HOMOGENEITY OF TMV 65 



almost equal to that observed for the principal component composed of 

 particles with lengths calculated to be about 270 ni[i. 



On the other hand, we might test the assumption that the particles 

 have different lengths by attempting to attach limits to the variation in 

 particle lengths in a given solution. This approach has been followed 

 by Williams and co-workers* (in press) using new electron micro- 

 scope techniques and by Schachman (in press) using the ultracentri- 

 fuge. The electron micrographs were made on aliquots of a solution 

 obtained by heating the expressed juice to 56°C. The ultracentrifuge 

 studies were made on solutions prepared in the conventional manner of 

 differential centrifugation. Since one method is used to study particles 

 in the dry state and the other is used to examine collections of particles 

 in solution, it is interesting to compare the results. 



By means of special techniques Williams and co-workers were able 

 to examine micro-drops of dilute solutions in a manner which per- 

 mitted them to measure every particle present in a given drop. Most 

 electron micrographs of tobacco mosaic virus show a large percentage 

 of particles with lengths about 300 m|i and a small percentage of 

 shorter particles of varying lengths as well as a small number of 

 longer particles. In their studies, Williams and co-workers found that 

 70% of the particles seen in the micrographs had a length close to 

 300 m[x. Since each drop pattern contained only about 15 particles 

 they were able to measure all of the particles, and they found that the 

 particles of abnormal length can be summed to equal precisely a small 

 whole-number multiple of the value, 300 m\i. This work required the 

 use of very dilute solutions and the study of many individual patterns, 

 so that the summing of particles would be confined to only a few par- 

 ticles in a given field. Williams and co-workers concluded that over 

 96% of the virus particles exist either as monomers of length about 

 300 mu or as multiples of two or three times this length. Presumably 

 then, the short particles seen in most electron micrographs arise from 

 rupture of 300 mu length particles when a drop containing these par- 

 ticles is dried preparatory to examination in the electron microscope. 



Despite the limitations of the ultracentrifuge in the study of rod- 

 like particles, it seemed of interest to re-examine the problem of the 

 homogeneity of the virus particles with respect to length in the light of 

 the progress in the experimental techniques for the purification and 



The author is indebted to Dr. Robley C. Williams for his permission 

 to use these data before their publication. 



