4 AN INTRODUCTION TO THE STUDY OF VIRUSES 
this, Bawden and Pirie (1937) showed that the virus was a nucleo- 
protein. They also succeeded in crystallizing the virus of tomato 
bushy stunt which was the first virus to be crystallized in a three- 
dimensional form. Other plant viruses which can form true crystals 
are those of tobacco necrosis, southern bean mosaic, and turnip yellow 
mosaic. The discovery of crystallizable viruses has enabled X-ray 
diffraction studies to be made, and photographs taken on the electron 
microscope of single virus crystals afford information on the molecular 
arrangement of the crystal faces. The application of electron micro- 
scopy to the study of viruses has greatly increased our knowledge of 
the size and shape of virus particles and may perhaps throw some light 
on the multiplication mechanism of viruses. The tissue-culture of 
animal viruses, the egg-membrane technique mentioned above and 
their passage through unusual animal hosts have led to the production 
of valuable vaccines, of which that in use against yellow fever is an 
outstanding example. 
As to the exact nature and origin of viruses, various theories have 
been put forward. One propounded independently by Green (1935) 
and by Laidlaw (1938) visualizes a virus as a retrograde organism 
which, having lost the power of making substances essential to its 
growth, is entirely dependent on the cells of the host for such sub- 
stances. On this theory the viruses have developed parasitism to its 
highest possible degree, and in Laidlaw’s words, are living a 
“borrowed life.” 
Another theory has it that some viruses have no extraneous origin 
at all but are a product of the cell itself. The virus most quoted in . 
support of this theory is that of potato paracrinkle which is present in 
all plants of the potato variety King Edward but has no known natural 
means of spread. It can, however, be transmitted to other non-tolerant 
varieties of potato by grafting and in them it produces a severe disease. 
Some circumstantial evidence in support of the de novo origin of plant 
viruses is given by the unexplained appearance from time to time of 
apparently “new” viruses in plants growing under more or less 
controlled conditions. This question has recently been discussed by 
van der Plank (1948). The whole question of the heterogenesis, or 
de novo origin, of viruses was debated in a symposium held at 
Leeds (1946). 
