Jan. IS, "915 



Mosaic Disease of Tobacco 



297 



Table I. — Effect of dilution uj>on the infcctivily of the virus of the mosaic disease of 



(ofcocco— Continued 



These tests show beyond question that the virus of the mosaic disease 

 when diluted to i part in i ,000 of water is quite as efifective in producing 

 infection as the original undiluted virus. A dilution of i part in 10,000, 

 however, gives evidence af attenuation. At greater dilutions than this 

 the chances of infection are very greatly reduced. In dilution experi- 

 ments of this sort, where increasing attenuation of the virus is taking place, 

 it is obvious that no sharp line of demarkation can be found beyond 

 which chances of infection do not exist. Since the solutions are punc- 

 tured into the leaves with a sharp needle, the quantity of virus taken up 

 and actually introduced into the plant tissues must be exceedingly small, 

 especially for the higher dilutions. It is of interest to consider this fact in 

 connection with the enzymic theory of the mosaic disease, which has been 

 advanced to explain the nature of the disease. This theory assumes that 

 the mosaic disease develops when certain oxidizing enzyms normally 

 present in the plant increase in amount or in activity as a result of various 

 external conditions affecting nutrition and growth. 



It is somewhat difficult to reconcile this theory with the fact that a tiny 

 drop of \nrus diluted to i part in 10,000 can readily produce the mosaic 

 disease. It must be assumed that the immeasurably small quantity of 

 oxidizing enzyms carried by this drop is sufficient to increase the normal 

 oxidase content already present in the plant to the extent of a permanent 

 pathological reaction resulting in the mosaic disease. This is highly im- 

 probable, since it is well known that the oxidase content of healthy indi- 

 viduals normally varies to a measurable degree in response to various en- 

 vironmental changes. 

 G0733°— 15 2 



