THE MOSAIC DISEASE OF TOBACCO. 3 



inoculation with the virus. In order to study this phase of the ques- 

 tion thoroughly the writer has inoculated tobacco plants at all stages 

 of growth from the cotyledon stage until the plants have matured 

 seed. 



INITIAL SYMPTOMS IN YOUNG PLANTS. 



In very young plants at the two-leaved or three-leaved stage and 

 in much larger plants in the rosette stage the first visible symptom 

 of the mosaic disease usually appears as a slight downward curling 

 and distortion of the smallest, innermost leaves, which at the same 

 time become more or less chlorotic. 



As these leaves increase in size, small, abnormally dark-green spots 

 and areas quite generally make their appearance. If the plants are 

 given plenty of room and an abundance of plant food to stimulate 

 active growth, these dark-green portions of the leaves develop rapidly 

 into large, irregular, crumpled swellings, or blisters, more or less 

 characteristic of the so-called " savoyed appearance." At this period 

 these leaves very frequently grow to a size disproportionate to the rest 

 of the plant, becoming long, narrow, sinuous, or otherwise enlarged, 

 distorted, and irregular. On the other hand, if the young plants 

 affected with the disease are badly crowded and starved in the seed 

 bed, growth proceeds very slowly, so that the leaves may show only 

 a slight mottling. Under such unfavorable conditions of growth tiny 

 plants showing on the leaves the dark-green spots characteristic of 

 the true mosaic disease may remain unchanged in size and appear- 

 ance for months. 



Although young plants sometimes appear to outgrow the marked 

 distortion and deformity affecting the immature leaves at this age, 

 the mottled phase of the disease makes its appearance and persists 

 with more or less intensity until all leaf development has finally 

 ceased. (PI. I, fig. 1, B.) 



DEVELOPMENT IN PLANTS APPROACHING MATURITY. 



Should tobacco plants approaching maturity become infected with 

 the mosaic disease, a characteristic phase known as " mottled-top " 

 or "gray-top" develops. (PL I, fig. 2.) Although the number of 

 leaves affected and the intensity of the markings is considerably in- 

 creased if the disease develops at a comparatively early period in the 

 life of the plant, attenuated mottling in one or two of the small top- 

 most leaves is the only evidence of the malady in plants which 

 develop the disease just prior to final maturity. In many experi- 

 mental tests made by the writer upon greenhouse plants, typical 

 phases of " mottled-top " have almost invariably appeared whenever 

 the mosaic virus has been introduced into healthy plants just as the 

 small, immature flower head was beginning to emerge and to elon- 



