MOSAIC DISEASE OF TOBACCO. 85 



Roots. 



In the same manner roots of mosaicked and healthy plants were ex- 

 ammed at various times under all conditions of growth and severity of 

 disease, and in every case the root structure was found to be normal. 

 Root tips from healthy and diseased plants showed absolutely no differ- 

 ences in structure. It might be anticipated that, as the disease first mani- 

 fests itself in the dividing cells of the leaves, there might be a supple- 

 mentary differentiation, so to speak, of the meristematic tissue at the 

 growing point of the root, functioning co-ordinately with that of the aerial 

 part of the plant. No such condition was observable, however, and, so 

 far as the writer has been able to find, there is no manifestation of local 

 cell disturbances in the root such as are found in the leaf tissue. 



The causal agent of the disease, however, as has previously been noted, 

 is without question present in all parts of the plant, and it should not be 

 stated that it is confined to those parts which show structural variation. 



Fungi and the Mosaic Disease. 



Almost from the first it has been established that no fungi are asso- 

 ciated with the cause and development of the mosaic disease of tobacco. 

 In no ease where careful work has been conducted under conditions elimina- 

 ting the possibility of accidental infection has any fungus been found 

 associated with the trouble. Cultures of fungi obtained occasionally 

 from leaves have always been traceable to careless manipulation or ex- 

 ternal infection, and the fungus obtained failed to infect healthy plants, 

 no matter what methods of inoculation were used. 



The writer has occasionally obtained cultures on various media such as 

 oat agar, tobacco leaf agar and prune agar, from the tissue of the so- 

 called "rusted" spots which are sometimes a late manifestation of the 

 last stages of the mosaic; but, as with previous investigators, it was found 

 impossible to infect healthy plants from these cultures, either by needle 

 pricks, spraying, or inserting the fungus into incisions in the leaf or stem. 



These experiments with fungi were made merely to demonstrate to the 

 writer's own satisfaction that they could not be the causative agents of 

 the disease, as there might be a possibihty that they were latent in the 

 plant during the earlier stages of the disease and only developed super- 

 ficially during the later stages. 



According to Jenkins ^ and others these rusted spots which are some- 

 times observed are primarily caused by a drying out and disintegration of 

 the cell tissue, which has been weakened -by the disease and which thus 

 forms a suitable medium, under favorable conditions, for the develop- 

 ment of secondary fungi and micro-organisms. This view is also held by 

 the writer as a result of observations extending over a series of years. 



» Jenkins, E. H.: Studies on the Tobacco Crop of Connecticut. Conn. Agr. Exp. Sta. Bui. 

 No. 180, p. 56 (1914). 



