58 AN INTRODUCTION TO THE STUDY OF VIRUSES 
stunt and tobacco necrosis viruses, also becomes non-infective after a 
few wecks but full serological activity is retained. From such sap, 
non-infective crystalline nucleoproteins can be isolated that are 
indistinguishable in their physical, chemical, and serological properties 
from normal virus preparations (Bawden, 1943). 
Some plant viruses are inactivated by freezing, but again it depends 
upon the conditions under which the freezing is carried out. Thus 
the virus of tomato bushy stunt, in dialysed and isoelectric solution, 
is inactivated and coagulated by freezing; the more concentrated 
the solution is, the more readily it is coagulated. But at neutrality, 
it is more stable towards freezing than it is at its isoelectric point 
(Bawden and Pirie, 1943). 
In the plant and in extracted sap, this virus is protected from inacti- 
vation but more prolonged freezing under those conditions may 
destroy infectivity without causing denaturation or loss of serological 
activity. 
The virus of tobacco ringspot behaves in a similar way to bushy 
stunt virus when frozen, and tobacco necrosis virus is inactivated in 
the plant, if the leaves are kept dry and frozen for a week or more. 
When purified preparations of tobacco necrosis virus are frozen, the 
solution loses its characteristic opalescence and there is a reduction in 
infectivity and serological activity. 
Bawden and Pirie (1940) have tested the effects of alkali and fifteen 
simple organic substances on three plant viruses, tobacco mosaic virus, 
tomato bushy stunt virus, and potato virus X. The viruses were all 
in purified form. Of these three viruses, that of bushy stunt is the most - 
resistant to denaturation and potato virus X the least. The effects of 
alkali on tobacco mosaic virus are complex; gentle treatment may 
increase infectivity, slightly more severe treatment causes loss of 
infectivity but not loss of serological activity, and more severe treat- 
ment causes loss of all characteristic properties. In the case of bushy 
stunt virus, the range of pH over which inactivation occurs, is wider, 
and crystalline non-infective preparations can be made from alkali- 
treated material. Apparently similar preparations can also be made 
from sap which has “aged” for some months. 
In the presence of alkali, sodium dodecyl sulphate destroys all the 
viruses tested, separating the nucleic acid from the protein. 
The substances tested, besides the above, were urethane, guanidine, 
pyridine, picoline, lutidine, aniline, nicotine, phenol, salicylic acid, 
and benzoic acid. With the exception of nicotine and arginine, all 
