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CONTRIBUTIONS TO THE BIOLOGY OF RHIZOBIA. 



II. The motility of Rhizobmm 7mitabil€. 



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In previous communications' I stated that Rhizobium mittabile vas 

 absolutely non-motile. Further investigations have proved this to be 

 incorrect. The former conclusion vras based upon the observation of 

 organisms grown in and upon neutral culture media. In neutral or 

 very slightly alkaline solid beef gelatine or beef agar there is abso- 

 lutely no active motion on the part of the organisms, whether cultures 

 are recent or old. Nor do the organisms undergo any great change in 

 size or form. As soon as the Rhizobia of sweet clover are transferred 

 to acid media they undergo great changes in size and form. They are 

 much smaller and more uniform in size. Branching forms are very 

 rare, and the sporoids or highly refractive peripheral bodies are few 

 and indistinct, but the cytoplasm is quite unequally distributed, as shown 

 in the stained organisms. As a rule the cytoplasm is collected at either 

 end, also at the middle, giving in the stained material the semblance 

 of spores. 



The motion is quite characteristic. It is a rapid, jerky, to-and-fro, 

 and rotary motion and continues during the period of active septation, 

 so that two or three incompletely divided organisms are seen moving 

 about in the liquid media. If the medium is very slightly acid, only 

 a few organisms show motility; or more correctly speaking, perhaps, 

 in such media motion exists for a brief period only. In media which 

 are quite acid most of the organisms show motion. In fact, the num- 

 ber of motile forms is proportionate to the degree of acidity. When 

 motile forms are transferred to neutral or slightly alkaline solid media 

 they develop into large, branching and non-branching, non-motile 

 forms. Early in the experiments the presence of motile forms in what 

 were supposed to be pure cultures of Rhizobia of sweet clover led me 

 to believe I had to deal with contaminations with foreign bacteria. In , 

 fact, whole series of cultures w^ere rejected as a consequence of this 



^Beitrag zur Kenntnis der Rhizobien : Ber. Deutsch. Bot. Gesell. i2:il-i7' ^^94- 

 Contributions to the biology of Rhizobia, I. Rhizobium mutabile in artificial culture 

 media. Box. Gaz. 34 : 109-113. 1902. 



56 [JANUARY 



