66 II. FUNDAMENTAL STRUCTURE OF PROTOPLASM 



hand can form longer threads through end-to-end association. It seems 

 possible, however, that thread-like particles of approximately 0.3 jj, in 

 length are the most common and stable. Knight and Oster (45) claimed 

 that the most majority of virus particles (20 to 40 per cent) fall into a 

 group size of 15x280 m/i and less than 5 per cent being extraordinarily 

 short or long. Rawling et al. (46) also published the opinion that the 

 majority of particles are about 0.3 jj. in length, the longer particles 

 being aggregations of these and the shorter units being due to breakage 

 of some of the rods during the drying of the specimen before examina- 

 tion in the electron microscope. 



4. Various Shapes of Plant Viruses 



Many plant virus particles appear, as mentioned above, to be rod- 

 shaped, but this never holds true for all the plant viruses. It is gene- 

 rally accepted that the particles of plant viruses such as those of 

 southern bean mosaic, tomato bushy stunt, tobacco necrosis, and turnip 

 yellow mosaic, are of globular form having diameters of approximately 

 30 TCifx (32) (47). If virus particles are produced from elementary bodies 

 of protoplasm on the alteration in its structure by a virus, there should 

 be an intimate correlation between the rod-shaped particles and these 

 globular ones. 



In the study on the effect of high frequency sound vibrations on 

 tobacco-mosaic virus, Oster (48) found that the vibrations break the 

 virus rods, reducing the length first to 140 m/^, then to 70m/z until at 

 the end of 64 minutes of continuous exposure to the vibrations one 

 eighth of the original length being the most common. If the molecular 

 weight of the unit protein forming the rod is 150,000, then the particles 

 having one eighth of the original length will be composed of protein 

 fragments whose molecular weight is approximately 20,000 ; this value 

 appears to be close to the molecular weight of Svedberg's unit, 17,600. 

 If protein of certain cells was liable to break down into such units by 

 the disturbance in its structure on the infection with a virus and there- 

 by if the virus action was retained in these fragments, then the virus 

 would be obtained as such extremely minute particles. 



Oster suggested that only particles of 280 m/t in length were in- 

 fectious since a falling off of infectivity seemed concomitant with the 

 breaking down of the original rods. However, as discussed in the 

 previous Part, even with tobacco-mosaic virus there are evidences that 

 the particles broken to a certain extent can retain some infectivity. If 

 protoplasm proteins in some plant cells affected by certain viruses other 

 than tobacco-mosaic virus were readily decomposable to smaller mole- 



