18 BULLETIN 879, U. S. DEPARTMENT OF AGRICULTURE. 
nearly isodiametric and less in number per unit area than in the 
normal leaf. 
The spongy parenchyma of these yellowed areas is also more 
compact and the intercellular spaces smaller than in the green 
portions. 
The chloroplasts of the cells of the yellowed portions are decidedly 
smaller than in normal cells and are often pressed so closely to the 
walls of the cell as to be almost invisible. In contrast to this the 
chloroplasts in the dark-green areas are so large and numerous as to 
seem to fill the cells. The vascular elements of mosaic leaves show 
little variation from the normal. 
FRUITS. 
On the fruits, the dark-green warty growths and the abnormally 
light 3^ellowish green of the surrounding surface are the chief external 
symptoms. Here, the cells directly beneath the epidermis assume 
the same abnormalities that occur in the palisade cells of the dis- 
eased leaves. In the raised green areas, these cells are slightly 
longer and narrower than in normal fruits, and the chloroplasts are 
larger and crowded closely together. In the yellow portions of the 
diseased fruits these subepidermal cells are more nearly square in 
longitudinal section and their chloroplasts are much reduced in size. 
The other tissues of the fruit, including the vascular elements, 
appear to be normal, the raised character of the green tissues being 
evidently a result of hyperplasia rather than hypertrophy. 
CAUSE OF THE DISEASE. 
The cause of cucurbit mosaic, like that of all other infectious 
chlorotic diseases, is unknown. While the infectious character of 
the trouble is readily proved, there is no definite indication of the 
nature of the infective principle or its origin. 
RELATION TO SOIL AND WEATHER CONDITIONS. 
Extended observations on the mosaic of cucumbers in the field 
have shown that the physical or chemical nature of the soil appar- 
ently has no relation to the origin of the disease. Mosaic will de- 
velop with equal rapidity on clay, muck, or sandy soils, and applica- 
tions of lime, wood ashes, stable manure, or commercial fertilizers 
have little appreciable effect. It has also been impossible to con- 
nect environmental conditions with the disease in any way which 
indicates that it originates from any unfavorable condition of soil, 
temperature, or humidity. 1 
1 Recent investigations indicate that environmental conditions, particularly temperature, may affect 
the susceptibility of the plant to mosaic, and it is therefore probable that the rapidity with which the 
disease spreads is partially determined by the conditions of environment. 
