SOME ASPECTS OF THE PLANT VIRUS PROBLEM^ 



By KfiNNCTH M. Smith, D. Sc, Ph. D. 

 Potato Virus Research Station, School of Agriculture, Cambridge 



[With 2 plates] 



There appears to be rather a tendency on the part of botanists to 

 consider the study of plant viruses a dull subject and one without any 

 sure foundation in fact. It is hoped, therefore, in this short article 

 to show that, on the contrary, the subject is not only an intensely 

 interesting one, involving problems of fundamental biological im- 

 portance, but is also of extreme economic importance and that plant 

 virus workers really have a definite problem in hand. 



No one at the present time knows what a virus is, and this un- 

 certainty as to its nature adds, perhaps, to the interest of the study. 

 In speaking of a virus, stress is usually laid upon certain properties 

 which are mainly negative in character such as inability to see the 

 virus with the microscope, impossibility of cultivating the virus on 

 media outside the host, and the fact that viruses cannot be held back 

 by the usual bacteria-proof filters. Improving methods of technique, 

 however, are showing that some of these qualities are merely relative 

 and it is already possible to photograph some viruses by means of 

 the ultraviolet light microscope and to devise filters which will allow 

 viruses to pass or hold them back at will according to the pore size 

 of the filter. 



In speculating upon the nature of viruses, whether of animals or 

 plants, as a whole, it is well to remember that they are a hetero- 

 geneous collection of disease agents, and it is by no means certain 

 that they are necessarily all of the same nature. At one end of the 

 scale is the virus of Psittacosis or parrot fever, the particle-size of 

 which is 250 millimicrons (1 millimicron equals one-millionth of a 

 millimeter) and which is in consequence within the range of the 

 ordinary microscope. This virus appears to have a definite life cycle 

 and is presumably a living organism. At the other end of the scale 

 is the virus of foot-and-mouth disease which has a particle-size of 

 about 10 millimicrons and is only two or three times the size of an 

 oxyhaemoglobin molecule. It is difficult to conceive of this as a 

 living organism. Certain plant viruses are also very small ; the par- 



1 Address to section K, British Association meeting, Norwich. Reprinted by permission 

 from Science rrogres.«, No. 119, January 1936. 



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