1919] 



ALLEN — AZOTOBACTER CHROOCOCCUM 3 



and more particularly in the soil humus was difficultly soluble 

 in water. The work of Krzemieniewski has rightly been the 

 subject of much discussion, and no wholly satisfactory ex- 

 planation has been brought forward yet to explain the remark- 

 able results obtained on the addition of humus to Beijerinck's 

 mannite culture solution. The explanations that have been 

 suggested are, however, well worthy of note. 



Kaserer ( '10 ) conceived the idea of humus supplying cer- 

 tain rare or unusual inorganic constituents to the culture 

 medium which were required in very small amounts by 

 Azotobacter, but entirely essential nevertheless to their 

 proper development. After many experiments with media 

 containing iron, aluminium, manganese, and silicon, he con- 

 sidered that he had very nearly duplicated in a synthetic way 

 the remarkable results of Krzemieniewski, which success he 

 attributed to the presence of iron and aluminium silicophos- 

 phates which furnished iron and aluminium in soluble form to 

 the bacteria. He believed, however, that he had not yet at- 

 tained the best possible combination of required mineral nutri- 

 ents. Later ( 'll a ) he elaborated to a considerable extent on 

 this theory, postulating that all bacteria require these rather 

 unusual elements and compounds to a certain extent, but that 

 the requirements in this line of the oligocarbophilous and of 

 the oligonitrophilous forms were especially high. The decoc- 

 tions and broths on which the ordinary saprophytic and path- 

 ogenic bacteria are ordinarily grown contain sufficient of the 

 above compounds to cover the needs of these organisms, hence 

 their presence has remained undetected. He predicts that 

 before the ideal culture medium is attained minute amounts 

 of other substances will have to be introduced, e. g., copper, 

 zinc, arsenic, iron, and titanium. He also believes that the 

 necessity of these unusual elements for green plants cannot 



be denied with certainty. 



Remy and Rosing ('11) made an extended study of the 

 cause of the results obtained from humus by Krzemieniewski. 

 After confirming certain important points in regard to the 

 beneficial action of humus, they proceeded to search for an 

 explanation for such action. Kaserer 's solution containing 



