22 BALD 



each infectable area to cause an infection. The infectabihty of these 

 areas is assumed to follow a normal frequency distribution. 



Kleczkowski's hypothesis assumes that the type of distortion of the 

 lesion - dilution relationship illustrated in Table I may become much 

 greater because of the greater variability of the infectable areas on the 

 inoculated leaves. Such an explanation does not easily account for the 

 common experience that dilutions from different samples of the same 

 virus inoculated on similar sets of plants may give dissimilar lesion- 

 dilution curves. If Kleczkowski's hypothesis were correct, differences 

 in the form of dilution curves obtained in parallel trials on similar sets 

 of plants should be relatively slight. 



Kleczkowski has shown in experiments with purified samples of 

 virus at concentrations between i and 20 gms. per litre, that numbers 

 of lesions increase with virus concentration far above the level at which 

 the maximum, A^, would usually be supposed to lie. He therefore dis- 

 counts any fit dependent on a value of A^ that would give agreement 

 with a Poisson series or with a Poisson series modified for aggregation. 



Details are given by Kleczkowski of 4 experiments with purified 

 tobacco mosaic virus in which concentrations of 1 to 20 gms. per litre 

 were tested as well as lesser concentrations. The results of these 4 ex- 

 periments are plotted in Figure 1 as log local lesions against log con- 

 centration in grams per litre. The scales for log lesions in each experi- 

 ment were adjusted on the ordinate so that the values for the higher 

 concentrations fell on a single curve relating lesions and concentration. 



The results presented in this way indicate that the 4 samples of 

 tobacco mosaic virus induced a similar progressive increase in relative 

 number of local lesions with increasing concentrations of virus between 

 0.1 and 20 gms. per litre. Coincidence at these concentrations of the 

 curves from the different experiments suggests a common mechanism 

 for the relationship between lesions and concentration. This mechanism 

 is presumably not the same as that inducing dissimilar curves with 

 more dilute virus suspensions. Amounts of 0.1 to 20 gms. of tobacco 

 mosac virus, like somewhat smaller quantities of certain inert materials 

 (see above), alter the physical properties of the inoculum. The virus 

 itself acts as the spreading or wetting agent in the inoculum. The curve 

 in the upper ranges of concentration presumably demonstrates the rise 

 in value of A^ as the wetting power of the inoculum increases. Virus is 

 enabled to make contact with more and more infectable areas on the 

 leaf surface as virus concentration is raised. 



Values of A^ for inocula at lower concentrations may be derived 



