Abstract1. The mosaic pattern is indistinct on the very young leaves; becomes prominent with the growth of the leaves; then becomes less distinct with age. 2. The chlorotic areas tend to become green with age. This is probably due to the action of the sunlight on the chloroplasts. 3. The chloroplasts in the chlorotic areas are fewer in number and smaller than in the green areas. There is no evidence of disintegration. 4. The tendency of the chlorotic areas to become green with age is due to increase in number and size of the chloroplasts. 5. The chlorotic areas can be detected by cytological studies before the unfolding of the leaves. 6. The chlorotic areas increase in size by cell division and cell growth and not by the infection of surrounding cells or by a disintegration of the chloroplasts of the surrounding cells. 7. The formation of apparently new chlorotic areas in leaves is probably due to very small infected areas which increase in size with the general growth of the cells. 8. Green areas in a chlorotic area can be detected by cytological studies before they can be detected by the unaided eye. 9. The small size and number of chloroplasts is common to both sugar cane and tobacco when infected with mosaic. 10. The nuclei of diseased cane cells are usually enlarged or deformed, but this is not true of tobacco.
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