INTERFERENCE 33 



bacterial viruses our inability to settle this point is due to the phe- 

 nomenon of mutual exclusion: unrelated viruses simply do not multi- 

 ply wdthin the same cell, therefore we can not tell whether, if they did 

 multiply within the same cell, they would compete for a certain ma- 

 terial. In plant viruses it should be possible in principle to settle this 

 point but it does not seem to me that it has in fact been settled satis- 

 factorily. In the preceding discussion a number of cases are cited in 

 which mixed infection with unrelated strains apparently gave no mu- 

 tual exclusion. Such experiments are not decisive for deciding whether 

 or not mutual exclusion occurs. They correspond to mixed infections 

 of whole bacterial cultures with two unrelated viruses. From such 

 experiments mutual exclusion could never have been inferred, since in 

 whole cultures some cells will yield one virus, some the other, thus 

 obscuring the mutual exclusion phenomenon which concerns the indi- 

 vidual cell. The case of TMV and severe etch, cited by Bawden, where 

 inclusion bodies corresponding to both viruses are regularly seen in the 

 same cell, is convincing, but not sufficient to prove the rule. We should 

 be prepared to find that some pairs of unrelated viruses parasitize dif- 

 ferent subcellular structures, and therefore do not exclude each other. 

 Dr. Bald stressed the contrast between the local lesion technique in 

 plant as opposed to animal viruses. In animal virus assay on egg mem- 

 branes we deal with a continuous layer of infectable cells, while in 

 plant virus assays on leaves only a very limited number of leaf cells 

 may be made directly infectable through injury. Dr. Bald pointed out 

 that this may be the reason for most of the unsatisfactory features in- 

 herent in the method of local lesion counts. It seems to me that this 

 defect implies also an advantage which may not have been sufficiently 

 appreciated. If it is true that the primarily infectable cells with which 

 the inoculum makes contact are few and widely separated, or, if inocu- 

 lation methods can be specifically designed to produce widely spaced 

 lesions, then this method affords an ideal set-up for the study in indi- 

 vidual cells of mutual exclusion, and of other phenomena of mixed 

 infection, like multiplicity reactivation and recombination. The leaf 

 surface may be smeared with a mixture of the two viruses in relatively 

 high concentration so that every infectable cell is actually exposed to 

 both viruses. At the same time one can be certain that only very few 

 cells, well isolated from each other, will be infected. The first genera- 

 tion of mixedly infected (or, at least, mixedly exposed cells) is thus 

 automatically implanted in host tissue suitable for cultivation of the 

 crop of viruses from these primarily affected cells. The analysis of the 

 resulting local lesions would correspond to the analysis of plaque con- 

 tents as used by Hershey and by us in earlier studies of recombination; 

 perhaps it may not even be entirely out of the question to perform 



