NATURE OF VIRUSES 



Virus Diameter or appropriate size (A) 



Psittacosis 4,500 



Vaccinia 2,100 X 2,600 

 Herpes simplex 1,500 



Influenza 1,150 



T-1 bacteriophage 500; tail 1,200 

 Southern bean mosaic 310 



Bushy stunt 260 



Lansing polio 250 



Foot and mouth disease 100 



Tobacco mosaic 2,800 X 120 



Transforming factor 



r — > III pneumococci 3,400 X 85 



bean mosaic, tobacco necrosis, and tomato bushy stunt — are 

 the shapes in solution or suspension accurately known. Of 

 these, the first consists of long rods, 2,800 A, or more, in length, 

 with probably a hexagonal cross section 120 A across. The 

 other three are very accurately spherical. Electron micrographs 

 of dried viruses aboimd, and, from these, three shapes seem 

 to cover all ol)served classes: roughly spherical; rod-like; or 

 sperm-like, with nearly (but nt)t accurately) spherical heads 

 and quite long tails. It is very unwise to suppose that we know 

 enough to generalize as yet. 



Mutation of Viruses 



Viruses are apparently capable of definite mutation. Various 

 means for recognizing these exist, for example, in terms of the 

 symptoms they produce or in terms of the range of host in 

 which they will multiply. There are supposedly 50 types of 

 tobacco mosaic virus. 



Virus mutations are found in various ways. The usual muta- 

 genic agents are effective on viruses, notably ultraviolet light, 

 chemical mutagens, and thermal action. The application of 

 genetic methods of study to viruses is perhaps the most powerful 

 presently available technique. A multiple infection of a bac- 

 terium with two different mutants of a bacterial virus yields 

 differentiatable progeny from which many genetic facts can 



