l8 BALD 



withdrawn through the protoplasmic bridges into neighboring un- 

 injured cells. 



At the same time as the wounds are made, moieties of the inoculated 

 virus suspension must make contact with them. The number of con- 

 tacts will depend on the wetability of the plant cuticle and the spread- 

 ing power of the applied inoculum. When inoculum is rubbed over 

 a leaf surface during the process of inoculation, there is not complete 

 contact between the film of liquid and the leaf surface. The number 

 of suitable entry points made by rubbing the leaf may be greater 

 than the number with which the inoculum actually makes contact. 

 Evidence of this is obtainable by adding to the inoculum soluble or 

 insoluble substances that improve its wetting power without otherwise 

 affecting the infection process. As the wetting power of the inoculum 

 increases, the numbers of lesions rise, presumably because the inocolum 

 makes contact with more suitable points of entry into the epidermal 

 cells. If equal amounts of the inert wetting agent are added to in- 

 oculum diluted to different virus concentrations, the rise in numbers of 

 lesions is proportionately the same at all dilutions. 



Presumably, once contact is made between virus suspension and 

 the protoplast of a wounded cell, the physiological condition of the 

 plant will affect (a) the degree of entry of virus particles into cells, 

 (b) the chances of a virus particle reaching those regions of the cell or 

 attaching itself to those molecular configurations in the host proto- 

 plasm, which allow virus multiplication to occur. Experiments on the 

 effects of light on host plants before and after inoculation indicate sig- 

 nificant alterations in the reaction of host plants to infection. These 

 and other experiments also indicate that, of the many virus particles 

 in the moieties of inoculum that make contact with the naked proto- 

 plasts of host cells, only a small proportion infect and multiply. 



There is an obvious analogy between the initiation by viruses of 

 local lesions on leaves of susceptible host plants, and the development 

 of bacterial colonies on solid media. This analogy led Youden, Beale 

 and Guthrie, and Bald to formulate the relation between relative con- 

 centration of virus and numbers of lesions, on the assumption that 

 numbers of lesions would conform to the term of a Poisson distribution. 

 Some samples of virus when tested by dilution, inoculation, and lesion 

 counts conformed very well. Some did not. To cover cases of diver- 

 gence, Bald derived a second equation on the assumption that at higher 

 concentrations virus particles may be more or less aggregated, and 

 that single particles and aggregates of more than one particle are 



