1922] 



ROSEN — A BACTERIAL DISEASE OF FOXTAIL 353 



shown the importance of hydrogen-ion concentration in the me- 

 tabolism of numerous and diverse organisms. 



Clark in 1915 summed up the matter as follows (p. 109) : "The 

 rate of enzyme action, the stability of colloidal structures upon 

 which cellular life depends, the solubility of many physiologically 

 important compounds, as well as the structure and composition 

 of media and the color of the indicators used in the adjustment 

 of their reactions all are dependent in greater or less degree upon 

 hydrogen-ion concentration." 



While it is absolutely essential that plant pathologists recognize 

 the importance of the determination of hydrogen-ion concentra- 

 tion, yet it should be recognized that other factors are equally as 

 important and when it is desired to know the buffer strength of 

 a medium it even becomes necessary to use total acidity titra- 

 tion. That there is danger of attributing too much to hydrogen- 

 ions has been pointed out in recent articles by Brown (II, '21 a ), 

 Jones (II, '20), and Traube (II, '21). 



In spite of the very considerable amount of data that has ac- 

 cumulated on this subject, many plant pathologists, as has al- 

 ready been stated, have paid little attention to it. With very 

 few exceptions, Wolf and his associates in particular (II, '21), 

 those who have dealt with bacterial plant pathogens have more 

 or less completely ignored all the recent advances in this phase 

 of bacteriology. Having presented data showing the necessity 

 for determining the hydrogen-ion concentration of bacteriological 

 media the writer will attempt to outline very briefly the prin- 

 ciples involved and the methods used. This has already been 

 done in a number of articles and books ; the aim in presenting the 

 matter here is to make available to plant pathologists the infor- 

 mation at hand and to call attention to the more important ref- 

 erences. 



PRINCIPLES INVOLVED IN THE DETERMINATION OF ACID AND 



ALKALI 



To define the terms acid and alkali in relation to the color re- 

 action of some organic indicator is not satisfactory. Thus, to 

 say that a solution is acid because it shows no color in the pres- 

 ence of phenolphthalein does not give much information, since 

 by using another indicator, such as Congo red, the same solution 

 may be said to be alkaline (see Washburn, II, '10). The satis- 

 factory basis for a definition of acidity and alkalinity is furnished 



