Bacteria in their Relation to Vegetable Tissue. 25 



Arthur 1 has drawn attention to this point and suggests that these 

 differing degrees of susceptibility are due to physical causes. 



Not only has mechanical exclusion, as a cause of immunity, been 

 shown in the pear-blight disease, but other bacterial diseases, as well, 

 illustrate the operation of this cause. In his inoculation experi- 

 ments upon " La Jaune des Jacinths," Wakker 2 infected, besides a 

 number of susceptible varieties of the hyacinth, one variety (Norma) 

 which was regarded under natural conditions as immune from the 

 disease. This he found succumbed as readily to the malady as the 

 naturally susceptible varieties when the infective material was 

 introduced into the leaf of the plant, showing that the natural immu- 

 nity of the variety was dependent upon its epidermal tissue. 



These two cases, cited above, are examples where immunity of the 

 plant toward a specific parasite is dependent upon a physical means 

 of exclusion, but the same cause is also operative in regard to the 

 general resistance of the plant against all forms of germs. Under 

 ordinary circumstances we do not find that saprophytic organisms, 

 even decomposition bacteria, are able to enter the normal, healthy, 

 intact plant structure, yet we have shown in the preceding pages 

 that these germs when they once get a foothold into the plant tissue 

 are able to survive for a long time, and where the vitality of the 

 host is much reduced may even cause a disorganization of tissue. 



If the thin cutinized layers of epidermal cells afford such an effectual 

 barrier to the entrance of micro-organisms, the more resistant corky 

 layers of the mature plant will be even more efficacious in excluding 

 germ life in general. This is demonstrated by the complete inability 

 of most bacterial diseases 8 to penetrate the resistant tissues of the bark. 

 These act as an efficient barrier against the entrance of any organisms, 

 except through the natural openings (lenticels) and wounds. 



Although the cuticular and cortical layers of the plant function 

 as the chief hindrances to the entrance of germs, fungal as well as 

 bacterial, the fully developed walls of the inner cells also inhibit 

 the spread of pathogenic forms. The young fruit of the pear 

 cannot be successfully infected after it has reached a certain size, as 



'Arthur : Proc. Phil. Ac. Nat. Sc. 1886, p. 340. 



2 Wakker: Arch, norland, d. Sc. ex., T. XXIII, p. 18. 



3 Savastano thinks that B. olese-tuberculosis must make its way through the bark 

 tissue in order to reach the succulent cambium where it thrives. But this idea is 

 conjectural and is not based upon experimental proof. 



