NATURE OF VIRUSES 11 



not SO bad. Lack of accuracy did not prevent the founders of 

 modern pliysics from measuring the charge on the electron, or 

 Avogadro's number, accurately enough to move forward fast 

 in that subject. The i)hysical study of viruses is today his- 

 torically where atomic physics was in 1900. 



This dilution technique is often all that is available for 

 animal virus work. For bacterial viruses a very powerful method 

 is available. The idea used exploits the fact that virus infection 

 of a bacterium results in dissolution, or ly.si.s, of the bacterium, 

 producing a clear solution. If agar jelly, nutrient medium, and 

 bacteria are mixed together and poured into a thin layer on a 

 glass petri dish, the result will be a whitish, cloudy growth of 

 bacteria. If a .small number of virus particles are added to a 

 similar uiixture and it is poured, then, after about 8 hr, the 

 cloudy growth will ai)pear, but around each virus there will be 

 a clear spot, or plaque, which is readily seen. The number of 

 these plaques is, then, an exact count of the viruses. This is a 

 method of tremendous sensitivity, comparing very favorably 

 with any of the techniques of physics. As will be seen later, 

 many elaborations of this means of assay can be devised. Of 

 course, to cover a wide range, dilution must also be employed, 

 but this can be done quite accurately. 



For plant viruses, a method which is rather similar in char- 

 acter, but unfortunately not so simple or accurate, is available. 

 This is the method of local lesion counting. Some host plants 

 are so sensitive to virus infection that, as the virus spreads, the 

 plant is locally killed, and the infection then ceases because it 

 has been over-efficient. The killed ])atches, or local lesions, are 

 then easily seen and counted. The difficulty in the method lies 

 in plant variability and in the serious problem of cell invasion. 

 Virus will not penetrate a plant cell until it is damaged, and the 

 damage must not be too severe or the cell dies. The technique 

 used is to rul) a little fine Carborundum powder over the leaf 

 and then follow with a smearing of the virus solution to be 

 assayed. Half leaves can be used for comparison. In addition, 

 plant viruses show a marked tendency to aggregation. The 

 theoretical basis for lesion counting is discussed by Bald (in 



