1922] 



ROSEN — A BACTERIAL DISEASE OF FOXTAIL 349 



and media were used which seemed worthy of trial. The aim 

 has been to utilize the current methods of plant pathologists and 

 at the same time take cognizance of technique employed by bac- 

 teriologists in general and by the medical men in particular. 



The necessity of utilizing methods which are not commonly 

 used by plant pathologists became evident early in the work, 

 and having in mind the busy life of the ordinary pathologist, it 

 will be necessary to indicate the reason for the adoption of any 

 new scheme. 



Using nutrient agar testing +10 or +15 on Fuller's scale (10 

 or 15 parts per 1000) the writer found that from diseased foxtail 

 material he would at times obtain bacterial colonies which pro- 

 duced a very conspicuous reaction, a colorless zone followed by a 

 marked whitish area (pi. 28, fig. 1), and then perhaps at the 

 same time, on other plates, or at other times, using the same 

 medium, colonies were obtained without any such features. Mor- 

 phologically, the colonies as well as the individual bacteria ap- 

 peared exactly alike, and inoculation experiments showed that 

 both were pathogens producing the same kind of spot. Two 

 explanations for this variation seemed plausible, first, that there 

 existed 2 strains of the same species, one producing a precipitate 

 and the other not, and second, that the conditions in the nutrient; 

 media were different. It was soon found that a colony that pro- 

 duced a white precipitate may, on transferring to another plate 

 or tube, show no such character. It appeared therefore that there 

 must be some slight difference in culture media even though it 

 was made up at the same time and received the same treatment 

 throughout. 



In an effort to solve this, and mindful of the extensive data 

 that were accumulating on hydrogen-ion concentration in rela- 

 tion to various phases of bacterial activity, the writer instead 

 of titrating by the Fuller scheme, using phenolphthalein as the 

 indicator, used the indicators recommended by Clark (II, '20) 

 in which the color change with reference to acidity or alkalinity 

 had been definitely worked out in terms of hydrogen-ion concen- 

 tration. The color obtained by adding a few drops of indicator, 

 like brom thymol blue, to a tube of melted, nutrient agar was 



then compared with the color chart given by Clark (following p. 



40), and the color in the series for brom thymol blue matching 



best with the tube was taken as the hydrogen-ion concentration 



in terms of Ph. This of course is a rough way of measuring true 



