THE MOSAIC DISEASE OF CUCURBITS. 17 
becomes whitish gray in appearance (PI. II, C). This leafless and 
whitened condition of the basal parts of the stem is one of the most 
easily recognized symptoms in the late stages of the disease on the 
cucumber. Such stems seem more brittle than those of normal 
plants and break more easily. The whitening is due apparently to 
the loss of leaves and resultant exposure to the weather, since it 
occurs on healthy plants if the lower leaves are removed. The cu- 
cumber stem is not mottled, but it often develops a yellowish green 
color, somewhat lighter than the dark green of normal stems. Some 
cucurbits, particularly the squash and pumpkin, occasionally show a 
slightly mottled appearance of the stern in addition to the shortening 
of the internodes found on all cucurbits. This mottling consists of 
pale-green circular patches, about a centimeter in diameter, which 
occasionally have a slightly sunken appearance. 
ROOT SYMPTOMS. 
The roots of mosaic cucumber plants show no external evidence 
of the disease except in its later stages, when the number of finer 
rootlets is much reduced and the larger roots tend to show a faint 
yellow color accompanied by an unusual brittleness. Other cucur- 
bits examined have shown no differences between the roots of 
healthy and diseased plants except for a reduction in the number 
of finer rootlets. 
PATHOLOGICAL ANATOMY. 
ROOTS AND STEMS. 
Neither the roots nor stems of mosaic cucurbits, so far as examined, 
show any internal symptoms of the disease. There are no lesions, 
and sections of these parts have the same appearance as those of 
normal root and stem tissues. 
LEAVES. 
The leaves of mosaic cucurbits exhibit many variations in their 
external symptoms, the size of the yellow areas varying with different 
hosts, but the internal symptoms are essentially the same in all 
cases. The dark-green portions are slightly thicker than the normal 
leaf tissue, and this fact accounts for the blistered and distorted 
appearance which is very marked in the leaves of cucumbers and 
squashes. The intervening yellow areas are somewhat thinner than 
the green portions, but are of nearly the same thickness as the tissue 
of normal leaves. 
When stained sections of the leaf are examined, the palisade cells 
of the green areas are found to be crowded closely together and to be 
somewhat narrower and longer than those of the normal leaf. The 
palisade cells of the yellowed portion, on the other hand, are more 
185118°— 20 2 
