June, 1913.] The Mosaic Disease of the Tomato. 
163 
leaves and shoots to become infected. In regard to the first 
argument of the virus theory,_ we see it is not quite in accord with 
our present knowledge of colloidal diffusion; he eliminates a 
possibility. The second statement is an assumption, rather than 
a known fact, for the behavior of the injected juice is problematical. 
Regarding the amounts of juice required for inoculation he 
says, (1898, p. 5), “a small drop injected into the plant at the 
right place will cause numerous leaves and shoots to become in¬ 
fected. If these diseased areas are then crushed and the juice 
injected into healthy plants they may become diseased. ” From 
the fact that pouring juice upon the soil causes the disease to ap¬ 
pear first upon the youngest leaves, he concludes that the virus 
has a definite course in the plant. He applied juice and pieces 
of diseased tissue to agar plates and allowed the virus to diffuse. 
He carefully separated the upper and lower strata of such agar 
and used it for inoculation purposes and produced the disease 
in each case although the disease appeared more slowly when the 
lower strata was used. It seems strange that this author did 
not get a bacterial growth from such plates as Iwanowski did. 
Lodewijks (1910) hypothesizes a virus in these diseased plants 
which continually disturb merismatic regions. In normal regions 
an antivirus is produced which helps to neutralize the virus, 
like a toxin and an anti-toxin. The formation of this virus 
and anti-virus is influenced by external conditions; when the 
former is produced in excess, the plant becomes mosaiced and if 
the anti-virus is more abundant immunity results. Westerdijk 
(1910) speaks of a vims in tobacco and tomato, but does not 
express her opinion as to their nature. She believes that the 
vims of tobacco is distinct from that of the tomato. She says 
(1910, p. 19), “There are, therefore, two different infectious 
substances; they affect only their respective hosts.” In her 
histological studies she excludes organisms as a cause, saying, 
(p. 8), “No organisms were found, neither in the yellow nor 
blue-green areas. ” 
(3). The Physiological Theory .—Perhaps the most varied, 
but generally accepted theory is the Physiological one. Some 
investigators explain this disease as an enzymic trouble, while 
others simply say that it is of a physiological nature, without 
mentioning any specific factor or group of factors which can be 
definitely correlated with it. Sturgis (1899), in his first work 
on tobacco mosaic states, that artificial injuries or abnormal 
conditions, whereby the functions of the plant are disturbed, 
are probable factors in producing this disease. Soil and atmos¬ 
pheric conditions are important agencies according to his views, 
and he says that mosaic disease is more prevalent in heavy soils. 
Hunger (1902), believes this disease to be physiological, occurring 
when the plants are in a weakened condition, predisposed plants 
