[Vol. 10 



196 ANNALS OF THE MISSOURI BOTANICAL GARDEN 



agency of disease. In the present paper we shall use the term 

 virus in the general sense just referred to. The ability of the 

 agency or " organism" to pass through the pores of a standard 

 Berkefeld or Chamberland filter is the usual criterion. Some 

 would undoubtedly define a virus as an ultramicroscopic organism, 

 probably of bacterial nature. This, however, was not the final 

 view of Beijerinck ('99a) who postulated a "contagium vivum 

 fluidum" as the cause of the mosaic disease of tobacco. While 

 his agar filtration studies may now be regarded as inadequate, 

 the capacity of the infectious agencies of several mosaic diseases 

 to pass through certain standard filters under certain conditions 

 has now been demonstrated by Iwanowski ('92), Beijerinck 

 ('99), Allard ('16), Doolittle ('20), Duggar and Karrer ('21), 

 and others. After summarizing an interesting study of the prop- 

 erties of the virus of tobacco mosaic Allard ('16a) is convinced 

 that "there is every reason to believe that it is an ultramicro- 

 scopic parasite of some kind. " If this evidence of the "filterable" 

 nature of the disease is admitted, it would bring the causal agency 

 into a class possibly composed of a rather miscellaneous group 

 of bodies, since there are well-known analogies in the agents of 

 animal disease. The "virus" view in one form or another has 

 been widely held, but the favorite idea has been an ultramicro- 

 scopic organism. 



THE AMOEBA OR PROTOZOAN THEORY 



It has been pointed out that both Iwanowski and Hunger drew 

 attention to the presence in mosaic plants of bodies which they 

 interpreted as amoeba-like. These, however, were of relatively 

 infrequent occurrence. More recently, in a study of the mosaic 

 disease of sugar cane in the tropics, Matz ('19) has found certain 

 cells of the affected tissue filled with a granular matter. Using 

 reliable cytological methods, Kunkel ('21) confirms the presence 

 of such cells in a mosaic disease of corn, to which, however, he 

 seems very justly to attach no significance. Kunkel does find 

 in the cells of diseased corn peculiar plasma-like bodies in the 

 vicinity of the nucleus. He has also been able to distinguish 

 similar structures in the " diseased " areas of Hippeastrum (Kunkel, 

 '22) also affected with a mosaic disease. This is a very clean-cut 



