FuRTin-R RrsrARCHrs in DisrAsrs oi- Plants ')-l9 



We must expect to continue to find animal and plant parasites with 

 peculiar methous of propayation ditlicult to discover and to fmd parasites 

 much smaller than any microort;anisms now known to us. There are plant 

 viruses, the particles of which are so small that an ordinary haclerium 

 swimming among them would be almost like a whale among minnows, or 

 a Zeppelin among cockchafers. Tlicre is an immense lee-way for living 

 things between the size of the largest molecules and that of the smallest 

 known organisms. There are many hherable virus diseases in plants and 

 more arc being discovered every year. The virus of the tomato streak is 

 exceedingly infectious and sometimes kills in three or four weeks yet we 

 have not been able to isolate any organism. The same is true of the tobacco 

 mosaic and yet the least particle of the juice of a diseased tobacco plant 

 will infect a healthy one, and some of the particles of this virus will pass 

 through a filter with'pores only 3/100 micron in diameter (B. M. Duggar). 



In March and April of that year, he had tried to isolate, by the 

 use of culture media, a parasitic organism from tomato streak. 

 At one time he had what he described as a " gray white bacterium." 

 "Parasite.'*" he asked himself. He described his culture media 

 thus: " Various agars, Cohn, Fermic, steamed potato, milk, litmus 

 milk and bouillon of juice and tissue fragments from the interior 

 of the stem under the stripes taking the utmost care to keep out 

 surface contaminations." Other agar plates with various prepara- 

 tions were used. Miss Brown continued the study when other 

 researches required most of his time. 



In 1926 Smith expressed ''^ some of his final views on this 

 subject. He believed that " possibly, in the end," the " flagellate 

 protozoan diseases," (concerning which there was by that time 

 " quite a literature ") , and virus diseases in plants would be shown 

 to be " one." But he still referred to them as " two groups." 

 Plant virus diseases were to him very " interesting," and he recog- 

 nized that, while the cause or causes yet remained in doubt, much 

 had been " learned respecting signs of these diseases, host plants, 

 and methods of transmission. Scepticism," he said, 



will not down until these assumed flagellates and amoeba can be cultivated 

 on artificial media and reinoculated with production of the disease. AUard 

 (1914-1918) showed that tobacco mosaic is spread by aphides, and we 

 now know that many of these diseases are introduced by the bites or punc- 

 tures of insects, that some of them have several host plants and that in 

 some of these plants they produce no signs. ... So far, trypanosome-like 

 flagellates have been found principally in plants with a milky juice. Atten- 



^* Fifty years of pathology, op. cit., 35-36. 



