446 STATE BOARD OF AGRICULTURE 



insect transmission of protozoan animal and human diseases. Further, the 

 paucity of definite experimental proof of the absence of visible organisms in 

 plant tissues affected with mosaic, is evident from an examination of the 

 literature. Finally, the apparent ultra-microscopic size of infectious parti- 

 cles which pass through the bacterial filters seems to have been the barrier 

 which has hitherto deterred extensive attempts by cytological methods to 

 demonstrate visible organisms in constant association with mosaic plants. 



In this investigation the finding of definite protozoan organisms in con- 

 stant association with mosaic plants and their absence from healthy ones 

 indicate that they are probably the factor so long sought as the cause of these 

 diseases. The fact that the mosaic diseases are cross-inoculable among 

 groups of closely related plants, and the finding of similar organisms in 

 mosaic bean and clover plants makes this seem even more probable. 



The work of Elmer (10), who claims to have obtained successful results in 

 cross-inoculation studies of mosaic from Cucurbitaceae, Solanaceae, and 

 Leguminosae, will need extensive confirmation under carefully controlled 

 conditions, especially since other investigators have reported negative results 

 from similar attempts, and also because of the diversity of form shown by 

 the flagellates from diseased plants in these different famiUes. The possi- 

 bility of great polymorphism within the same species, and the adaptation of 

 organisms to changed conditions must, however, also be kept in mind. Until 

 these organisms can be isolated and grown in pure culture and inoculated into 

 healthy plants, the evidence of their causal relation to the diseases in question 

 necessarily will be circumstantial. Such studies are now in progress and also 

 cytological work in an attempt to find in the aphids which transmit the 

 causative agents, forms belonging in the life-cycle of these flagellates. 

 Success in this last venture would greatly strengthen the evidence now 

 available. 



Concerning the filterable nature of the infectious agents of mosaic diseases, 

 investigation is much needed of the process by which colloidal and other par- 

 ticles pass through the pores of these bacterial filters. Some of the very 

 slender forms of the bean flagellates, less than 0.3 micron in diameter, might 

 pass through the medium filters. There is also the possibility of '' symplastic " 

 forms somewhere in the life-cycle. In this connection Schaudinn (11) has 

 expressed the opinion that some stages of trypanosomes investigated by him 

 were small enough to pass through bacterial filters. Moore and Breinl (12) 

 have asserted that infected blood retained its infectiousness after filtration. 

 The work of Fry (13) with Trypanosoma hrucei is in line mth these obser- 

 vations. He states that T. hrucei can throw off particles which, when liber- 

 ated in the blood, have a motility of their own. This extrusion of granules 

 he regards as of a vital and not degenerative nature. The filterable nature of 

 certain spirochaetes which, in size, are comparable to the small flagellates 

 present in bean mosaic, and the possibility, as expressed by some bacteriolo- 

 gists, that the organisms which are said to pass through bacterial filters, 

 actually grow through the pores, disclose a field for investigation, which un- 

 doubtedly will yield important results. 



SUMMARY 



Using modern cytological methods, protozoan killing and fixing solutions 

 and protozoan stains, an intensive study has been made of bean mosaic, 

 clover mosaic, tomato mosaic and potato leaf-roll. Definite protozoan 



