1917] 
FREIBERG—-MOSAIC DISEASES 203 
greenhouse records are available for the cool greenhouse, but 
since no heat was supplied, the temperatures were in the 
neighborhood of those recorded in the monthly meteorological 
summaries given above. 
The question, of course, arises as to whether plant b 
is really affected with mosaic. The leaves are not truly 
mottled, but are venated. The malformation is characteristic, 
particularly of some of the new shoots appearing on old dis- 
eased tobacco plants. When discussing the effect of tempera- 
ture on the plants shown in pl. 14, it was stated that the leaves 
passed through the venation stage to true mottling or appar- 
ent recovery. It therefore seems that the venation stage is a 
transitional stage of mosaic, and as far as external appear- 
ances go, it is in this stage that we find plant b in pl. 15, fig. 1. 
The plants shown in pl. 14 assumed a healthy appearance by 
the gradual darkening of all the spaces between the veins, 
while in the case of diseased leaves only some of the veins 
“тип together,’’ the area between others remaining chlorotic 
and the leaf becoming truly mottled. The histology of this 
has not been worked out satisfactorily, but ‘‘recovery’’ in 
strongly venated leaves seems to be effected by a lengthening 
of the palisade cells and a normal elongation and growth of 
the cells in general. In the chlorotic areas the palisade cells 
remain short, division is infrequent, and elongation of the 
cells in general is retarded. The relation of growth to the 
development of the disease is of great importance. 
Another interesting observation on temperature relations 
is the following: Plate 15, fig. 2, shows a plant, the lowest 
shoot of which (c) is slightly mottled. The other shoots at 
the base (а and b) show no mottling whatever. Shoots а and 
b appeared during the time when the temperature of the 
greenhouse was low and sunlight was not abundant. Shoot 
с appeared during the early part of January when the tem- 
perature was higher and the illumination better. The plant 
was taken into the laboratory on January 10 to be photo- 
graphed, and on account of the inclemency of the weather was 
allowed to remain there until January 13, when it was 
observed that some of the large leaves were becoming 
