108 E. MARKHAM 



The diffusion coefficient (D) of the mixture has been measured approxi- 

 mately and has a value of about 1,1 X 10~' cm.^/sec. This would give a 

 particle diameter in the hydrated state of 38.7 m/i. This is probably an 

 overestimate because X-ray scattering measurements gave a diameter of 

 284 A for the bottom component (the corresponding D would be 1.5 X 10"' 

 cm.^/sec). The top component gives an indication of being hollow, the 

 external and internal diameters being 280 and 210 A. 



Seen on the electron microscope the particles are evidently near spheres, 

 which when examined frozen-dried have a polyhedral appearance. If air- 

 dried they collapse, the top component being less stable than the bottom 

 component. 



Yields and chemical analyses of the virus have not yet been given. It 

 would appear that the material is rather scarce and difficult to deal with, 

 but it is to be hoped that a more detailed investigation of this interesting 

 virus will be made. In particular, the possibility of removing the nucleic acid 

 by a relatively mild treatment would make the virus very useful for X-ray 

 crystallographic studies, if crystals of the virus can be obtained (which 

 should be possible). 



XI. The Tobacco Necrosis Viruses 



The tobacco necrosis viruses were first encountered in the glasshouses 

 used for virus research in Cambridge in 1935 (Smith and Bald, 1935), where 

 they were found affecting the roots of many of the experimental plants. 

 When emulsions of such roots were inoculated on to the leaves of the plants, 

 local necrotic lesions appeared after a few days (Fig. 19). It was characteristic 

 of such lesions that they remained localized and did not cause systemic virus 

 infection. The host range was enormous, almost all plants being susceptible. 

 The origin of the virus was at the time obscure, but it now seems likely 

 that it infected the plants through the soil, which was probably incom- 

 pletely sterilized (the virus stands dry heat at 90°C. in soil without being 

 completely inactivated). 



For many years the tobacco necrosis viruses were thought of as laboratory 

 curiosities, but in the late 1940's it was found that a serious disease, stipple 

 streak, of beans {Phaseolus vulgaris) in Holland was caused by tobacco 

 necrosis (Bawden and van der Want, 1949), and it was also found causing 

 systemic infections of tulips (Kassanis, 1949), Primula obconica (Bawden and 

 Kassanis, 1947), and a number of other plants. The viruses responsible are 

 stable in the soil for long periods of time and are spread about by the com- 

 merce in such perennial plants as tulips and daffodils. The term "tobacco 

 necrosis" was first given to this group of viruses because of the symptom 

 picture in tobacco, but it is now realized that the disease is caused by a 



