﻿19031 BRIEFER ARTICLES 57 



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erroneous belief. The fact, however, that these cultures retained the 

 general macroscopic characters of those which showed no motile forms 

 led me to believe that they were, after all, not contaminations, but 

 simply the product of a change in media, a conclusion fully verified by 

 subsequent tests and experiments with media varying in degree of 

 acidity. The discovery is also in harmony with recent observations of 

 Arther Meyer,' and has further very important bearings upon my for- 

 mer conception of species of Rhizobia.^ Rhizobia which I formerly 

 described as R, Frankii^ R. Rrafikii minus ^ and R. Frankii majus are 

 in all probability merely motile forms of R, mutabile. But, as yet, the 

 question of species and varieties of Rhizobia is far from settled. The 

 more recent investigations point toward the existence of well-estab- 

 lished varieties of one or at most a few species, of which R, mutabile^ 

 will unquestionably prove to be the chief or dominant type, as it occurs 

 in the root tubercles of the greater number of leguminous plants. 



R. mutabile cultures in acid solid media have a grayish tinge from 

 the very first, which color becomes lighter, finally changing to brown- 

 ish gray, and the cultures become less liquid and tenacious ; otherwise 

 they are as in neutral solid media. Rhizobia also tend toward motil- 

 ity in liquid neutral media. Observations lead to the conclusion that 

 motility is the result of unfavorable conditions as to food supply, on 

 the one hand, and an increased plasticity resulting from an abundance 

 of liquid environment. Motility thus enables these organisms to com- 

 pete more successfully in the struggle for existence. The motile forms 

 are, in all probability, the normal, more highly specialized organisms, 

 while the larger, branching, more polymorphous forms are degenerate 

 types, adapted to a fixed and abundant food supply of the host plant. 

 At least this seems to be a justifiable theoretical conclusion from the 

 purely structural or anatomical point of view. Further experimenta- 

 tion is necessary to determine whether the motile forms are function- 

 ally more highly specialized, and whether these have a greater free 

 nitrogen assimilating power or function. 



The following is a summary of the experiments : 



I. Rhizobium 7nutabile is non-motile in most neutral media, espe- 

 cially in solid neutral media. 



=" Kurze Mittheilung iiber die Begeisselung der Bakterien. Centralbl. f. Bact. u. 

 Parasit. 31:15. 1902. 



^Observations on some American Rhizobia. Bull. Torn Bot. Club 19:203-218. 



1902. 



*The R. Leguminosarum of Frank. 



