20 S. G. WILDMAN 



are left then with the probability that TMV must travel via the protoplasmic 

 bridges (plasmadesmeta), which comiect plant cells with each other, and 

 which represent a route whereby the necessity for penetrating the cell wall 

 could be circumvented. 



Information on the size and frequency of plasmadesmetal comiections is 

 too scanty and unsatisfactory to permit more than the impression that their 

 diameter might be less than the long axis of an intact TMV particle (0.3 /u,), 

 which would perhaps require orientation of virus rods before they could 

 escape from one cell into another, I believe it is more plausible to thuik of 

 transfer being accomplished by infectious nucleic acid, particularly since 

 e\'idence from radiation inactivation experiments suggests that mtact virus 

 particles are not present during the earliest stages of the multiphcation of the 

 infectious material. One piece of unpublished experimental information may 

 bear on the transfer problem. When the area of still green cells which sur- 

 romids the necrotic area of the lesion, but which is in the throes of degenera- 

 tive changes, is dissected, and tested for the presence of extractable infect- 

 ivity, the cells display much less infectivity than a comparable group of cells 

 that are part of the necrotic area. Further confirmation of this finding could 

 be interpreted to signify that nucleic acid has indeed invaded the cells and 

 induced the degeneration as the result of its midtiplicative activity, but the 

 infectious material has not yet acquired a protective coat of protein sub- 

 units, with the result that the infectivity is vulnerable to inactivation during 

 the extraction process. Consistent with this notion is the fact that infectious 

 nucleic acid derived from extracellular TMV^ is highly susceptible to mactiva- 

 tion by juices derived from ruptured plant cells. 



M. Is Tobacco Mosaic Virus Extracted from Lesions Identical with 

 Tobacco Mosaic Virus Extracted from Systemic Hosts? 



Our knowledge of the chemical composition, physical behavior, and con- 

 struction of TMV is, as far as I am aware, confined to virus that has been 

 extracted from systemic infections. The question remains as to whether the 

 virus formed in a lesion is in all respects identical with that derived from 

 systemic hosts. We have some mipubhshed information to suggest con- 

 formity. By extracting a large number of lesions, the presence of rod-shaped 

 particles could be ascertained by electron microscopy. The infectivity was 

 removed from a solution by centrifugation in a manner comparable to the 

 sedimentation behavior of TMV extracted from a systemic host. Lesion virus 



^ X-ray inactivation experiments by Ginoza and Norman (1957) and Laufifer et al. 

 (1956), indicate the molecular weight of infectious nucleic acid to be of the magnitude 

 of 2 million. Gierer (1957), on the basis of inactivation experiments with RNAase, also 

 arrived at this order of size. Ginoza (1958) has presented a provocative conception of a 

 TMV particle constructed from a single strand of nucleic acid. 



