6 , 



Predisposition. 



typical characters associated with the quality of rust ^ s ^ a " ce ' 

 would appear therefore to have a share in bringing albout this, re suit. 



rust-resistant wheat, the leaves have a tough cuticle, and the straw 

 is well glared often with a glaucous bloom. The flag is narrow 

 not broad, erect not drooping, stiff and firm, not soft and flabby. 

 Still all these characters may be present, and yet the climatic conditions 

 may overrule them and weaken the power of rust-resistance. It will thus 

 be 'seen that a variety of causes may contribute to rendering a plant im- 

 mune, and that not only the chemical and other properties of protoplasm 

 come into play, but there is also the mutual reaction of the living proto- 

 plasm of host and parasite to be considered. 



As far as the rust fungus is concerned, there is no such thing as a per- 

 fectly immune wheat-plant, for, given the necessary conditions of situation, 

 heat, moisture, and spores at the right season, and ati some time or another 

 rust will appear. We express this by saying that there are rust-resistant 

 wheats, but not " rust-proof." 



Animals are sometimes rendered immune, or, at least less susceptible 

 to disease, by repeated infections, so that they become gradually accus- 

 tomed to the effects of the parasite, and the important question arises, Can 

 plants also be submitted to " protective inoculation " ? Inoculation witl 

 attenuated bacteria is the simplest method with animals, and this treatment 

 retards their vegetative development, and so lessens their injurious effects. 

 This artificially-induced immunity only lasts a certain time, varying; in 

 different cases. Ehrlich and Huebener 1 have shown, from numerous 

 experiments upon animals, that an immunized mother can impart immunity 

 to her offspring, but it is not lasting. 



In the case of plants, as in animals, there seem to be two opposing 

 forces at work. On the one hand, the protoplasm of the fungus is 

 endeavouring to overcome the resistance to its entrance offered by the host- 

 plant, while on the other hand the host is more or less successfully resisting 

 the inroads of the fungus. There are no known cases of a plant becoming 

 "immune" or "partially immune" by inoculation with the parasitic fungus 

 to protect it against further attack, but advantage has been taken of the 

 great sensitiveness of certain spores to copper salts to protect the organism 

 against them. (Note 6, p. 75.) 



Chemotaxis is the name given by Pfeffer 1 to a form of sensitiveness 

 which certain organisms possess towards certain chemical substances. This 

 power, which certain nutritive and other substances have of attracting 

 bacteria and other organisms towards them, is known as -positive 

 chemotaxis, and of organic substances with a high nutritive 

 value which are positively chemofactic, may be mentioned asparagin an< 

 peptone, while sugar, which is one of the best food stuffs and rich( 

 sources of energy, has but little attractive power. The power of othe 

 substances, on the contrary, to repel bacteria, is know as negative chei 

 taxis, and free acids and alkalies, as well as alcohol, have this effe 

 Glycerine, as far as known, is an inactive substance. Massee 9 has stal 

 that immunity is owing to the absence or small proportion of the substance 

 chemotactic to the parasite in the plant not attacked, and if a plant can 

 be impregnated with some substance which is negatively chemotactic, and 

 at the same time does not affect the utility of the plant, then immunity 

 against parasitic fungi may be obtained in this way. 



Laurent 1 , acting on this principle, conducted a series of experiments 

 to test the possibility of producing potatoes which would be proof against 

 Phyto-phthora infestans. He grew very susceptible varieties in pots, to 

 the soil of which sulphate of copper was added, and when the tubers were 

 harvested, some were cut in two and their cut surface placed in contact with 



