32 BAWDEN-DELBRUCK 



same cells, but only with tobacco mosaic and severe etch viruses is there 

 positive evidence for this. In plants simultaneously infected with these 

 two viruses, intracellular inclusion bodies characteristic of each virus 

 regularly occur in the same cells. 



In summary, it can be said that related strains of bacterial viruses 

 probably interfere with one another in the same manner as do related 

 strains of plant viruses, but the lysis of infected bacteria prevents the 

 testing for any phenomenon comparable to the protection afforded to 

 plants systematically infected with one strain against the effects of a 

 second. Serologically unrelated plant viruses interact with one another 

 in a variety of different ways, but there is nothing similar to the mu- 

 tual exclusion phenomena found with unrelated bacteriophage. This 

 again may reflect host differences rather than differences between the 

 viruses that attack the two kinds of host. The bacterial cell is obviously 

 more drastically affected by infection than cells of higher plants in 

 which systemic infection occurs. The early effects of infection with one 

 bacteriophage probably injure the host metabolism too severely for an- 

 other bacteriophage to be able to multiply. 



COMMENT 

 M. Delbruck 



The foregoing discussion by Mr. Bawden explains satisfactorily the 

 apparent discrepancy between the behaviour in mixed infections of 

 pairs of related plant viruses on one hand and bacterial viruses on the 

 other. In both cases interference occurs of a type which is best ex- 

 plained in terms of competition for intracellular material available in 

 limited amount. In simultaneous infections with related strains of 

 viruses the two viruses prorate the available material and give corre- 

 spondingly reduced crops. 



At first sight this looks like a competition for something that might 

 be termed a "precursor" of virus. However, such an inference would 

 be weakly founded. In the first place, we do not know whether this 

 material is of the nature of building material for the virus or of the 

 nature of equipment necessary for the synthesis of the virus. In the 

 second place, if it is a question of building material, we do not know 

 how large a part of the virus is involved; perhaps it is only a matter of 

 one or another small molecular group in the virus. In the third place, 

 we do not know whether the material is specific, i.e., whether the 

 material for which one group of related viruses compete is or is not the 

 same as that for which another group of related viruses compete. In 



