150 Pathologist U. S. Department of Agriculture 



organism of which has not yet been isolated so as to know its form and 

 mode of life. The infective power of the disease from plant to plant 

 under the artificial conditions of sap mixture has been established with 

 certainty. Under natural conditions there is no plain infection from plant 

 to plant. The seeds of diseased plants can produce sound plants. 



From 1887 until 1894 Smith had been investigating peach 

 yellows, another disease believed today quite positively to be due 

 to a virus, and, although no parasitic bacterium had been isolated, 

 a theory of a bacterial origin had been studied. This will be 

 elaborated further. His interest in Mayer's study centered about 

 the iatter's efforts to demonstrate bacteria in accordance with 

 Koch's canons. None of the bacteria which Mayer found, when 

 inoculated into healthy plants, produced the disease. But the juice 

 of diseased plants, when inoculated, did so, and by 



rubbing up a plainly diseased leaf in a few drops of water, taking up a 

 little of this thick, green emulsion in a glass tube drawn out to capillary 

 size, and sticking it into the thick midrib of an old leaf so that it remained 

 without reaching through to the back side, sound plants became badly 

 diseased in nine cases out of ten. 



Filtration experiments indicated as possible causes of the disease 

 either chemical substances or "organized bodies small enough to 

 pass through the pores of the paper. A clear filtrate was finally 

 obtained by using a double filter," but, wrote Smith,*^- '" fluid passed 

 through this possessed no infective power. Evidently the cause of 

 the disease was filtered out and could not be a chemical ferment, 

 for it is opposed to all known peculiarities of enzymes to be 

 filtered out of solutions. The common method for the concentra- 

 tion of an enzyme, i. e., precipitation with not too strong alcohol 

 from the crude juice and re-solution in water, was tried. This led 

 to no substance which had infective power." Besides, that " an 

 enzyme multiplies from itself" was unheard of. Nor could fungi 

 be the cause since "they must assume at some stage some more 

 easily visible form " and are too large to pass through filter paper. 



In 1892 D. Iwanowski of Russia demonstrated that the virus of 

 tobacco mosaic may be passed through a Chamberland filter with- 

 out losing its power to infect.*^^ By 1893 he had published at least 

 twice on his researches on this malady, but Smith mentioned 

 neither publication in his review of Mayer's work. By 1898 



"''Op. at., 384-385. 



"' E. F. Smith, Fifty years of pathology, op. at., 21. 



