14 BULLETIN 40, U. S. DEPARTMENT OF AGRICULTURE. 



although it has been impossible to produce observable mosaic symp- 

 toms in the male parent. 



After many trials the writer finally secured an abundance of 

 viable seed from a single capsule of Nicotiana tabacum fertilized 

 with pollen of N. viscosum. This hybrid, in size of plant, in leaf and 

 blossom characters, etc., is an intermediate form between the two 

 parents. The most vigorous and persistent methods of inoculating 

 the mosaic virus into plants of this hybrid have invariably failed to 

 produce observable symptoms of the disease. 



These hybrids, like the male parent, N. viscosum, are attacked by a 

 very destructive rot at the points of injury if the mosaic virus is 

 punctured into the stems. 



MOSAIC DISEASE IN DIFFERENT SPECIES OF NICOTIANA. 



In the genus Nicotiana the degree of expression of particular 

 mosaic symptoms depends to a considerable extent upon the species 

 affected with the disease. 



All pink-flowered varieties of Nicotiana usually indicate the pres- 

 ence of the disease by a more or less conspicuously blotched appear- 

 ance of the corollas in addition to various foliage symptoms. So far 

 as the writer's observations extend, no other plants manifest the dis- 

 ease in this characteristic manner. 



The mottled phase of the mosaic disease is particularly pronounced 

 in plants of the species Nicotiana longiflora, N. silvestris, and N. 

 plumbaginifolia. (PL V, fig. 2, and PL VI, fig. 1.) This distinctive 

 appearance of the foliage in its extreme form occurs in N. longvfiora. 

 Likewise, in affected plants of this species the tendency to produce 

 abnormally long, sinuous, ribbonlike leaves is especially marked. 

 In mosaic plants of the species N. longiflora and N. silvestris, as a 

 result of abnormal division in the region of the apical cells, bifur- 

 cated leaves not infrequently arise. These forms have not been 

 observed in any varieties of N. tabacum affected with the disease. 



Nicotiana rustica (from seed known as the D'Amersfort) by ex- 

 perimental inoculation readily becomes affected with the mosaic 

 disease. In plants of this species the development of the disease is 

 particularly malignant, oftentimes in young plants completely dwarf- 

 ing the plants and inhibiting practically all foliage growth and 

 flower formation. (PL VII.) The mottled phase is less evident in 

 mosaic plants of this species than in any other species observed. 

 The " savoyed " appearance of the leaves, however, is an especially 

 striking feature of mosaic in plants of this species. Directly or in- 

 directly, as a result of the disease many affected plants are fre- 

 quently attacked by a progressive decay of the tissues, which some- 

 times involves the entire plant, killing it to the ground. 



