592 Third European Joi/rney 



but also their immunity or semi-immunity to crown gall. These 

 tests and examinations also were to last through the years 1924 

 and 1925; and from their results they would publish their paper 

 on Begonia lucerna, "A Begonia Immune to Crowngall: with 

 observations on other immune or semi-immune plants." ^^ All of 

 the begonias were found to contain oxalic acid and to be " rather 

 resistant to crowngall inoculations," but their final conclusion 

 read: 



We started out with the assumption that the pH controlled tumor forma- 

 tion and that no tumors could be produced on plants having; a greater 

 acidity than pH 5.00, but the final experiments show that we have been 

 able to produce crowngall tumors on three begonias having a very much 

 more acid juice reaction than any the organism will tolerate in fluid cul- 

 tures. The only conclusion we have come to is that mass action must control 

 infection, /'. e., if you can once get a tumor started in a tissue, even in a very 

 acid tissue, it will make its own milieu and continue to grow in spite of the 

 inhibiting pH. This probably would be true even on Begotiia lucerna. 

 Considering these results, it is premature to state that no crowngall tumors 

 can be produced on the plants mentioned and supposed hitherto to be 

 immune owing to their acid juice. Further experiments should be made, 

 but it is still believed that crowngall tumors on such plants are non-existent 

 in a state of nature. Other factors than acidity of course may determine 

 resistance in some of these plants. 



Smith's idea that bacterial growth may be inhibited by pH 

 alone is now known to be sound. Under definite conditions, bac- 

 terial growth and gall development in tissue culture are inhibited 

 at about pH 4.5. Oxalic acid, furthermore, is now high on the 

 list of organic acids known to be inhibiting and bactericidal at 

 high concentrations.^' 



Another discovery made in September of 1923 was " some very 

 astonishing fasciations in flower head of a Dahlia." ^^ Immediately 

 he had this photographed, and poured plates for their study. In 

 1921 he had prepared an explanatory note on " Fasciation of plants 

 due to bacterial infection." ^* This was to accompany the pub- 

 lication of four or five photographs of crown gall inoculations on 

 nasturtium. Early in 1922 he had studied at the microscope sections 



'^^Phytopathology 16(8): 491-508, Aug. 1926, and reprint. 



^^ A. J. Riker, Studies on the influence of some environmental factors on the 

 development of crown gall, ]our. Agr. Res. 32(1): 83-96, 1926; Hildebrandt and 

 Riker, op. cit., 83 ; memorandum to author by Dr. Riker. 

 Diary, Sept. 12. 

 Diary, Dec. 23. 



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