THE MOSAIC DISEASE OF CUCURBITS. 65 
shortening of the petioles and internodes of the stem. Mosaic 
fruits of the cucumber are mottled with green and yellow and often 
develop dark-green wartlike outgrowths. The Summer Crookneck 
squash also shows a mottled and warted appearance, but the fruits 
of most other cucurbits are little changed. 
Nearly all species and varieties of the genera Cucumis, Cucurbita, 
Lagenaria, Luffa, Momordica, Trichosanthes, Ecb allium, Benineasa, 
Micrampelis, and Sicyos are susceptible to the disease, but the 
Citrullus species seem to be partially resistant. 
The pathologic anatomy of the mosaic leaves shows a distinct 
variation from the normal in the tissues of the mottled leaves. The 
palisade cells of the green portions of such leaves are longer and 
narrower than similar cells in the yellow portions. The spongy 
parenchyma is more compact in the yellowed areas, and the chloro- 
plasts are somewhat smaller than those of the green portions of the 
leaf. Similar differences appear in the cells directly below the epi- 
dermis in mottled cucumber fruits, but all the other tissues of the 
fruit are normal in appearance. The structure of the stems and roots 
of mosaic plants does not differ from that of healthy plants. 
No visible causal organism has been associated with cucurbit 
mosaic, and the disease appears to be unrelated to soil conditions. 
The juice of mosaic plants contains an infective principle, or virus, 
however, which possesses certain definite properties. 
The expressed juice of mosaic plants is rendered noninfectious if 
heated above 70° C. The power of infection is also destroyed by 
formaldehyde, phenol, and copper sulphate in 0.5 per cent solutions 
and by mercuric chlorid in a strength of 1:2,000. A 10 per cent 
solution of chloroform will also render the virus inactive, but neither 
5 per cent chloroform nor 10 per cent toluene are effective. 
The juice of mosaic diseased plants may be diluted to 1 : 10,000 and 
still retain the power of infection. Filtration of the expressed juice 
of mosaic plants^ through a Berkefeld filter does not remove this 
power of infection, but Chamberland filters have rendered the filtrate 
noninfectious. The expressed juice of mosaic plants rarely remains 
infectious longer than 24 to 48 hours, and the virus is rapidly destroyed 
by desiccation. 
The infective principle, as far as it has been determined, possesses 
many properties of a living organism, and it appears possible that the 
disease may be caused by an ultramicroscopic parasite. 
The mosaic is highly infectious and can be produced by intro- 
ducing the expressed juices or crushed tissues of a mosaic plant into 
slight wounds in healthy plants. Inoculations may be made at any 
point in the stem or leaf, including the leaf trichomes. Infection 
can also be produced through the fruit, but has never resulted when 
18511S°— 20 5 
