SOME REPRESENTATIVE VIRUS DISEASES 33 
for this curious development of concentric rings on the leaves of 
infected plants, though various explanations have been put forward. 
Whatever the explanation, the phenomenon itself is common enough 
and there are at least a dozen unrelated viruses which give rise to this 
type of symptom. In one or two cases, the rings are not confined to 
the leaves. but develop also on the flower petals and even on the 
fruit. 
In both mosaic and ringspot diseases there is often an initial reaction 
on the inoculated leaf or the site of infection. This takes the form of 
local lesions which may be either rings or solid spots of dead cells. 
An internal symptom is also associated with these diseases and is 
known as an “intracellular inclusion.” It consists of a rounded mass 
of cytoplasm, frequently vacuolated and in close association with the 
nucleus. 
Various types of distortion are common in plant virus diseases and 
may be accompanied by mottling in some cases. There is, for example, 
a strain of the tobacco mosaic virus which distorts the leaves instead 
of mottling them. Other viruses give rise to leaf rolling, suppression 
of the leaf blade, distortion of the veins (Plate IV), and the production 
of abnormal or false blossoms. 
As we shall see later in describing some animal virus diseases, tumours, 
and outgrowths of tissue are characteristic of one or two virus infections. 
The commonest type of virus outgrowth in plants is known as an 
enation and consists essentially of a new leaflet growing out from the 
under surface of the normal leaf. Palisade tissue is formed in the veins 
and this proliferates to form the new leaflets. 
Since these enations are organized growth, they cannot be compared 
with the true virus tumours or cancers which occur in animals. There 
is, however, at least one apparently true virus tumour in plants. This 
is caused by the wound tumour virus and affects plants of the legumi- 
nosae (Plate V). This unusual virus was described by Black (1945, 
1946, 1947), who suggests that the following facts are of particular 
importance when comparing this plant virus tumour with the forma- 
tion of tumours generally. No causal micro-organism can be detected 
in the tumour tissue. The disease is not contagious and requires the 
intervention of a specific insect vector for its spread. Apparently several 
factors may play a vital secondary role in starting tumour growth in 
plants invaded by the virus. Wounding is one of these factors; here- 
dity of the host plant is another, whilst the age of the wounded tissue 
also appears to be important. The tumour tissue never reverts to 
