94 AN INTRODUCTION TO THE STUDY OF VIRUSES 
of this is shown by some breeding work on the tobacco plant. The 
virus of tobacco mosaic, which causes a disease in tobacco of great 
economic importance, develops only local lesions without systemic 
spread in a related species, Nicotiana glutinosa. 
The gene which thus localizes the virus has been transferred to the 
tobacco plant so that infection with the virus is confined to the point 
of contamination and the disease is unable to spread from plant to 
plant. 
4. Treatment of Virus-diseased Plants 
Once a plant has become infected with a virus, it is usually impos- 
sible to cure it, but there are one or two exceptions to this rule. In 
certain cases where the infecting virus has a low thermal inactivation 
point, a plant may be cured by subjecting it to heat treatment. Kunkel 
(1936) was able to cure peach trees, infected with the viruses of peach 
yellows, little peach, red suture, and rosette. The trees were kept at 
a temperature of about 35°C for a fortnight or more and the time 
necessary was longer for large trees than for small and it was easier 
to destroy the virus in the top of the tree than in the roots. That the 
virus was actually destroyed was demonstrated by grafting a scion 
from a cured tree on to a healthy one which remained healthy. Cured 
trees could be re-infected with the same virus, which shows that it 
was not a question of attenuation of the virus by the heat. It also 
demonstrates that there is no acquired immunity of the type we have 
discussed in the animal viruses. 
Later, Kunkel (1941) showed that certain plants infected with the 
virus of aster yellows could be cured by similar means. It was only 
certain plants, however, not including the aster, which could survive 
being grown at 40°C for two wecks. 
There seems to be only one case known of the chemotherapy of 
plant viruses. Stoddard (1942) states that he cured buds from peach 
trees affected with X-disease by soaking them in water solutions of 
quinhydrone, urea, and sodium thiosulphate. 
5. Protective Inoculation 
Since, so far as we know, there are no antibodies formed in plants, 
there is no vaccination in the manner applicable to the animal virus 
diseases. There is, however, one form of inoculation possible and it 
is mentioned here, though at the moment its interest is purely academic. 
We have seen in Chapter VIII how one virus will protect a plant from 
