24 H. L. Russell. 



we would not consider these two cases similar. The one is simply 

 the normal resistance which the animal offers to an organism which 

 has no power under any circumstances to produce a diseased condi- 

 tion in its body. The other is a case of immunity to a certain 

 degree against the pathogenic organism. 



We cannot, however, compare the phenomena of animal immunity 

 too closely with that of plants, as we find that the causes which tend 

 to produce this state are unlike in the two kingdoms. A much 

 closer similarity exists between the immunity of plants from bacteria 

 and from fungi. Not only are the phenomena presented quite 

 homologous in character, but the causes which are operative are no 

 doubt more or less closely related. 



It will be hardly possible to classify the causes which may tend 

 to produce the resistance and immunity of the plant from its bacterial 

 foes in any satisfactory and complete manner. A tentative classifi- 

 cation, however, may be suggested along the lines of physical and 

 chemical action ; including under physical sources all those mechan- 

 ical contrivances, such as epidermal covering, cutinization of tissues, 

 and the secondary thickening and lignification of cell membranes, 

 which enable the plant to ward off injurious external forces. Under 

 chemical sources we would class, not only the reaction of the tissues 

 and the nutritive conditions, but the resistant action due to the living 

 protoplasm itself. 



That one of the possible causes of the partial immunity which 

 some plants exercise toward parasitic forms is dependent largely 

 upon the mechanical obstructions which the plant offers to the 

 entrance of the germ, is seen in the examples of immunity which 

 have already been mentioned. 



Certain strains of the common pear, under ordinary conditions of 

 cultivation, are quite refractory toward the blight when subjected to 

 natural infection which takes place in the blossom through insect 

 visitation, but these varieties when artificially inoculated sub-epider- 

 mally with bacteria yield readily to the disease. This can scarcely 

 be accounted for on any other ground than that the blight bacteria 

 are unable to gain a foothold on account of some peculiarity of the 

 external plant membranes. This may be so slight that one cannot 

 detect any histological difference in the exterior cells, yet one variety 

 will be considerably more refractory under natural conditions than 

 the other. 



