1917] 
FREIBERG—-MOSAIC DISEASES 205 
conditions. Plants grown on plots or in the field mature and 
die or are cut down or killed by frost, which also precludes 
the possibility of observing total or apparent recovery. 
RELATION oF LIGHT то THE Mosaic DISEASE 
The effect of intensity of illumination or shading upon the 
development of the disease has been studied by Westerdijk 
(710), Sturgis (’00), Chapman (713), and others. А determi- 
nation of the effect of colored light upon the mosaic disease 
of tobacco was attempted in the work of Lodewijks ('10) and 
more recently in the experiments of Chapman (716). The 
work of Lodewijks, repeated by Chapman, is in brief as 
follows: 
It was desired to note the effect of red and blue light upon 
the development of the mosaic disease. The tops of badly 
mottled plants were therefore covered with hoods made of 
red and blue cloth which were allowed to remain for about 
30 days. At the end of this period it was noticed that shoots 
under red hoods were somewhat less mottled, while those 
under blue hoods showed little or no evidence of the disease. 
It is very evident, however, that a red or a blue cloth hood 
does not give us a red or blue light, and all that can be ex- 
pected in these cases is a difference in the shading effect of 
these hoods. In order to determine the effect of different 
light waves, it would be necessary to grow the plants under 
glass as nearly monochromatic as possible. The results re- 
ported by Lodewijks and Chapman are entirely in accord with 
what one would expect if plants were shaded. The red hood 
would shade the plants to a certain extent, growth and meta- 
bolic activity would be less than normal, and the disease would 
be less pronounced. The blue hood, however, would absorb 
more light than the red hood, the shading effect would there- 
fore be greater, and the mottling would be reduced corre- 
spondingly. 
The effect of light on the development of the disease is no 
meager problem. There are at least two distinct phases. It 
is a well-known fact that the mere absorption of light by 
