THE BIOCHEMISTKY OF PLANT VIRUSES 91 



viruses, because the isolation of the experimental material was performed 

 at least as carefully as was the isolation of any other plant virus, and it 

 certainly originated from a single lesion. There is also little possibility of 

 the histidine being a terminal grouping on the polypeptide chain which is 

 removed by specific enzyme systems in the tobacco plants, because by 

 altering the growing conditions it is possible to maintain the "bean type" 

 characters in virus grown in tobacco. This may be accomplished by growing 

 the tobacco plants at a somewhat elevated temperature (30°C.), The yield 

 of virus at this temperature is rather low. 



Apart from the purely chemical differences noted above, the two forms of 

 the cowpea virus have markedly different characteristics as determined in 

 other ways. The immunological relationships are fairly remote, the electro- 

 phoretic mobilities are different, and, most striking of all, the X-ray diffraction 

 patterns exhibit differences which are greater than those between the type 

 strain and cucumber virus 4 (Holmes and Frankhn, 1958). When one 

 considers that cucumber virus 4 has been thought by some not to be a strain 

 of the tobacco mosaic virus at all (Knight, 1954), this is all the more 

 surprising. 



0. Strains of the Tobacco Mosaic Virus 



It is now evident that what we recognize as a typical tobacco mosaic virus 

 by its symptomatology is one of a number of strains of this type of virus 

 which have established themselves successfully in commercial tobacco crops. 

 Whether the virus is primarily a parasite of the tobacco plant is a matter 

 of some doubt. Certainly the related ribgTass virus of Holmes (1941) is 

 widespread both in America and in Europe (Harrison, 1956), and it has been 

 suggested that the ribgrass {Plantago lanceolata) was introduced to eastern 

 United States as a weed by the white settlers. It is possible, therefore, that 

 the virus originally met its present host quite recently, just as it appears 

 likely that, in Europe, the bulk of the tobacco mosaic type of viruses affecting 

 the tomato crops originated from smoking tobacco which was infected with 

 tobacco mosaic virus; in the small time in which such an association has been 

 formed, the tomato mosaics have adapted themselves to tomato to such an 

 extent that they are only transmitted with some difl&culty to tobacco, in 

 which they usually only produce a localized infection. 



There are also strains of tobacco mosaic virus which infect numerous 

 leguminous plants in the tropics: one, which has already been mentioned, 

 occurs in cowpea, and another is fomid on Bombay hemp (Crotalaria) 

 (Raychaudhuri, 1947); but from a scientific point of view the most interesting 

 of these odd strains, and possibly the most divergent, is the cucumber virus 3, 

 and its yellow mutant, cucumber virus 4. 



