentration. As a 

 generalization, if one carries out a large number of experiments and then plots 

 the niunber of local lesions produced or the fraction of hosts showing systemic 

 infection against the logarithm of the concentration of virus used as innoculum, 

 one obtains sigmoidal curves of the general shape shown in i'igure 37* 



.8 



JL 6 



LOG (X) + LOG (v") 



FIGURE 37 - GRAPH SHOEING PROBABILITY 0? VIRUS 

 INF3CTI0N PLOTTED AS A FUNCTION OF LOGj, g VIRUS 

 COITCENTRATION. SOLID LINE DERIVED FROM LA^'S OF 

 CHANCE, BROKEN LINES DSRI'/ED FROM HOST VARIATION 

 THEORY. (M.A.Lauffer and Vi. C.Price, Arch. Biochen. 

 8, 449 (1945) ). 



ihere are two theoretical approaches which can lead us to curves of this 

 shape. The first is that if one makes the assumption that a virus Infection can 

 be initiated by the chance occurrence of a virus unit in a location on the host 

 which affords a favorable medium for that virus particle, then one can show by 

 an elementary application of the laws of chance that the relationship between 

 the number of infections obtained and the concentration of virus should be that 

 given by the solid line on figure 37. This theoretical approach is based solely 

 upon the assumption that a single virus unit favorably situated can initiate in- 

 fection. It does not in any way depend upon the number of virus units which 

 must be applied to a host before infection is likely. For exeimple, there is con- 

 siderable evidence to indicate that infection can occur in tobacco plants only 

 when virus comes in contact with a cell on the leaf surface which has been in- 

 jured sufficiently to allow the virus particle to enter, but not sufficiently 

 to cause the death of the cell. Thus when one is dealing with infection of to- 

 bacco plants by tobacco mosaic virus, one must consider not the total number of 



