THE MOSAIC DISEASE OF TOBACCO. 31 



concerned with the origin of the disease in the seed bed ; the other with 

 the origin and spread of the disease in the field. Observations at Ar- 

 lington, Va., during the season of 1912, indicate that if suitable insect 

 carriers are active, mosaic plants in a field serve as a source of infec- 

 tion to all surrounding healthy plants. Should the origin of these 

 diseased plants trace back directly to the seed bed, it is plain that any 

 means of keeping mosaic infection from seed beds is at the same time 

 a most important means of controlling the introduction and possible 

 spread of the disease in the field. Should the disease originate 

 directly in the field, the matter of successful control would be very 

 difficult. 



In various field experiments different investigators have shown 

 conclusively that laborers in the practical operations of worming, 

 suckering, and topping tobacco plants readily communicate infection 

 from diseased to healthy plants. For this reason it is evident that the 

 prompt removal of all mosaic plants is a most important means of 

 preventing the spread of the disease in the field by human agencies. 

 Since insect agencies may also act as carriers of infection in the field, 

 the early removal of mosaic plants serves as an important check to 

 the spread of the disease from this source. In whatever manner in- 

 fection may be carried to healthy plants, it is obvious that mosaic 

 plants must always be considered infection centers so long as they 

 are allowed to remain in a field. 



NATURE OF THE DISEASE. 



Various theories have been advanced to explain the primary origin 

 of the mosaic disease of tobacco. The view most generally accepted 

 defines the disease as a disturbance of the enzymatic equilibrium in- 

 duced by unfavorable conditions of growth. An enzymatic disease, 

 is physiological in its nature, has it origin within the protoplasmic 

 complex, and results in a serious and sometimes permanent impair- 

 ment of the assimilative functions. 



Although it has been shown by previous workers that the oxidase 

 and peroxidase content of mosaic leaves is higher than in normal, 

 healthy plants, this fact alone does not warrant its being considered 

 the initial cause of the disease, for it might well be an effect rather 

 than a cause. If it be considered that oxidases play an essential 

 part in the metabolism of the plant, it would seem reasonable to sup- 

 pose that the living cells suffering from lack of nutrition as a result 

 of the disease in question would be stimulated to secrete an increased 

 quantity of these enzyms, for it is a well-recognized physiological 

 principle that cells suffering from lack of nourishment develop an in- 

 creased amount of enzyms. It has been shown in the Office of To- 

 bacco and Plant-Nutrition Investigation, for example, that during 



