1 62 The Ohio Naturalist. [Vol. XIII, No. 8, 



He describes the bacterium which he used successfully for 

 inoculation purposes, but did not make thorough studies of its 

 habits. It is 0.3 mic. long; in fresh cultures it forms quite long 

 threads or chains. It may liquify gelatin under certain conditions, 

 staining it black. He concludes by saying, that the question of 

 the artificial culture of this microbe of mosaic disease needs further 

 study. Hunger (1905), however, reports that he succeeded at 

 times in obtaining minute bodies which he says might be taken 

 for bacteria. But he says (p. 264), "In fact, I was able to obtain 

 minute bodies at times following out the technique in a few cases 

 even the Plasmodium -like bodies. Unfortunately, however, I 

 cannot regard these as bacteria or zoogloa, since it is shown that 

 both of these bodies disappear when phenolchloralhydrate is 

 u.sed in connection with heat, all remaining cell structures remain 

 undisturbed." In a recent article, Allard (1012), believes that 

 Aphids are carriers of mosaic disease in case of tobacco. Accord- 

 ing to his experiments, he would not place this malady in the 

 category of purely physiological diseases. He says, that facts at 

 hand strongly suggest the presence of a living, active micro- 

 organism. 



In order to reach definite conclusions in a pathological problem 

 of this nature, experiments must be conducted on an extensive 

 scale. The organism should be isolated, grown on various media 

 and its cultural characteristics properly recorded. Proper checks 

 with inoculation experiments are absolutely necessary. An ex- 

 periment without accompanying controls is of little value. The 

 original organism must be reisolated after inoculation and its 

 presence conclusively demonstrated in the host, before its con- 

 nection with the disease can be considered established. Inasmuch 

 as this has by no means been accomplished, the bacterial theory 

 cannot be considered as more than a working hypothesis. 



2. The Virus Theory. — The "contagiuin vivium fluidum" 

 or virus theory seems to be a kind of variation of the bacterial 

 theory. Beyerinck (1S9S) abandoned the bacterial theory and 

 proposed this in its place. He says (p. 5), "this is not brought 

 about by a microbe, but through a "contagium vivium fluidum." 

 He regards the virus as a soluble substance and not a corpuscular 

 body. It remains inert in dead organic material, but when mixed 

 with the cell plasma, it increases in quantity, but docs not lose 

 its individuality, hence the name. He regards the Flecken- 

 krankheit of tobacco as a mild form of the disease, largely con- 

 fined to the chloroplasts, while in the more intensive fonns the 

 protojjlast as a whole is involved. His theory is based upon two 

 considerations. (Ij. The virus must be a liquid and not a 

 corjHiscular body, because it diffuses through agar, which is im- 

 possible for a corpuscular body. (2). He believes that it must 

 increase in the plant, because a sinall drop causes numerous 



