THE MOSAIC DISEASE OF TOBACCO. 



so that the leaf is little more than a twisted midrib. In other in- 

 stances a long, sinuous, ribbonlike leaf is produced by the growth of 

 a very narrow and irregular leaf blade (PL II, fig. 1, C.) A very 

 peculiar and irregular leaf shape sometimes arise when the lamina 

 fails to develop beyond certain abnormally thickened veins, which 

 become the margins of the leaf. (PI. II, fig. 1, D.) 



A very distinct and irregular phase of mottling is usually produced 

 in immature leaves of young plants by the development of abnormally 

 dark-green swellings, or blisters. These surface prominences may 

 be so small as to appear like mere dots, or so large and confluent as 

 to involve the greater portion of the leaf, and they sometimes occur 

 in definite lines along the midrib or lateral veins of a leaf. Other 

 leaves may appear perfectly normal in color, except for a few dots 

 scattered irregularly here and there. (PL II, fig. 2, A, B, and 0.) 



In later manifestations of the disease the immature top leaves and 

 suckers become beautifully mottled; hence, the term "mottled-top." 

 This phase of mottling is typically developed in upper leaves which 

 have almost completed their growth and is not generally associated 

 with the development of surface swellings, or blisters. 



Individual plants inoculated at the same age and in identically the 

 same manner with the same virus frequently exhibit markedly dis- 

 similar phases of the disease. In one plant the disease is of a par- 

 ticularly malignant type, greatly dwarfing and distorting all the 

 leaves ; in another plant the symptoms may be shown only as a slight 

 mottling of the leaves throughout the growth of the plant. 



BLOSSOMS OF DISEASED PLANTS. 



The presence of the mosaic disease in all varieties of Nicotiana 

 iabacum usually affects the intensity and distribution of the pink 

 coloration of the blossoms in much the same manner that it affects 

 the intensity and distribution of the green coloration of the leaves. 

 For this reason an examination of the blossoms affords one of the 

 surest indications of the presence of the disease in a tobacco plant. 

 So far as the writer is aware, this distinctive appearance has not 

 before been mentioned. As a rule, in blossoms of mosaic plants the 

 normal pink coloration is present only in lines, specks, or conspicuous 

 blotches. In most instances these markings are very irregular in dis- 

 tribution, sometimes involving a portion or all of the lobes of the 

 corolla. (PL III, fig. 1.) 



A rather striking and symmetrical color pattern is sometimes 

 afforded by a blossom in which the normal pink coloration occurs 

 only as a fine line in the sinus of each corolla lobe. Other affected 

 plants produce blossoms all of which are devoid of color, so that 

 they appear quite white or very pale. More rarely, the presence of 



