18 H. L. Rmsell 



increased powers of development to the attacking organism. It is 

 possible that the same set of causes may produce this double effect, 

 as, for instance, such meteorological conditions as excessive moisture, 

 to the extent that it not only interferes with the normal action of 

 the vital processes of the plant, but gives also the optimum condi- 

 tions for the development of the parasite. But while the resultant 

 of these forces may have a doubly deleterious action on the plant, it 

 does not of necessity follow that this should be the case. 



It will have been seen that the resistance which a plant offers 

 toward its enemies in general is broad and wide-reaching in its effects ; 

 not so with immunity in regard to a specific disease. The one is a 

 general condition of normal, healthy vegetable life ; the other, the 

 expression of a restricted group of vegetable organisms toward the 

 cause of a specific malady. The term immunity will then be restricted 

 to those plant-organisms which do not succumb to the action of a 

 germ that is able to call forth a genuine infection in another related 

 species. Thus we may define Immunity in plants as the ability of 

 the organism to repel the attacks of a germ which produces a path- 

 ological condition in a closely allied form. By a closely allied form, 

 we mean a species or variety which stands in close taxonomic affinity 

 with the form which is a natural host for the germ in question. 

 Now, the limit of this immunity must, of necessity, be a somewhat 

 variable one. Whether the term should be restricted in its applica- 

 tion to those species grouped in the same genus or subgenus, or 

 whether it should embrace the limits of a whole family, will differ in 

 different cases. 



Our knowledge of the ability of bacterial germs to produce in 

 plants a diseased condition in different species is yet somewhat 

 limited, but the observations already on record are further strength- 

 ened by analogous cases with fungal diseases. 



We know that some fungi are very restricted in their development, 

 and that outside of a single host-plant they are unable to call forth 

 any diseased condition, even under the most favorable opportunities. 

 This is the case with Fusicladium pirinumj which causes the destruc- 

 tive pear-scab both in this country and Europe. Sorauer 1 claims 

 that under no conditions, even during the seasons which are most 

 favorable for the growth of the fungus, does it ever exceed the 



1 Sorauer : Landw. Versuchsstat. XXVI, 327. 



