MOSAIC DISEASES 147 
artificial agencies. The expressed juices have been care- 
fully examined for the presence of a micro-organism 
likely to cause the disease, but without success. 
In the past many theories have been propounded to 
account for the disease. The first suggested that it was 
of a bacterial nature, and this was supported by Mayer 
(33), Iwanowski (25), Hunger (24), and Boncquet (12). 
Iwanowski states that in the cells of tobacco leaves from 
a diseased plant he found bacteria, amceba-like bodies, 
colourless lamelle, and waxy crystalline deposits. 
The enzymatic theory next found favour with 
investigators, among whom may be mentioned Woods 
(49), Chapman (14), and Freiberg (22). They considered 
the cause of the disease to be an excessive development 
of oxidizing enzymes, which prevented the production 
of the green colouring matter. In this respect the later 
work of Allard in 1916 is interesting. He showed that it 
is possible to destroy the oxidase by hydrogen peroxide 
without destroying the infectiousness of the juice, and 
that on destroying the virus without changing the oxidase 
no infection is obtained. 
The virus theory, which is most generally accepted 
to-day, is the outcome of Beijerinck’s theory (8) of a 
contagium vivum fluidum. Although he first favoured 
the bacterial theory, he later suggested that the infective 
principle is soluble in water. The term “ virus”’ as used 
to-day includes the existence of an ultra-microscopic 
organism or an infective principle of an unknown type 
in the expressed juices of an infected plant. 
Of late years a new theory—the amceba theory—has 
been expounded by Matz (32) and Kunkel (27). In their 
published results they suggest the presence of amceba- 
like bodies as the cause of mosaic disease of the sugar-cane 
and maize. 
Many attempts have been made to isolate the 
causal organism in pure culture on artificial media 
by methods well known to bacteriologists, but without 
success, 
