Bacteria in their Relation to Vegetable Tissue. 19 



narrow limits which form apparently its fixed boundaries. On the 

 other hand, the host-distribution of some diseases is known to be 

 dependent largely upon certain climatic conditions. During years 

 with the normal amount of rainfall or seasons of drought, the fungi 

 only attack the ordinary hosts upon which it is a natural parasite, 

 but when an excessively rainy season supervenes, the conditions 

 being then much more favorable for the development of the parasite, 

 the fungus is reported upon many new hosts, usually, however, 

 allied species within generic or tribal limits. Swingle 1 gives an inter- 

 esting series of results which he found with the Peronosporese on 

 the Euphorbise in Kansas, corroborative of this statement. Here 

 the ordinary boundaries which usually limit the spread of the fungus 

 were broken over during those years that were particularly favorable 

 for the development of the parasite, and the same parasitic species 

 was often found on entirely new hosts. 



These two extremes indicate the impossibility of establishing any 

 hard and fast line for the limits of a disease, and, consequently, the 

 limits of immunity in the latter example would be much wider than 

 in the former. 



So far as is at present known, among the bacterial plant-diseases 

 we have no malady that is able, either in a state of nature or by arti- 

 ficial injection, to produce a pathological condition in plants belong- 

 ing to different natural families. Such a condition may not be 

 impossible, however, and further research may give us examples 

 which have a wider range of host-plants. The majority of them are 

 naturally limited in their distribution, even within generic and often 

 within specific boundaries. 



The two phases to which this exemption of plants from bacteria 

 is due, immunity and resistance, although acting distinctly with 

 reference to different germs, may be resident even in the same plant 

 organism. A plant may be resistant or totally insusceptible toward 

 an ordinary saprophyte or even a bacterial parasite whose host-plant 

 is in a distant family, and yet may or may not possess immunity 

 from another species of bacteria which is a natural parasite upon a 

 closely related species. 



The presentation of a few examples of what is meant by this will 

 suffice to illustrate this distinction between immunity and resistance. 



'Swingle : Kans. Acad. of Science, Vol. VI, 1887-88. 



