BACTERIA FIXING ATMOSPHERIC NITROGEN 131 



on artificial culture media in the presence of acid phosphate, 136 sodium 

 succinate and glycerol, 137 caffeine 138 and cumarine. 139 According to 

 Barthel, 140 caffeine and other vegetable alkaloids, like guanidine, pyridine 

 and chinoline, will stimulate the formation of involution forms in pure 

 culture; he suggested, therefore, that the formation of these so-called 

 bacteroids in root nodules is due to the presence of alkaloids in the plant. 

 The bacteroids are never so large and numerous on the artificial culture 

 media as in a young nodule; they are produced, either in the medium, 

 or in the nodule due to specific nutrition or to unfavorable conditions; 

 in that stage they are hardy and multiply rapidly. According to Zipfel, 

 the branching forms are not degeneration forms, but may be looked 

 upon as a normal and necessary stage in the life of the organism with 

 specific biological functions; they are formed from rods and change 

 again into rods when inoculated into proper media. 



Five stages in the life cycle of the Bad. radicicola, through which it 

 passes under cultural conditions, were recognized. 141 



1 . Non motile, pre-swarmer form, obtained in 4 to 5 days when a culture of the 

 organism is placed in a neutral soil solution. 



2. Larger, non-motile coccus. The pre-swarmer coccoid changes in the pres- 

 ence of saccharose, certain other carbohydrates and phosphates, by increasing in 

 size until the diameter has doubled. 



3. Motile, swarmer stage, when the cell becomes ellipsoidal and develops high 

 motility. 



4. Rod-form, as a result of the further elongation of the swarmer, with decreas- 

 ing motility. 



5. Vacuolated stage. When available carbohydrates become exhausted or the 

 organism is placed in a neutral soil extract, the cell becomes highly vacuolated and 

 the chromatin divides into a number of bands. Finally these bands become 

 rounded off and escape from the rod as the coccoid pre-swarmer. The pre- 

 swarmer stage is usually formed from normal rods in calcareous soils, when 



136 Stutzer, A. Die Bildung von Bakteroiden in kiinstlichen Nahrboden. 

 Centrbl. Bakt. II, 7: 897-912. 1901. 



137 Buchanan, R. E. The bacteroids of Bacillus radicicola. Centrbl. Bakt. II, 

 23: 59-91. 1909. 



138 Zipfel, H. Beitrage zur Morphologie und Biologie der Knollchenbakterien 

 der Leguminosen. Centrbl. Bakt. II, 32: 97-137. 1912. 



139 Fred, 1911 (p. 127). 



140 Barthel, C. Contribution a la recherche des causes de la formation des 

 bacteroides chez les bacteries des Legumineuses. Ann. Inst. Past. 35: 634-647. 

 1921. 



141 Bewley, W. F., and Hutchinson, H. B. On the changes throughwJufllLthe 

 nodule organism (Ps. radicicola) passes under cultural conditionfl^JtyWr./&gr>\ 

 Sci. 10: 144-162. 1920. /<^O or ' *o VN 



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