INTERFERENCE 3I 



alone. Symptoms will be intermediate between those caused by the 

 two strains separately and the yield of each strain, and the symptoms, 

 will reflect the relative proportions, of the two strains in the inoculum. 

 It has not been established that the two strains multiply together in in- 

 dividual cells, but there is no reason to assume that they cannot. The 

 protection of a plant by one strain against another is a later phenom- 

 enon, produced when one strain has previously multiplied to a high 

 virus content. It seems there is a maximum amount of any one virus 

 that a cell can produce, and if it has already produced this amount with 

 one strain another serologically related one cannot become established 

 and multiply. This phenomenon is one for which tests cannot be made 

 wdth bacterial viruses, because when one strain has multiplied fully, 

 lysis occurs. There is nothing comparable to a systematically infected 

 plant with bacteria, and the apparent differences beween the behaviour 

 of strains of plant viruses, on the one hand, and of bacteriophages, on 

 the other, probably reflect differences in host behaviour. Strains of both 

 kinds of viruses probably interfere with the multiplication of one an- 

 other to a comparable degree. 



Despite the seeming similarity between the plant protection phe- 

 nomenon and that of mutual exclusion, it is in mixed infections wdth 

 unrelated viruses that plant viruses and bacteriophages show greatest 

 differences. There is nothing known with plant viruses that is exactly 

 comparable with the mutual exclusion phenomenon. Various kinds of 

 interactions are known between different plant viruses, but there are no 

 pairs known in which infection with one precludes infection with an- 

 other. The nearest approach is provided by tobacco severe etch virus 

 and potato virus Y; plants infected with the first do not support the 

 multiplication of the second, but the effect is not reciprocal and plants 

 infected \vith virus Y are as susceptible as uninfected plants to severe 

 etch virus. As severe etch virus multiplies, virus Y decreases and the 

 etch virus can replace and supplant virus Y in tissues where it was 

 already fully established. 



Mixed infections with pairs of unrelated viruses are common with 

 plants, but only a few have been studied quantitatively. Often, though 

 not always, such mixed infections produce symptoms that are more 

 severe and of a different type than those produced by either virus acting 

 alone. With some pairs, both viruses reach as high a concentration as 

 they would if present alone; with other pairs, one or other virus may be 

 reduced in amount, whereas in mixed infections of severe etch and 

 dodder latent mosaic virus, the concentration of the dodder virus may 

 be greater than in comparable plants infected with it alone. It is likely 

 that in mixed infections the different viruses occur together in the 



