4492 ALLARD: Mosaic DISEASE OF TOBACCO 
Since in the process of topping, suckering, etc., all mosaic 
portions removed from the plants are thrown upon the ground, 
Van Bijlert was convinced that the path checked the further spread 
of the disease through the soil by preventing rains from washing the 
virus of this material to the roots of neighboring healthy plants. 
He considers that coolies are very largely responsible for the 
wholesale spread of mosaic in a field by carrying infection on their 
hands from plant to plant during the usual field operations of 
topping, suckering, etc. | 
Van Bijlert strongly recommended the laying out of paths one 
meter wide around experimental plats, not only to afford an easy 
means of access to-differént portions of the experimental field, but 
also to serve as an important means of preventing the spread of 
mosaic. 
Koning (15), in 1899, largely confirmed the conclusions of 
previous investigators in Europe. In extensive field experiments 
he proved conclusively that in ordinary topping operations the 
disease is readily transferred from diseased to healthy plants. In 
fact, he claimed that in this manner as high as 88 per cent. of the 
healthy plants became affected with the disease. He likewise 
claimed that in some instances kainit and Thomas slag tended to 
diminish the extent of the disease. 
Woods (18), in 1899, made a study of various morphological 
and physiological differences between healthy and discolored 
tissues in leaves affected with the mosaic disease and later (29) 
brought out certain facts relative to the disease, namely, that it is 
infectious, that excision of affected parts does not check its de- 
velopment in other parts, that the virus is generally distributed 
throughout the plant, and that interlacing root systems do not _ 
necessarily communicate the disease from a mosaic plant to a 
healthy plant. Like Sturgis (17) he concluded that the disease 
was a physiological response to certain unfavorable conditions. 
Woods went considerably farther than Sturgis, since he sought 
to define the actual pathological changes induced in plants as a 
result of this malnutrition, which he thought must involve the 
normal enzym activity of the plants. With regard to this point 
Woods (29) makes himself clear as follows: 
“The disease is not due to parasites of any kind, but is the 
