40 A STUDY IN CEREAL RUSTS 



10,000 to 1 to 2,000. They were added to Sach's medium in these pro- 

 portions. Minnesota No. 163 was used for all the trials. Copper sul- 

 fate could not be added in larger amounts than 1 to 5,000, since it 

 stunted the plants when more was added. Copper carbonate could not 

 well be used in greater concentration than 1 to 2,000. Iron sulfate 

 did not dwarf the plants when used at the rate of 1 to 2,000. None of 

 the solutions used diminished the amount of rust very appreciably when 

 used in such concentration as to permit of normal development of the 

 host plant. There was a slightly smaller amount of rust on plants 

 grown in the medium to which copper sulfate had been added in 

 amounts of 1 to 4,000 and 1 to 2,000. However, a very good infection 

 was secured on all of them, even on those which never grew to a greater 

 height than one inch. Neither was there any less mildew on any of 

 the plants. None of these substances, apparently, can immunize wheat 

 against rust, even when used in such concentration as to dwarf the 

 plants to one-sixth their normal size. 



These experiments show that in the case of Puccinia gramims, 

 since it is a very highly specialized, obligate parasite, there is a very 

 intimate relationship between host and parasite, and whatever is con- 

 ducive to the health of host is ordinarily conducive to the vigorous de- 

 velopment of the parasite also. This applies not only to susceptible 

 forms but also to forms uncongenial to a biologic form and to resistant 

 varieties. 



THE NATURE OF RESISTANCE 



The work of Cobb, Eriksson, Ward, and others on the nature of 

 resistance has already been mentioned. The theory which Ward de- 

 duced from his extensive work on parasitism was that there are en- 

 zymes or toxins and antitoxins produced by host or parasite or both. 

 His work on "A Lily Disease" (1888) showed that in all probability 

 Botrytis secretes an enzyme which enables it to live more successfully 

 on the host. Pfeffer had already given the name chemotaxis to the at- 

 traction certain substances seemed to have for certain growing plant 

 parts. Miyoshi (1894, p. 21) claimed to have been able to observe a 

 very definite chemotropism when a Tradescantia leaf was injected 

 with a wheat-leaf decoction and then inoculated with Uredo linearis 

 (Puccinia graminis). The same author decided (1895) that a large 

 number of fungi responded to chemical attraction. Massee (1904, 

 p. 7) attached a great deal of significance to chemotaxis. He asserted 

 that infection depended on the presence in the plant cell of positive 

 chemotactic substances and further that "in the future we shall be justi- 

 fied in defining an immune plant as an individual in which the positive 

 chemotactic substance, necessary for facilitating the entrance of the 

 germ-tubes of a given parasitic fungus into its tissues, is absent." On 



