MOSAIC DISEASES 145 
a general rule only a small proportion of infected plants 
go “blind,” the majority continuing to produce foliage 
bearing typical lesions throughout the season. It has 
been observed that infected plants may produce normal 
foliage after a time, but they cannot be said to have re- 
covered because their juices are still infectious. Infected 
plants continue to bear fruit during the season, but the 
crop is considerably less than that from healthy plants. 
The flowers of diseased plants are frequently smaller and 
paler in colour than normal flowers, and mottling of the 
fruits may occur in advanced stages of the disease. 
As in the case of the tomato, the resistance of the host 
plant and its environmental conditions are important 
factors in determining the severity of the external 
symptoms. Thus the variety Butcher’s Disease Resister 
grown in this country shows only slight symptoms in 
comparison with other varieties commonly cultivated. 
The temperature of the soil and air, as will be shown later, 
are also important factors influencing the progress of the 
disease. 
Pathological Anatomy of Diseased Plants 
A critical microscopic examination of the tissues of 
plants suffering from mosaic disease reveals several 
interesting facts. Woods (50) in 1902 was the first to 
observe structural differences between the light and dark 
green parts of mosaic-infected leaves. He found that 
the palisade cells of the lighter areas did not develop 
normally but were cuboidal in shape, being only a third 
or half as long as the palisade cells from the dark areas. 
Also, the cells from the lighter areas contained an abnormal 
amount of starch. Recently Dickson (17), working with 
rapidly growing leaves of highly infected plants of 
different varieties, obtained interesting results. He 
found that the thickness of the light areas of the leaf was 
in the majority of cases only two-thirds that of the dark 
areas, whilst the dark green areas were rarely thicker 
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