The Ohio Naturalist. 
[Vol. XIII, No. 8, 
156 
tological studies of mosaiced tomato, but did not find any char¬ 
acteristic abnormalities. No striking differentiation was seen 
between the yellow and adjoining green or healthy tissues. I 
did not find stages as described by Woods (1900), where the 
palisade parenchyma was undeveloped or the presence of con¬ 
spicuous cuboidal palisade cells as described by Iwanowski (1903) 
for tobacco. Although at times in the yellow areas this tissue 
appeared slightly less developed then usual. The yellow areas were 
slightly thinner than the adjacent green areas, especially in older 
leaves. The epidermis appeared normal. No difference was detected 
in the number or size of the chloroplasts in the yellow and green 
areas. That they were well supplied with starch was apparent 
from the slides and especially in the sections from the older tissue. 
Potato .—Sections of yellow, adjoining green and healthy 
tissue of potato mosaic, were fixed in weak chromacetic fluid 
and imbedded in the usual manner. A microscopic study showed 
that the yellow areas were thinner at all ages; in some cases they 
were only 90 mic. thick as compared with 120 mic. in the normal 
leaf. (See Fig. 1, 2, pi. VIII.) This thinness was largely due to a 
shortening of the palisade cells which were of a striking cuboidal 
form (Fig. 1, pi. VIII). Sections from mottled areas were easily 
distinguished by the shape and size of the palisade cells. The 
cuboidal cells began very abruptly in some sections, while in 
other cases there was an intergradation between them and the 
normal palisade cells. In the yellow areas as a rule, these cells 
were generally quite regular in shape, but sometimes there was 
less regularity. Their length varied from one-half to one-third 
that of normal cells and their thickness was usually slightly 
greater. The spongy parenchyma appeared normal in all areas, 
except that in the yellow regions, there were somewhat fewer 
chloroplasts. Figure 3, pi. VIII, represents a green area, adjoining 
a yellow spot. The palisade cells are slightly shorter than in 
Fig. 2. The chloroplasts throughout the yellow regions in living 
material were a pale yellowish-green, but contained considerable 
starch. 
Characteristics of Mosaic Disease. 
Infectious -Investigators who have conducted inoculation 
experiments with this disease on tobacco find it transmissible 
by means of the juice. Mayer (1886), Sturgis (1899), Hunger 
(1905) and others, have shown that it must be classed as infectious 
rather than contagious, for the mere presence of a diseased plant 
in a healthy plot does not cause the disease to spread. Numerous 
investigators have inserted diseased leaf tissue into healthy plants 
and produced the disease; in grafting healthy and diseased plants, 
similar results were obtained, Iwanowski (1903), Woods (1902) 
and Hunger (1904, 1905). Heintzel (1900) states, that he got 
