TRANSMISSION AND SPREAD OF VIRUSES 
roots of normal-looking plants. The method of spread, so far as 
known, appears to be as follows: pieces of dead leaves from infectea 
plants, still containing virus in an infective state, fall on to the surface 
of the soil in which the virus finally becomes liberated. In the soil it 
comes into contact with the roots which it enters. We have here an 
interesting parallel to the infection of pigeons with the fowl-pox 
virus; it may be remembered that whilst the drinking water con- 
taining the virus remained free of grit, infection did not take place 
and it was only after the addition of sharp pieces of grit that the virus 
was transmitted. Similarly with the virus of tobacco necrosis, if the 
plants are grown in water culture containing virus, the roots do not 
become infected, but if the plants are in soil or sand, the root hairs 
become damaged during growth movements, and the virus enters 
through the small wounds thus produced. 
There is a virus disease of wheat in the U.S.A., known as rosette, 
which is an apparent example of mechanical contamination, though 
the exact method of infection is still obscure. Healthy wheat seedlings, 
transplanted to soil in which diseased wheat plants have been growing, 
become diseased in their turn. If, however, the soil is sterilized, 
either by heat or other methods, the plants do not become infected. 
Soil-inhabiting insects or other arthropods do not appear to be re- 
sponsible for the spread of the virus and further work is needed on the 
mechanism of infection in this case. 
Transmission by Grafting 
In the case of plants, grafting is a certain means of transmitting 
plant viruses provided organic union between scion and stock is 
effected. Grafting is the only method by which the paracrinkle virus 
from potatoes of the King Edward variety can be transmitted and 
is indeed the method by which it was discovered. 
Transmission of some plant viruses can also be effected by means 
of the parasitic plant, Cuscuta sp., and this is really a kind of grafting. 
By using Cuscuta it is often possible to transmit a virus from a host 
which is unsuitable for study to one which is more convenient. For 
example the virus of cranberry false blossom, which is difficult to study 
in the cranberry, can be transmitted by means of Cuscuta to more 
suitable plants such as the tobacco or tomato plant. This could be 
done in no other way since the virus is not sap-transmissible and has 
a specific insect vector which will not feed upon the tobacco or tomato, 
and grafting between cranberry and these two plants is not feasible. 
2—(T.502) 
