66 SCHACHMAN 



Study of the virus. Furthermore, it was important to study solutions 

 of virus particles as a common study to that performed by Williams 

 and co-workers on dried specimens. Boundary spreading experiments 

 show that our recent preparations are much more homogeneous than 

 those studied earlier by others and by ourselves. The enhanced homo- 

 geneity of these preparations is due presumably to the salt concentra- 

 tion used during the isolation and study of the virus. Since the method 

 of purification involved the same type of differential centrifugation as 

 that used previously, particles of % the length of the normal particles 

 would not have been eliminated. Because of the inadequacy of accu- 

 rate diffusion data it cannot be stated categorically from the spreading 

 experiments that the virus is or is not homogeneous. Therefore, another 

 approach was used. It happens that a physical anomaly in ultra- 

 centrifugation magnifies the effect of trailing components in ultra- 

 centrifuge patterns (Johnston and Ogston, 1946). Thus a 50-50 mix- 

 ture of tobacco mosaic virus and a smaller particle might appear to be 

 a 10-90 mixture. This effect seemed to provide a means of detecting 

 small amounts of shorter particles in the presence of large amounts of 

 virus particles. A model substance of particles with lengths about % 

 that of the virus was obtained by chemical degradation. Mixtures of it 

 and virus were then studied in the ultracentrifuge. The results showed 

 clearly that two parts of the % length particle could be detected in the 

 presence of 98 parts of virus. It can, therefore, be stated that our best 

 preparations of tobacco mosaic virus can not contain more than about 

 1 % of % length particles. This test implies that our mixed "impurity" 

 must itself be relatively homogeneous, otherwise we would not detect it. 

 Since most electron micrographs do show % length particles in quanti- 

 ties greater than 1%, it would appear that they arise from breakage of 

 the uniform virus particles. 



In closing, it is important to note that both of these studies were 

 made on preparations obtained from the expressed juice of diseased 

 tobacco plants. Other workers obtain greater yields of virus due to 

 additional treatment of the pulp remaining after expressing the plant 

 juice. Studies of the type described here must be performed on the 

 virus solutions prepared by these other methods before a detailed 

 statement of comparison can be made. 



In summary then, the recent work using new electron microscope 

 techniques and ultracentrifugal analyses indicates that the virus par- 

 ticles obtained from the expressed juice of diseased tobacco plants 

 possess a remarkable degree of uniformity with respect to length. 



