VIRUSES 46 



a leaf is shown in ifigure 1. It is now known that this bright yellow spot rep- 

 resents a lesion produced by a mutant of tobacco mosaic virus which arose spon- 

 taneously, ihe development of such spots is fairly common. Jensen has isolated 

 more than ^0 strains of tobacco mosaic virus from such yellow spots. I'hese stra- 

 ins are all slightly different in their biological nature, that is, they produce 

 slightly different symptoms of various sorts. Some of these strains isolated by 

 Jensen have been found to mutate further. At present many more than a hundred 

 strains of tobacco mosaic virus are known. 



Strong pieces of evidence for the possible molecular nature of certain 

 plant viruses are the facts that these materials are very simple chemically and 

 that they may be homogeneous with respect to size and shape. However, not all 

 viruses are homogeneous chemically and uniform in size and shape. In general the 

 viruses which cause diseases of man and animals are relatively more complex. For 

 example, it was shown in a preceding chapter that the size of influenza virus 

 varies about a mean. JJ'urther, this virus was shown to be made up of protein, 

 fat and carbohydrate. There is even some reason to believe that the influenza 

 particle has a cell membrane. In addition to that, it was shown to contain an 

 amount of water comparable to that of ordinary living organisms such as yeast 

 and bacteria. Somewhat similar evidence is available for vaccinia virus. It 

 has been shown to be composed of protein, carbohydrate and fat, and in addition, 

 to contain blotin, riboflavin, copper and other constituents of enzyme systems. 

 This virus also contains water and behaves in some ways as though it had a cell 

 membrane. Thus, the evidence of chemistry and physics, while supporting the hy- 

 pothesis that some viruses may be molecules, also leads to the conclusion that 

 some viruses very definitely resemble organisms more than simple protein mole- 

 cules . 



Aether viruses are molecules or living organisms, one of the Important 

 questions concerning them is how they multiply. Very little evidence has yet 

 been amassed to provide an answer to this question. However, insight into the 

 fundsimental aspects of this matter can be gained from an answer to the simpler 

 question of whether Infection by a virus is induced by the interaction of the 

 host with a single virus particle or with many virus particles, while there is 

 disagreement concerning this matter, considerable experimental evidence can for- 

 tunately be brought to bear upon it. 



Symptoms by viruses are generally of two sorts, localized and systemic. 

 yor example, in dealing with plant viruses, one can have a systemic infection 

 such as is realized when tobacco mosaic virus is allowed to infect a Turkish 

 tobacco plant. The entire plant shows signs of disease. Conversely, when to- 

 bacco mosaic virus is rubbed onto the leaf of a plant of ITlcotiana glutinosa , 

 a weed closely related to tobacco, necrotic lesions are formed on the rubbed 

 leaf. The virus does not spread through the plant. In animal virus diseases 

 a similar differentiation can be made. In a disease like influenza in the human, 

 the symptoms and signs are general. However, in a disease such as Herpes simplex, 

 or the common cold sore, the virus seems to produce local lesions on the face and 

 lips . 



When one Inoculates a susceptible host with virus solutions at different 

 levels of concentration, one invariably finds that at relatively high concentra- 

 tions virtually all of the hosts inoculated succumb to disease. At very low 

 virus concentration, on the other hand, virtually none of the hosts succumb, and 

 at intermediate virus concentrations, some but not all of the host individuals 

 succumb. v*hen local lesions are produced by the virus, then one finds that at 

 high virus concentrations the maximum numbers of lesions is produced; at extreme- 

 ly low concentrations no lesions were produced; and at intermediate concentra- 

 tions intermediate numbers of lesions - depending upon the virus concentrations- 

 are produced. This state of affairs is illustrated by some data obtained with 

 tobacco mosaic virus. In Figure 36 can be seen four leaves of Nlcotiana glutin - 

 osa. 



