1923] 



DUGGAR & KARRER 



195 



\ 



Iwanowski ('03) describes the presence of bacteria-like a: 

 as amoeba-like bodies within the tissues. The bacteria 



described 



of the 



wall, and extremely minute in size. Being the first to demonstrate 

 the filterable character of the tobacco mosaic agency, it is rather 

 interesting that he (Iwanowski, '92) ultimately concluded that 

 bacteria were causally related to the disease. Strangely enough, 

 he did not employ the skill in determining this point that he 

 applied to other aspects of the problem. In recent years, in spite 

 of the rapid advances in the culturing of bacteria, the bacterial 

 view has gained few, if any, consistent adherents. A small nitrate- 

 reducing streptococcus found in mosaic-affected tobacco is briefly 

 referred to by Boncquet ('16, '17). Bacteria-like bodies were 

 also identified by Dickson ('22), and he endeavored to culture 

 the organism. By his method, bits of affected leaves were cut out, 

 and after short disinfection intervals in alcohol and mercuric 



bichloride, crushed in tubes of bouillon 



by 



him there could be no certainty that surface organisms were 

 killed. Dickson, however, secured infection by inoculation from 

 these tubes after clouding occurred. Similar results, as he indicates, 

 might be obtained by this method, in view of the amount of the 

 agency originally inserted, whatever the nature of the infective 

 particles, and the clouding with bacteria may have been entirely 

 from secondary or surface forms. 



It is rather significant that there are so few adherents of the 

 :terial nature of mosaic diseases. On the other hand, there 

 10 great amount of published evidence against the bacterial 



ba 



viewpoint. This is certainly not wholly due to lack of effort to 

 find bacteria, but rather to two facts: first, in at least half a 

 dozen cases personally known to the writers, where extensive 

 studies were made, the negative evidence was considered of too 

 little consequence for publication; and second, acceptance of the 

 filterable organism view tended to discourage search for bacteria. 



tained. 



may be regarded at present as wholly 



THE VIRUS OR FILTERABLE VIRUS THEORY 



The connotation of " virus" is fairly definite, inasmuch as the 

 term is now generally restricted to homologize with a filterable 



