274 ORIGIN OF STRUCTURES AND FUNCTIONS 



the end of 1935 when W. M. Stanley^'' succeeded in isolating 

 it in crystalline form. This virus, as well as a whole series of 

 analogous viruses producing diseases in higher plants, has 

 been studied by Stanley himself and also by F. C. Bawden, 

 N. W. Pirie, R. Wyckoff, J. D. Bernal, R. Markham, H. 

 Fraenkel-Conrat, G. Schramm and, in the Soviet Union, by 

 V. Ryzhkov, K. Sukhov, P. Agatov, A. Vovk, M. Gol'din 

 and many others. There has also been extensive progress in 

 the study of the bacterial viruses or bacteriophages, and 

 especially of the viruses of animals and man which are of 

 medical importance. The scientific literature on viruses has 

 grown to an immense size. We shall only refer here to a 

 limited number of review works which contain extensive 

 references to the literature. ^^^ 



The great advantage of studying tobacco mosaic virus, as 

 an approach to the solution of a number of general biological 

 questions, lies in the relative simplicity of its composition. 

 While the particles of other viruses such as the animal viruses 

 of the smallpox-psittacosis group contains lipids, carbo- 

 hydrates and other substances as well as nucleoproteins, the 

 crystals of tobacco mosaic virus are composed entirely of 

 nucleoproteins. But, unlike other nucleoproteins which 

 have been isolated from living things, the virus has the 

 specific property that when it is introduced into the living 

 cell of the plant it evokes in the host a turbulent process of 

 biosynthesis of the particular proteins and nucleic acids 

 which are characteristic of the virus but which are absent 

 from the healthy tobacco leaf. In this way the amount of 

 virus in the cells of a large plant may increase many million- 

 fold within a few days. However, nobody has succeeded in 

 producing this so-called ' multiplication ' of virus particles 

 under any other conditions or on any artificial medium. 

 Outside the host organism the virus remains just as inert in 

 this respect as any other nucleoprotein. Not only does it 

 show no sign of metabolism but nobody has yet succeeded 

 in establishing that it has even a simple enzymic effect. It 

 is clear that the biosynthesis of virus nucleoproteins, like 

 that of other proteins, is brought about by a complex of 

 energic, catalytic and structural systems of the living cell 

 of the host plant, and that the virus only alters the course of 



