June, 1913.] 
The Mosaic Disease of the Tomato. 
155 
Histology. 
Koning (1899 [bj, 1900) made histological studies of mosaiced 
leaves, but says that little is brought to light by microscopic 
examinations. Intercellular cavities occur between the pali¬ 
sade and spongy parenchyma of young and old tissue. In 
some cases he found the chloroplasts disorganized and cell walls 
disappearing. Bouygues (1903) reported the absence of the 
epidermis. In old spots the cell contents had disappeared. 
Woods (1900, p. 17) found that, “a study of the histology of the 
diseased leaves has now revealed a histological difference which 
makes it very clear that the light colored areas are not normal 
and that this difference consists in the fact that in badly diseased 
plants the palisade parenchyma of the light colored areas is not 
developed at all. All of the tissue between the upper and lower 
epidermis consists of a spongy or respiratory parenchyma rather 
more closely packed than normal. In moderately diseased plants 
the palisade parenchyma of the light area is greatly modified. 
Normally the palisade parenchyma cells of a healthy plant are 
from four to six times as long as broad. In a moderately dis¬ 
eased plant, however, the cells are nearly as broad as they are 
long, or at most not more than twice as long as broad. As a 
rule the modified cells of the leaf pass abruptly into the normal 
cells of the green area. ” He also found that the light colored 
areas in both tomato and tobacco contained more than the normal 
amount of starch. Heintzel (1900), does not mention any pecu¬ 
liarities in the palisade cells themselves, but observed the most 
striking differences in the intercellular spaces between the palisade 
cells and the spongy parenchyma of younger and older tissue. 
These intercellular spaces occur in the dark green, bloated regions, 
the older tissue having the larger spaces. He believed these 
spaces were filled with gas, because their dark color disappeared 
when they were put in alcohol. The chloroplasts were con¬ 
gregated irregularly in small groups. Iwanowski (1903) states 
that the green areas bordering the yellow are ‘abnormally healthy’ 
and that such regions show a vigorous development of all cellular 
tissue. The yellow areas on the other hand, are thinner and the 
palisade cells are not so well developed, being very much shortened 
and cuboidal in form. He speaks of intercellular spaces in the 
yellow areas. The chloroplasts in these areas are yellowish and 
while these regions are young, scarcely react to the starch test, 
but eventually all the chloroplasts come to contain as much starch 
as they can hold. 
Tomato .—Westerdijk (1910) says that a microscopic examina¬ 
tion of mosaiced tomato leaves show nothing worthy of mention. 
In the yellow areas the chloroplasts are yellowish and slightly 
smaller and have but little starch. The writer also made his- 
