THE EFFECT OF SOME MOSAIC DISEASES ON CELL STRUCTURE AND ON THE CHLOROPLASTS
Authors: Cook Melville T. 
Published: October 2020

1. This paper is a record of comparative studi es on mosaic and healthy plants of sugar cane, canna, tobacco, tomato and cowpea. They deal entirely with the cell structure and cell contents of the chlorotic areas as compared with the green areas of the same plants and with healthy plants. They do not include studies of lesions. The comparisons in every case were made on tissues of corresponding ages. 2. The cell structure of a very young leaf of a normal plant is very uniform, but the differentiation begins very early and is practically complete long before the leaves are full sized. 3. The chlorotic areas of mosaic leaves are thinner than the green areas of the same leaves or than normal leaves of corresponding ages. 4. The active agent inhibits the differentiation of the cell structure from the time that the two come in contact. If the active agent comes in contact with the meristematic tissue at an early period in its development, the structure remains undifferentiated; if at a later period, the tissues remain in the stage of development at time of attack. 5. The spread of the active agent through the leaf appears to be very uneven, which accounts for the mottling and the variations in development in different parts of the leaf. 6. The reaction of the leaf may depend on the age of the leaf at the time of infection, to the virulence of the virus or to the amount of virus; but the fact that we find variations in both thickness and cell structure in a single leaf and sometimes within a single area indicates that there may be an unequal distribution of the virus throughout the plant or throughout an organ of the plant. This is substantiated by the many records of plants (especially tobacco plants) in which one side only showed the symptoms and of leaves in which the symptoms were more severe on one side than on the other. 7. There is no reason to believe that the cell structure is modified by the virus or active agent. The tissue is inhibited in its development and remains at practically the same stage as when attacked by the virus. There is some growth but it is rarely if ever as rapid as in the green areas or in normal leaves. 8. There is no evidence that the chlorotic area increases in size as a result of the spread of the virus into the green cells adjacent to the chlorotic areas. The chlorotic areas increase in size as a result of cell division and growth of chlorotic areas. 9. The variations in growth of tissues in the bloating of the green areas or pocketing which is so common in mosaic tobacco and to the various deformities in other plants. 10. The development of the plastids is inhibited in both size and number. In the very young, growing leaves, it is difficult to locate them in either fresh or stained material but they develop rapidly with age and exposure to sunlight. The mosaic pattern gradually fades and in many cases cannot be detected in the older leaves. 11. The chloroplasts increase in size and number with age and exposure to sunlight. 12. The author did not find any evidence of disintegration of the chloroplasts. 13. In cases of non-infectious chlorosis, the plants did not become green with age and the chloroplasts did not increase in size and number with age and exposure to sunlight.