298 Journal of Agricultural Research voi. iii. No. 4 



There seems to be no logical reason for considering that something ex- 

 ists in the constitution of all normal tobacco plants which is always 

 capable of producing the mosaic disease in response to suitable condi- 

 tions. This conception does not harmonize with the fact that even when 

 the virus of the mosaic disease is highly diluted and the infective sub- 

 stance becomes immeasurably small it is still capable of initiating the 

 disease when introduced into healthy plants. All evidence at hand points 

 to something in the virus quite extraneous to the protoplasmic constitu- 

 tion of healthy plants. Once introduced into the tissues of such plants, 

 this foreign substance rapidly increases in quantity and becomes actively 

 prejudicial to those physiological activities associated with normal nutri- 

 tion and growth. 



In the opinion of Woods and Heintzel this substance constituting the 

 active, pathogenic principle of the virus of the mosiac disease may be re 

 garded as purely chemical, nonliving, enzymic, and a normal constituent 

 of all healthy tobacco plants. According to Hunger, on the other hand, 

 the disease is caused by a toxic ferment not normally present in the cells 

 of healthy plants, but which develops in response to unfavorable condi- 

 tions of nutrition and growth. These theories are in complete agreement 

 in ascribing to the mosaic disease a spontaneous origin within susceptible 

 plants under favorable conditions. The development of this conception 

 is quite natural if a spontaneous origin is accepted, since this does not 

 admit of a consistent explanation on the basis of parasitism. At the time 

 these theories were evolved, little was known concerning organisms which 

 are smaller than the visible bacteria and yet responsible for parasitic dis- 

 eases. It was known that the visible bacteria could not pass through the 

 pores of certain filters. It was also discovered that passing the virus of 

 certain diseases through these filters did not necessarily deprive it of its 

 power to infect, although visible parasites were no longer present. At 

 this time this characteristic seemed to remove those diseases connected 

 with a filterable virus from that class of diseases definitely established as 

 bacterial in their origin. Until additional facts had been secured an 

 enzymic origin was perhaps the most plausible explanation for those 

 obscure diseases connected with a filterable virus and supposedly capable 

 of a spontaneous origin within certain plants. 



The writer's experiments, however, indicate that the mosaic disease 

 can not be induced to arise spontaneously in healthy plants by the opera- 

 tions of cutting back, repotting, or otherwise subjecting the plants to un- 

 favorable conditions. 



SUMMARY 



The virus of the mosaic disease when diluted to i part in i ,000 of water 

 is quite as effective in producing infection as the original undiluted virus. 

 Attenuation of the virus is indicated in dilutions of i part in lo.cxx) of 

 water. At greater dilutions infection is not likely to occur. 



