66 THE PHYSICS OF VIRUSES 



some extent this is due to first-order examination of dried viruses 

 in electron micrographs, wherein a roughly circular appearance 

 is classed as spherical. However, the X-ray scattering studies 

 described in this cha])ter show that, for three ])lant viruses in 

 solution, reasonably accurate spherical shapes are found. This 

 may well be true of many others. 



There are some long, rod-shaped viruses, but these are only 

 found among plant viruses. Among bacterial viruses, a combina- 

 tion of rod and sphere seems to be a plausible description, al- 

 though as careful studies are made, the heads appear to be only 

 very roughly spherical. In addition, the tails do not appear as 

 definite rods but seem to be rather variable both in length and 

 diameter. 



These physical shapes at once pose a sharp challenge to the 

 physicist. Virus can make more virus out of the host. Presuma- 

 bly, to some extent, the formation of dujilicates is by physical 

 forces — Van der Waals, valence, or electroionic. How does a 

 spherical object so influence its surroundings as to generate a 

 second spherical object? And if it doesn't directly do so, how is 

 its structure broken down for multiplication so that many 

 others can be assembled as s})heres from the parts? Physics and 

 physical chemistry has not yet encountered this type of crystal- 

 lization process in which a number of large atomic aggregates of 

 identical size and complex composition are formed from the 

 medium. The temptation is to say that virus multiplication imist 

 consist of rods or plates generating other rods or plates, using 

 the short range forces we already know. But in spite of this 

 "must," nature efficiently jjroduces spherical viruses. It would 

 seem that some extra process must be present. However, it is 

 unwise to invoke such a process until it is clear that the actual 

 multiplicative unit is indeed the spherical virus and not some 

 quite different sub-unit, suitably shaped for the action of 

 physical forces. Research is rapidly bearing down on this most 

 important question. 



References 



For general reference on motion studies, Svedberg and Pedersen's classic 

 The Vltracentrifuge (Oxford University Press, New York, 1940) is excellent. 



