16 H. L. Russell 



phenomena of a complex order. In the appendix to this paper will 

 be found a complete list of all the plant-diseases which are now 

 known to be closely associated with bacteria. A complete com- 

 pilation of this sort has not previously been made for the bacterial 

 diseases of plants, and a tabular review of this field of bacteriology 

 was thought to be desirable. Although the causal relation between 

 a specific organism and a plant-malady has not in every case been 

 satisfactorily and thoroughly demonstrated, there is no doubt but 

 that most of the plant-diseases mentioned may be rightfully ascribed 

 to the ravages of these micro-parasites. When we consider how 

 little attention has been paid to this branch of phytopathology, it 

 is no wonder that our information on this subject is meager. 



The list of diseases, although now limited, is rapidly and con- 

 stantly increasing, so that we may freely predict that with a more 

 thorough and exhaustive study of plant pathology from a bacterio- 

 logical standpoint, the number of diseases will be materially aug- 

 mented. Even now we possess sufficient data to qualify the 

 assertion that plants are not subject to diseases of a bacterial origin. 



A closer study of the general exemption of plant-structures from 

 the attacks of micro-organisms reveals the fact that the phenomena 

 heretofore embraced under the general term of immunity, are not of 

 the same character in all cases. Different phases of this exemption 

 seem to exist. One of these is the reaction of the plant toward 

 micro-organisms in general. The other is the ability of certain plant- 

 structures to withstand the inroads of a particular bacterial parasite. 

 Under the first head, the micro-organism is unable to gain a foot- 

 hold in the tissue of the plant, or, having once gained an entrance 

 accidentally, it is unable to cope successfully with the repellant 

 forces resident in the tissues. There is no susceptibility on the part 

 of the plant toward the germ in question. This is the action which 

 living tissue exerts in general. Where this action is overcome and 

 the micro-organism triumphs, we have the development of disease. 



Now, the ability of a single individual to withstand the attacks of 

 a germ capable of producing a disease in the tissue of another indi- 

 vidual of the same species, is evidently a different action. There 

 is a certain degree of susceptibility on the part of the plant to suc- 

 cumb to this enemy, as is evidenced by the fact that when the con- 

 ditions which maintain the natural balance of the forces inhibiting 

 the germ are disturbed, the germ is then able to successfully attack 



